Carbon Dioxide: An Open Door Policy

[Content note: reading this post might cause feelings of suffocation or provoke panic attacks in susceptible individuals. Epistemic status is very speculative.]

Last month I moved into a small cottage behind a big group house. The cottage is lovely. The big group house is also lovely, but the people in it started suffering mysterious minor ailments. Headaches, fatigue, poor sleep – all the things that will make your local family doctor say “Take two placebo and call me in the morning”. Using my years of medical training and expertise, I was able to…remain completely unaware of the problem while my housemates solved it themselves.

There’s been a flare-up of research interest in indoor carbon dioxide levels, precipitated by a Berkeley study (paper, popular article) finding that increasing CO2 concentration from the level of a well-ventilated building to the level of a poorly-ventilated building had profound effects on cognitive ability, cutting various test scores by as much as 50%. This was so dramatic as to be implausible, but seems to match the result of previous Hungarian studies and a later Harvard study on the same subject. The Harvard team later replicated their result with real workers in real offices and found that, controlling for other factors, workers in the best-ventilated offices scored about 25% better on cognitive tests than in the worst-ventilated ones. NASA got really interested in this research because spaceships require a lot of intellectual work and don’t have a lot of open windows. They’re still running tests but they say that “preliminary results suggest differences” between better- and worse- ventilated environments.

On the other hand, a 2017 study failed to find the effect, possibly because their cognitive tests were easier. And bloggers have pointed out that submarines have more CO2 than the worst terrestrial buildings, but don’t have any problems overt enough for the Navy to notice or worry. So it’s a crapshoot of contradictory results and considerations, just like everything else.

Aware of this research, my housemates tested their air quality and got levels between 1000 and 3000 ppm, around the level of the worst high-CO2 conditions in the studies. They started leaving their windows open and buying industrial quantities of succulent plants, and the problems mostly disappeared. Since then they’ve spread the word to other people we know afflicted with mysterious fatigue, some of whom have also noticed positive results.

When I heard about this, my first question was: didn’t any of these people notice they only felt bad at home? Shouldn’t it have been a big red flag when they went to the office, or went for a walk, and all their problems disappeared? This can’t be too big a deal, or else “I feel bad in my house, but fine everywhere else” would be a more common complaint.

My housemate Kelsey referred me to the work on CO2 and sleep. Right now this is just a few papers by a guy named Strøm-Tejsen, but the implications are pretty important. He notes that however bad your carbon dioxide levels are during the day they’re probably much worse at night, when you shut yourself up in a small room, close all the doors and windows, and just breathe for like eight hours straight. Normal outdoor air is about 400 ppm CO2 (more by the time you read this; thanks, fossil fuel industry!) A well-ventilated building during the daytime is about 700 ppm, and a poorly ventilated building during the daytime about 1400 ppm. But the average bedroom at night can be 2000 ppm or more. Friend-Of-The-Blog Gwern got a CO2 monitor to test these findings, and confirmed that while his daytime CO2 was around 500 ppm, nighttime CO2 in his bedroom could get as high as 3000 ppm. MIT’s Joel Jean discussed trying the same in this Medium post, with similar results:

I live in California, and Gwern presumably lives in some kind of formless cybermatrix, so we don’t have to worry about seasons. But Dr. Jean lives in Massachusetts, and he found that during the winter, indoor CO2 went up even further, in some cases exceeding OSHA’s rules for permissible workplace exposure:

If CO2 can affect sleep quality, that would explain how it could produce a whole-day effect. Strøm-Tejsen tests this on sixteen subjects and finds that “objectively measured sleep quality and the perceived freshness of bedroom air improved significantly when the CO2 level was lower, as did next-day reported sleepiness and ability to concentrate and the subjects’ performance of a test of logical thinking.” Good things about this study: subjects were blinded to condition, the paper contains a pilot experiment and a main experiment which mostly replicate each other’s results. Bad things about this study: the experiments were about n = 15 each, the researchers didn’t correct for multiple comparisons, and they admit to manipulating the statistics surrounding their logical reasoning tests to get better results. But if I just look at their tables and try to ignore their manipulation, I’m at least kind of impressed:

And experts seem to take their results seriously – for example, here’s NASA again. And we know from sleep apnea and studies that high physiological levels of carbon dioxide can cause sleep disturbances. I can’t figure out how to convert external ppm to internal likely level of carbon dioxide in the blood, but maybe this could provide a plausible mechanism.

I’m reluctant to be too numerical about all this, because everything about health has massive individual variability. Three people share one of the bedrooms at my group house (look, the Bay Area is really bad). One of them got the typical symptoms of excess CO2 really bad; the other two were fine. Some people are just going to be more sensitive to this kind of thing – the same way three people can drink the same amount of alcohol, two of them will get pleasantly buzzed, and one of them will black out.

I’ve tried sleeping with my door open the past few nights, and I haven’t noticed any difference. Probably I shouldn’t; my house is well-ventilated and I wasn’t feeling too bad beforehand. But I’ve started recommending a few of my patients with mysterious sleep issues try the same thing. It’s too early for results so far, and the science behind it is weak, but it seems like a cheap experiment.

Since the main source of CO2 is human exhalation, I’m most worried about buildings where many people are crammed into small spaces in close proximity (hello, Bay Area readers!). Since the main way CO2 gets cleared is through ventilation, I’m most worried about buildings made to strict environmental standards with great insulation (hello, Bay Area readers again!).

If you’re concerned about this, the best solution is to open a window or an internal door in your bedroom at night. If for some reason this is impossible, the second-best solution is to get certain succulents or other plants that participate in the ominously-named process of “dark fixation” – ie do their plant breathe-in-CO2-and-breathe-out-oxygen thing at night. This is also called “crassulacean acid metabolism” and Googling either term will get you a list of appropriate species. It will probably take like ten succulents to do much to CO2 levels, but a room full of succulents on every flat surface is also kind of #aesthetic.

I’m interested in more data on this, so if you’re planning on experimenting with changes to your nighttime air quality based on this post, please fill out this form to register for an informal quasi-experiment. I’ll follow up with a form for you to give your results in a couple of weeks.

Oh – and sorry for the content warning at the top, but I’ve felt kind of low-grade suffocate-y throughout writing this post, and had to go out and take a couple of breaths of fresh air a few times. Remember – perception is fundamentally Bayesian, and combines external sensation with internal expectations; this is why placebos can have such a profound effect on pain. Perception of air quality vs. suffocation seems to be especially susceptible to this, which is probably one of the major etiological factors behind panic attacks. Just repeat to yourself that it all adds up to normality: the air quality in your room hasn’t changed since when you were feeling just fine before you started reading this article, so you should be okay.

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213 Responses to Carbon Dioxide: An Open Door Policy

  1. amaranth says:

    googling does not get me appropriate species. can you recommend some plants to buy (reader)

    • MrBubu says:

      You are probably ok just buying any succulents. From Wikipedia

      The majority of plants possessing CAM are either epiphytes (e.g., orchids, bromeliads) or succulent xerophytes (e.g., cacti, cactoid Euphorbias), but CAM is also found in hemiepiphytes (e.g., Clusia); lithophytes (e.g., Sedum, Sempervivum); terrestrial bromeliads; wetland plants (e.g., Isoetes, Crassula (Tillaea), Lobelia;[11] and in one halophyte, Mesembryanthemum crystallinum; one non-succulent terrestrial plant, (Dodonaea viscosa) and one mangrove associate (Sesuvium portulacastrum).

      Ikea carries e.g. succulents as well as some euphorbias or crassula
      You can also see e.g. here

    • jastice01 says:

      I think the most common recommendation for this is Sanseviera Trifasciata aka “mother-in-law’s tongue” or “snake plant”

  2. tcheasdfjkl says:

    The problem with opening my window while sleeping is that then it gets cold and, worse, noisy, which ALSO interferes with sleep. :/ this is my least favorite tradeoff and a good example of what somebody recently called “galaxy brain: having competing access needs WITH YOURSELF”.

    A little while ago I borrowed a CO2 meter, found that my CO2 does get super high at night, got some succulents, observed CO2 that was lower but not low enough, got more succulents, and saw that actually the CO2 was just as bad as it had been. I now think I’d need a truly absurd amount of plant material to make a real dent and have hopefully temporarily given up on trying to solve this, at least while I solve things that are definitely problems as opposed to only maybe problems. Though if anybody has suggestions of how to improve ventilation without increasing noise, I’ll be happy to hear them.

    • sharper13 says:

      That seems to confirm what this guy figured, you’d need way too many plants to have much of an effect.

      It seems like the best way to go would be to get an oxygen concentrator and set it up to pull oxygen from outside to emit inside the room. Obviously, just concentrating the oxygen from within the room itself isn’t going to help. A machine for home use seems to range from $400 (used) to $2000 (new, higher end). They seem to be really designed for mask delivery, rather than general air delivery, though.

      • tcheasdfjkl says:

        How does one pull in air from outside without opening the window?

        • sharper13 says:

          When I’ve done it, I’ll generally use a conduit from the local home improvement store, basically a flexible pipe built for airflow. If you’re renting, then you’d have to use one designed for in-window use.

          Of course, for your purposes, probably best to just skip all that and pipe in regular air from outside using a fan, assuming the temperature is ok, or install a window AC unit if you need cooling as well.

          • Lapsed Pacifist says:

            AC units recirculate air, they don’t exchange it. You won’t get fresh air from a window AC unit.

          • The Nybbler says:

            A lot of window AC units have a little door that connects the outside to the inside, which you can open with a switch. I’m not sure how much ventilation you’ll get through it (probably not much).

          • noddingin says:

            @Lapsed Pacifist
            “AC units recirculate air, they don’t exchange it. You won’t get fresh air from a window AC unit.”

            The original window swamp coolers brought in fresh air only. You had to have a window open somewhere else to let the air out, or the house got muggy. This kind are probably still available.

          • toastengineer says:

            Presumably that’ll only work if you live in a place where evaporative cooling makes sense, though.

          • noddingin says:

            Regardless of cooling inefficiency, the swamp cooler is always bringing in a large amount of fresh air, filtered of (some) particles by going through the moistening pads (even with the water turned off). If you want more filtering, the structure that holds the pads is a good base for adding an extra filtering sheet, and the fan is strong enough to handle it.

      • zzzzort says:

        I don’t think that would work. Higher concentrations of CO2 =/= lower concentrations of O2. For one thing, the O2 concentration is almost 100x higher than the CO2 concentration, so the concentration of O2 isn’t changing much at all. For another, your body is able to sense the level of CO2, but not the level of O2, leading to fun effects like people suffocating in pure nitrogen environments without ever getting a feeling of suffocation.

    • sty_silver says:

      Have you considered just sleeping with earplugs? (And then open the window). Or even white noise, but earplugs are much easier to get used to.

      • Malarious says:

        I’ll second this; I picked up a pair of earplugs last week and I have no idea how I survived this long without them. Reading and sleeping is so much easier.

        This post has me considering picking up a CO2 monitor now. I always have my windows open (no air conditioning leaves few other options) but I’m also in a basement suite and the windows are 7 feet above the ground, so airflow isn’t great.

      • Elliot says:

        Earplugs are quite uncomfortable for me and tend to fall out in the night anyway (perhaps because I haven’t tried expensive ones), but I would recommend ambient noise. I have a tape of a car ride in the rain that I put on repeat through the night. There’s a lot of ASMR/Sleep tapes available depending on what you find useful. The sound is quite relaxing, and since I sleep with the window open it lowers my sensitivity to outside noise.

        • Jon S says:

          +1 to this, I disliked ear plugs a lot, but getting a whitenoise machine for my bedroom contributed to a large improvement in sleep quality.

        • Squirrel of Doom says:

          Regular cheap foam earplugs don’t fall out if you squeeze them tight and shove them all the way in, as intended.

          Maybe that’s what you consider “quite uncomfortable”, but then again maybe no one told you. I found them hardly noticeable after taking 1-3 night getting used to it. And they’re *very* effective.

          • Noumenon72 says:

            From working in a factory, the prescribed technique was to roll them, lift upon the top of your ear to straighten the ear canal, insert them until the end was near flush with the pinna of the ear, and wait until the memory foam expanded fully. When you do it right you can hear your own footfalls despite all the machinery.

          • The Nybbler says:

            Yep, @Noumenon72’s technique is illustrated on my packets of earplugs. They won’t fall out, but they tend to seal more reliably in one of my ears than the other. Also you only get a couple of tries inserting them before they won’t work any more. When they work they block out sound really well, but they’re also a little disturbing, I think they probably effectively amplify the sounds of your own body.

          • AG says:

            I’ve done this, but they still fall out in the middle of the night 100% of the time. And often end up in places I don’t expect them, so I kind of wonder if I’m manually pulling them out in my sleep.

          • Lambert says:

            You could try musician’s earplugs. I use Senner, but I’ve seen Alpine recommended often (both are available on Amazon. I find them far more comfortable than foam ones.
            (caveat: my normal use case involves being at the club a good few drinks in. That may well affect perceptions of comfort)
            My ears don’t tend to accept earplugs well. I use foam shell defenders to protect my hearing from power tools.

      • tcheasdfjkl says:

        I’ve considered earplugs but dragged my feet on actually trying them because when I’ve used earplugs in the past I found them really sensorily annoying. Plus, I already have trouble waking up to my alarm, and I worry that this will be even worse if my hearing is impaired by earplugs!

        That said, it is kind of silly for these predictions to prevent me from even trying a cheap intervention, so I probably will try this one of those days.

        • AG says:

          Use a vibration alarm (like from your phone) and place it on the bedframe.

        • toastengineer says:

          Or get ear-pluggy earbuds and plug them in to your phone, that’ll solve both problems. I’ve had to do that and it worked alright for me. There’s an app called Noise Wall that just makes brown noise, which I find is a close second-best to silence.

    • nameless1 says:

      Tell the neighbors to STFU 🙂 We have this problem in the summer, because some genius managed to design the flat so that A/C is nearly impossible, as it constists of balcony doors, glass walls, and walls to neighbors. Things like windows or normal walls not. So it is all wide open in summer nights and even silent discussion a problem. I mean, if someone is having a party at home, and as usual half the people are out in the balcony because that is where you can smoke your cigs without having to repaint the walls all the time, even if they try to be silent, they can’t be. But summer is a time of suffering and survival anyway, just walking to the tramway makes one drenched in sweat. I don’t do anything big just try to survive. In the rest of the year, leaving the window, or in our case, balcony door that is the farthest away from the bedroom open provides adequate ventillation and not much noise.

    • Vitor says:

      It might be more effective to get a small air purifier rather than a dozen succulents.

      I’ve bought (for unrelated reasons) a cheap air purifier that is quiet and includes an activated carbon filter. It cost 110 euros. According to what I’ve read on wikipedia, activated carbon basically absorbs CO2 when air concentrations are high, and releases CO2 when air concentrations are low. So the filter itself should not get used up.

      To reduce CO2 at night, you should probably leave it running 24/7, and open the window during the day when you’re away, effectively timeshifting your ventilation requirements.

      • Kibber Shuriq says:

        You’d have to replace your filter fairly often if it does work (which it probably will not). A human produces approximately 1kg of CO2 per day. Even if only 10% of that is produced during sleep (seems like a conservative estimate), that’s still 100gr per night. Your filter probably weighs just as much!

      • mustacheion says:

        Activated carbon is great for filtering lots of things, but I am highly skeptical that it would do anything at all for CO2. I don’t actually have any hard evidence for this claim, just my general science intuition.

        • Migratory says:

          I thought the same, but a brief search found a few articles attempting to optimize activated carbon for CO2 adsorption for industrial CO2 capture, so it looks like there’s some potential. I’m not sure how a non-optimized carbon filter will work for CO2, but the idea that it exists in some kind of equilibrium with atmospheric CO2 isn’t out of the question.

          I like this better than the succulent idea – those things grow so slowly that it’s hard to imagine they’re doing much.

    • mingyuan says:

      I’m right on Shattuck so the noise levels are insane, but I sleep with all of my windows open every night because I find that it’s worth it for the temperature and air (apparently sleeping in a cold room is actually good for your sleep). I use these earplugs (recommended by Wirecutter; best cheap earplugs I’ve encountered) and this white noise machine and I can…. usually stay asleep. Something like these sleep headphones might help too, though I haven’t tried them.

      But yeah having competing needs with yourself is the worst. I wish the world were 10000x quieter :/

    • Lapsed Pacifist says:

      I have a dual fan window unit which fits into a standard window sash, and I have it exhausting 24 hours a day. This pulls air from inside the house through my room and out. I always have a pleasant draft, the air is fresher, and I enjoy the white noise. It helps distribute the AC and heat from the center of the house and I can set it to one in /one out to exchange fresh air directly if I want.

    • Yosarian2 says:

      I wonder if sleeping with a fan in your room to circulate the air better might help?

  3. nameless1 says:

    In our school the teachers would open the windows every time we would become inattentive and it really invigorated our attention. I did the same when I was giving trainings and it worked. Fresh air is really a booster after the air in a crowded room becomes stale. If not CO2 then what?

    • Tatterdemalion says:

      My guess would be temperature, or conceivably breeze (which produces a cooling effect) – people are more likely to drowse and lose concentration when they’re too warm.

    • poignardazur says:

      Sensory stimulation? Your brain gets bored, thinks “We’ve been in this room for hours, we’re probably not going to get jumped by a tiger”, then the temperature changes, and your brain registers it as an environment change. (this is completely speculative, of course)

      • sharper13 says:

        Yeah, an interesting business experiment regarding change is that if you go through and change the light levels to make them brighter, productivity increases for a while, then drops back down again.

        Of course, if you then go through and make them dimmer again, again productivity increases for a while, then drops back down again.

        Turns out it’s not anything to do with the light levels per se, but that people do respond to changes in their environment.

    • Ann Socolofsky says:

      My ex-husband is a mechanical engineer. He focused on heating, ventilation and air conditioning. I had a classroom in medical school where I would always feel drowsy midway through the class. He looked over the room and told me that it did not have adequate ventilation to handle the number of people in the room. He thought that decreased oxygen and increased CO2 would occur when the room was fully occupied and lead to drowsiness. Apparently engineers have formulas for these problems and they use them to figure out adequate ventilation for a given room. It might be helpful to see if any engineers can provide input on the problems discussed above. I believe it is a concern to people designing buildings and they might have more information about what to do.

  4. outis says:

    When I heard about this, my first question was: didn’t any of these people notice they only felt bad at home? Shouldn’t it have been a big red flag when they went to the office, or went for a walk, and all their problems disappeared?

    Personally, I find it extremely hard to notice the causes of “general” malaises (anything that is not caused by trauma or by an obvious fast-onset disease). I get skin rashes sometimes, I guess I may be allergic to something? I often have a stuffy nose, same thing? I definitely sleep poorly, but who knows what the cause is? I have diarrhea semi-regularly, but maybe that’s normal? Maybe it’s caused by some type of food? I have no clue at all.

    As a matter of fact, I do sleep poorly and I am pretty sure the CO2 in my room is high. However, I can’t open the window because it’s noisy, and I can’t open the door because my flatmate’s cat would come in. I do try to crack the window during the day, so at least there is some air exchange.

    • poignardazur says:

      Same here. Sometimes I feel productive and alert and motivated, sometimes I feel depressed and despaired and everything seems impossibly complicated. Sometimes the cause is obvious (a breakup, severe burn-out, etc), sometimes I have no idea what it is.

      I try to follow most “how to sleep well” guidelines and do morning exercise to be safe. I guess I can add “have lots of oxygen” to the list.

      • mustacheion says:

        “Having lots of oxygen” is not at all symmetrical to “having low CO2”. While both gasses are exchanged in the lungs while breathing, the mechanisms for transporting them around the body are totally dissimilar. It would be possible to both have high CO2 levels and high oxygen levels by, for instance, breathing air that was 20% CO2, 80% oxygen. Normal air is 22% oxygen and about 3% of 1% CO2.

    • j1000000 says:

      Seconded. I can even remember two distinct cases where I had some new symptom that turned out in retrospect to have a very obvious cause, but I simply could not figure it out until a doctor pointed it out to me in one case and a friend in another.

  5. Anonymous Bosch says:

    It will probably take like ten succulents to do much to CO2 levels,

    Are there any studies of CAM plants in a room with sleeping humans? My blind hunch is that you’d need a lot more than that to make a dent; humans exhale about 2 lbs of carbon per day while a whole-ass mature C3-metabolizing oak tree still only absorbs about 50-100 lbs of carbon per year.

  6. summeroff says:

    A while ago I have seen a study that link cognitive decline to a level of bioeffluents in indoor air that usually correlates with a level of CO2.

    • jastice01 says:

      Do you have any more pointers on this? What kind of effluents? Some of them may well be amenable to plant or charcoal filtering.

      • summeroff says:

        You can find that study by the title “Physiological responses during exposure to carbon dioxide and bioeffluents at levels typically occurring indoors.”. Also maybe some useful can be found in related articles on pubmed.

    • DNM says:

      I am in a field related to the work in the article and have affiliation with one of the research centers cited (though did not work on this). The point you make is important. Some of the studies are examining the effects of CO2 specifically. Others are looking at spaces or times with high CO2 but not controlling for other things that are generally highly correlated with CO2. There’s lots of discussion currently about which of these things is the actual cause. First line defenses of extra ventilation (even just air circulation to disrupt your little bioeffluent bubble) are probably the same regardless of the culprit. Some others may not be (such as which plants are how helpful).

  7. fion says:

    My sleep is pretty bad. I get a good 8.5-9.5 hours sleep a night, but still feel tired all day. I’ve eliminated most of the likely candidates, so I’m intrigued to have a new one.

    The problem I have is similar to the person who commented about noise and cold, but for me it’s light. I sleep with the window open, but in order to block out light I have a thick blind that almost completely ‘closes’ it again, which I realise is pretty dumb. I’ll try leaving the blind partially open and opening my internal door and see if my sleep gets better or worse.

    • poignardazur says:

      Have you tried sleeping with a cap/hood over your eyes? It’s what I do, and it’s a lot better than a sleeping mask (it doesn’t slide off as easily).

    • ChrisA says:

      I use an airline sleeping mask – works pretty well.

      Also I wonder if you are trying to sleep “too much”. If I stay in bed too long, I can’t get to sleep the following night until late, and the problem starts to compound. I found that keeping as regular times as possible going to bed and getting up was better than longer in bed.

      • fion says:

        As poignardazur says, I find these slip off in the night and I get woken up by the dawn.

        The thing about sleeping too much is definitely an interesting one, but I think I’ve ruled it out. There have been times when I’ve been forced to get smaller amounts of sleep and I’ve never felt good on it.

        I am pretty good at regularity. 90% of the time I put my light off between 2245 and 2330 (after maybe half an hour of reading in bed) and I pretty much always wake up between 0830 and 0900, sometimes with the help of an alarm but normally not. (In case you’re doubting my maths at this point, it takes me at least 30 minutes to get to sleep. I think it all adds up. 😛 )

        • ChrisA says:

          That seems like a lot of sleep to me! Because of where I live, I go to bed around 10pm and I am usually awake by 5:30 (usually awake via alarm). I don’t find this too little. Not endorsing it, but there is plenty of stories out there that claim that too much sleep is unhealthy, here is one random article;

          • fion says:

            Thanks for the link. I’ve heard many of those sorts of arguments before and I’m a bit skeptical of how easy it is to establish causation. (The piece you linked acknowledges this…after browbeating the reader with lots of correlational evidence!) However, I will point out that my average is about 9 hours of sleep, which is the edge of the normal range. I’ve always assumed that I’m just towards one end of the bell curve in needing more sleep than average.

            I’ve also heard that alarm clocks should be avoided because they can interrupt deep sleep and that it’s better to wake up naturally. The book “Why We Sleep” by Matthew Walker says that if you’re being woken up by an alarm it’s a sign that you’re not getting enough sleep. But how do I limit my total sleep time without doing that?

            I sometimes wonder if the “oversleeping” people are doing that thing where you wake up, don’t feel like getting up, stay in bed with your eyes closed, fall asleep again, repeat. I never do this. As soon as I wake up I get up and stay up. This also makes me think I’m not oversleeping.

    • Hyperfocus says:

      When you eliminated sleep apnea, did you rule it out based on whether or not it was bad enough that insurance would pay for treatment/a sleep study? The reason I ask is because when I got tested (take-home oximeter), I was barely outside the range where a full sleep study would be paid for. That is, I was technically in the “normal” range. But since I knew I used to sleep much better, I tried one of the tongue-suction mouthpieces that prevent your tongue from rolling back and cutting off your air supply, just in case*. It has made a world of difference. Now the only time I wake up feeling like I haven’t slept at all is when I forget to put in the mouthpiece before I go to sleep.

      No disrespect intended to our host, but it seems that most doctors don’t care** about getting your health to 100% if you’re at 80%, as long as 80% is within the normal range, so it behooves us to take the lead on solving our “minor” health problems.

      *I cleared it with my primary care physician to make sure it was safe for me to do before I tried it.

      ** More accurately/charitably, they don’t have enough time per patient to care about getting them all to 100%.

  8. Michael Cohen says:

    I wonder how fast bubbling air through water pulls CO2 out of the air. Since bubbled CO2 is equilibrium with carbonate (CO3) and bicarbonate (HCO3) ions, one might be able to capture them with a precipitate. I looked at a solubility table, and some toxicity information, it looks like magnesium acetate mixed in water would be the best bet. Magnesium acetate is very soluble, and magnesium bicarbonate is not.

    Ba(OH)2 might work better kinetically (the basicity would lower the free energy of the intermediate state when you just have carbonate or bicarbonate dissolved and unattached to any positive ion). From what I can tell, I think thermodynamically, it might be slightly worse since only BaCO3 would precipitate, and not Ba(HCO3)2, but I’m not sure. Also, whereas magnesium acetate and magnesium carbonate are edible, barium is poisonous. This might be fine, since you’re not planning on drinking it, but I’d want to do more research into whether it could foul the air.

    Also, you can’t just get the best of both worlds by adding base to magnesium acetate, or you’ll precipitate out magnesium hydroxide instead of magnesium bicarbonate.

    EDIT: You might precipitate out MgOH anyway, which would compete with MgCO3. Also Barium is very heavy, so probably wouldn’t get into the air. If someone could find anything that confirms this, I think Barium would work better.

    • Michael Cohen says:

      Here’s something that would help the kinetics of capturing CO2 from the air: bubble it through blood. Added benefit: “Hey babe, want to see my blood fountain?”

    • ana53294 says:

      I imagine the machine would be really noisy, and it would have a similar effect to opening windows, though. Unless you get used to it as white noise.

    • ChrisA says:

      Air re-breathers are common in diving to remove accumulated CO2, lots of technology exists on this, see the Wiki article – I would guess that if this science on the effects check out probably someone would be selling a system for use in houses, you can have something that works by physical adsorption of CO2 onto a solid absorbent which is automatically being regenerated and vents the CO2 to the outside. Maybe for warmer climates you can imagine this could be combined with AC.

    • Michael Cohen says:

      Another idea: bubble air through algae-water under fluorescent light. Obviously the whole system needs to be covered since you want it dark in your room.

    • Lambert says:

      It’s called scrubbing. Used in space and on subs a lot.
      Back during Apollo, they used LiOH, but the current state of the art is LiO2.
      However, this process uses up the scrubbing compound, and is thus only suited to short term use.
      The ISS uses two different long-term scrubbers, the Russian Vozdukh and American CDRA.
      Both force air through zeolite molecular seives, which adsorb CO2. When they become saturated, the CO2 is vented by exposing the zeolite to vacuum.

      These systems are complex, expensive and hard to maintain. For these reasons, they only really see use in space, submarines and other situations where opening a window would lead to fairly immediate death.

      • Also, most of us don’t have access to vacuum to empty the scrubbers…

        • ChrisA says:

          You don’t need vacuum to regenerate the Co2 absorbent, just heating it up will work. Absorbing Co2 from gas is a common process in oil and gas and chemicals industry, and they do it this way.

      • David W says:

        Alternately, you could use the off-the-shelf solutions developed for labs that do IR spectroscopy, such as this or this or this.

        Would want to set it up with tubing so that the solenoid noise is outside the bedroom and the CO2-rich regeneration air goes outside, and you need some sort of air compressor. It’s got to be cheaper than trying to home-brew something, though!

    • robirahman says:

      Barium wouldn’t get into the air. If you’re interested, look up the vapor pressure of aqueous barium carbonate.

    • blarglesworth says:

      The cheapest way to do it would be to use calcium hydroxide aka slaked lime, pickling lime, kalkwasser, etc. It’s available in 50-lb bags from home improvement stores and in much smaller quantities for aquarium pH/calcium/phosphate adjustment, pickling foods, and the like. Get a large aquarium bubbler and a few gallons of water in a container (either an aquarium or just a 5-gallon bucket), add more than enough Ca(OH)2 to form a saturated solution, and bubble your room air through the aquarium. Ca(OH)2 + CO2 –> CaCO3 + H2O. CaCO3 is insoluble at high pH, so it would just collect at the bottom. No need to find the perfect chemical and jack up the price (and toxicity, for barium); Ca(OH)2 will do almost as well as others at a fraction of the cost.

      I don’t know if this works as well as I think it should, but I’m going to try it soon and see if it reduces my apartment’s CO2 levels. It’s possible that the carbonate jams up the bubbler or something like that, but in principle it should work pretty well as a DIY CO2 scrubber.

      • Tenacious D says:

        I know I’m late to this thread, but I searched to make sure someone had mentioned this. It doesn’t even have to be Ca(OH)2–any hydroxide compound should work (the others are more likely to remain in solution instead of precipitating though). My coworker who runs our lab uses KOH to strip CO2 out of gas mixtures, for example.

  9. ana53294 says:

    CO2 assimilation by CAM plants is not that high. Even assuming we have a very high fixation plant such as a Crassula, which absorbs 179.3 mmol/kg, that means just 8g/kg, and I assume that most indoor plants are not that much heavier than a kg.

    Open windows are much more likely to have an effect.

    • fion says:

      I think volume is a more intuitive way to do it than mass. 1 mol is 23 litres. A bedroom might be 50,000 litres. We’re interested in a difference on the order of 1000ppm, which is 1 in a thousand, so we’re looking at about 50l of CO2, which is about 2 moles. That’s 12kg of Crassula, using the numbers from the paper you quote. This seems like a lot of plant to me, but not implausibly much. Maybe 10-20 medium-sized plants.

      • Rm says:

        Aloe might be a better choice – Crassula is not big at all.

        Not to mention that you still need convection, which means open windows or some such.

        • Kibber Shuriq says:

          I could not find CO2 absorption rates for any Aloe species, but if it’s anything like the lower-end CAM plants from the study above, you’d need much more than 12kg of it to produce the same effect. Even another Crassula – ‘Moonglow’ – was 3.5 times less efficient than Crassula Himmaturi, so you’d need ~40kg of it to decrease CO2 by 1000 ppm by the end of the night.

          • Rm says:

            And I have definitely seen Aloe plants of about 3-4 kg or more grown on windowsills – people even throw them out because “they are a pain to re-pot”. Growing Aloe biomass is just easier than trying to grow big succulents (in the common meaning of the word).

            OTOH, Crassula might be more efficient due to higher surface/volume ratio…

          • Kibber Shuriq says:

            OTOH, Crassula might be more efficient due to higher surface/volume ratio…

            I’m no botanist, but it seems that the “efficiency” of a plant at capturing CO2 is mostly about how fast it can grow mass per unit of existing mass (since the capturing happens in the form of new plant matter). That would probably also be somewhat influenced by the plant’s chemical composition, including percentage of water in it, but overall the faster it grows, the more CO2 it should capture.

          • Rm says:

            re: efficiency – it is somewhat more complicated, but you would need a plant physiologist for this (not a botanist). I can only say that when people go after fast biomass production, huge side effects happen (Azolla caroliniana (besides, it’s a water fern, might be of use), Heracleum sosnowskyi, Pistia stratiotes), one of them being “what to do with all this biomass”.

            I can also see problems from increased humidity – the more “leaf” area grows, the more water is breathed out into the air. (You can’t have CO2 fixation without water, and water will escape or the plant will wilt.)

  10. Lambert says:

    In ‘Endurance’, Scott Kelley (the dude who spent a year on the ISS) talks a lot about CO2.
    N.B: All his figures are quoted in mmHg, which should be equivalent to 568 ppm, assuming ideal gas and all that. I’ll convert them to ppm here.

    He says he’s sensitive enough that he has learned to estimate the CO2 concentration to a ‘high degree of accuracy’.
    The maximum level of CO2 for the ISS, set by international agreement, is 3400 ppm, whereas USN submarines tend to avoid going over 1100 ppm.

    >At two mmHg (1100ppm) I feel okay, but at around three (1700ppm) I get headaches and start to feel congested. At four (2200ppm), my eyes start to burn and I can feel the cognitive effects. If I’m trying to do something complex, I actually start to feel stupid, which is a troubling way to feel on a space station.
    Endurance, p89

    He feels like NASA could really be doing more to keep CO2 levels down.

    • Scott Alexander says:

      “The maximum level of CO2 for the ISS, set by international agreement, is 3400 ppm, whereas USN submarines tend to avoid going over 1100 ppm. ”

      Wait, is this true? For some reason I thought subs were really high. Everything would make a lot more sense if they weren’t.

      • vaniver says:

        This news article cites this paywalled book from 2007 as saying:

        “Data collected on nine nuclear-powered ballistic missile submarines indicate an average CO2 concentration of 3,500 ppm with a range of 0-10,600 ppm, and data collected on 10 nuclear-powered attack submarines indicate an average CO2 concentration of 4,100 ppm with a range of 300-11,300 ppm,” according to a 2007 National Research Council report on exposure issues facing submarine crews.

        Interestingly enough, the NRC noted that a “number of studies suggest that CO2 exposures in the range of 15,000-40,000 ppm do not impair neurobehavioral performance.”

        • WafflesWaffles says:

          Can we take the people who did badly on the low CO2 tests and put them on submarines to see what happens?

        • fnord says:

          The apparent disconnect between very high concentrations on submarines (and space stations) not having noticeable effects whereas comparatively lower concentrations in Berkeley group houses have obvious ones might owe to chronic versus acute exposure. The navy studies I’ve seen (for example, one that tests 3% CO2) are for exposures over several days, and notice physiological adaptation even over that timescale. If CO2 levels spiked over a period of hours, there might not be time for the body to adapt.

      • Lambert says:

        ‘The Navy has their submarines turn on their air scrubbers when the CO2 concentration rises above two millimeters of mercury, even though the scrubbers are noisy and risk giving away the submarine’s location. By comparison, the international agreement on the ISS says the CO2 is acceptable up to six millimeters of mercury!’

      • mlogan says:

        The us navy apparently has a huge sleep deprivation problem. There was a This America Life episode about this 6 months ago or so. Any cognitive decline from co2 might be masked by the much larger decline from lack of sleep.

  11. Rm says:

    Crassulacean, not crassalacean.

  12. Deiseach says:

    It sounds like the animal suffering people should turn their attention from battery farm hens to battery farm humans in the Bay Area. I think the notion of modern energy efficient insulation meaning poor ventilation is correct, old houses were draughty which was bad for energy efficiency but well ventilated.

    • Evan Þ says:

      Bad for comfort, too, from what I’ve read of the Victorian Era.

    • jefftk says:

      It’s possible to build with heat exchangers to have good ventilation without losing all your heat to the outdoors.

      • The Nybbler says:

        Yes, but it’s not common. I’ve seen various articles about super-energy-efficient buildings with ventilation through counterflow heat exchangers, but they never seem to get past the pilot point. I suspect they run into issues of the models not working so well in a physical building with thermal cycling, deterioration of sealing with time, UV, ozone, etc, the need for wall penetrations, rain, rodents, etc. Especially with light construction in the US. And of course cost.

        • ahenobarbi says:

          My parents installed active ventilation system with a recuperator and it’s been working without any problems for 10+ years. It wasn’t exactly cheap but it wasn’t very expensive either (~5.5k$ (in central Europe)). And it’s great!

          They enjoy all the benefits of having windows and doors shut (silence, lack of smells, lack of draught) and fresh air all the time (well unless there too many guests).

    • outis says:

      The worst thing is when you’re in an office building in the late evening (because you work too much), and they turn off the AC (gotta be green), and it gets so hot you’re sweating. But outside it’s nice and cool. You could solve the problem in five minutes, without spending any energy on AC, just by exchanging air with the outside. But you can’t open the windows, and evidently the ventilation system does not actually ventilate that much.

  13. Bla Bla says:

    By coincidence I actually did something to decrease my bedroom CO2 levels recently and did feel a noticeable increase in the quality of sleep.
    I used to sleep with my window closed and my internal door open, but there was little air circulation as proven by the fact it took forever to cool down the bedroom with the AC located in another room down the hall.
    This summer I decided to cut on AC use and now I place a fan at my room door and I keep the window partially open.
    The purpose of the fan is not only to push air in from the hallway, but also to create a constant white noise that covers the loud, frequent sounds coming from the outside, sounds that made it very hard for me to sleep with my triple-pane, soundproof window open.
    The first strange thing I noticed is that when I wake up I don’t hear the fan for a few seconds. My sleeping brain simply ignores the constant noise and uses the mute button.
    The other was that I sleep less, but I wake up feeling better rested than I used to do. Gone are the sluggishness and fatigue that accompanied my waking up even when I overslept.
    I wasn’t sure about the cause, but CO2 reduction fits best.
    If I’m right that means that leaving the door open might not be not enough if your house is not well ventilated, and since I replaced the old window panes with insulated glazing I have little ventilation unless I open a window or use the AC or a fan.
    Now I only have to worry if sleeping with a source of constant noise nearby is bad for my hearing.

  14. justinowings says:

    Would sleeping with the fan on help—at minimum circulating air over your bed and locally dispersing concentrations of CO2?

  15. AnarchistMiracle says:

    And bloggers have pointed out that submarines have more CO2 than the worst terrestrial buildings, but don’t have any problems overt enough for the Navy to notice or worry.

    Then again, despite being pretty well-known among sailors, the Navy’s problems with overwork and sleep management didn’t become “overt enough for the Navy to notice or worry about” until they reached the level of ships crashing and people dying.

    So even if CO2’s affect on health and performance becomes a well-known fact, I still wouldn’t necessarily expect the Navy to do anything about it.

    • gwern says:

      The Navy argument here is stronger than it looks. The Navy isn’t concerned because there is no ocular trauma about problems at 2500ppm or lower, and because they sponsored experimental studies investigating CO2 at various levels with various measures in various ways, and found pretty much nothing. I covered this in my original research roundup in To quote myself

      Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants, National Research Council (2007), chapter 3 “Carbon Dioxide” (; many negative mental and physical effects at extremely high CO2 concentrations >50,000PPM; consistent statistically-significant effects below that tend to be harder to find but from the descriptions, they often were not using sensitive tests of higher cognitive functioning, a broad array of different measurements, and very small sample sizes; I suspect a meta-analysis grouping tasks by domain with some correction for ceiling effects might turn in a very different conclusion than their fairly sanguine conclusion that there are no cognitive impairments <40,000PPM and <25,000PPM is a perfectly safe limit.

      But arguing about whether 25,000 PPM is perfectly safe is just another way of saying that ‘Satish can’t be right that 2,500 PPM turns you into a drooling idiot’.

  16. Levantine says:

    I have something relevant to report at once: I live in Eastern Europe alone in a flat of about 50m^2, most inside doors open, and tend to keep windows closed at night. In the early hours of August 18 this year, I woke up feeling kind of suffocating. I opened the door-window next to my bed. The hours that followed were so extraordinarily pleasant that I decided to note these things down in my journal. There I’ve written down that in my dreaming I have shown ‘wider curiosity, more dynamics,’ and that the experience had ‘at or around the core, more healthiness.’ The previous evening I have had ‘not a great time’ spent in a ‘bad sitting posture.’
    Each of the following nights I also opened the window. The pleasantness repeated itself with a reduced intensity.

  17. nkurz says:

    NASA got really interested in this research because spaceships require a lot of intellectual work and don’t have a lot of open windows. They’re still running tests but they say that “preliminary results suggest differences” between better- and worse- ventilated environments.

    While there doesn’t appear to be a published paper from the NASA followup, I was able to find a Task Book update:

    The task is now complete. This double-blinded cross-over study was conducted with 22 healthy, astronaut-like participants at the Johnson Space Center. Four groups, each comprised of 4-6 individuals, were exposed to each of four concentrations of CO2 in a sequence that varied for each group so that the order of exposure to the four concentrations (600 ppm, 1200 ppm, and 2500 ppm, 5000 ppm) was balanced among the groups. Volunteer subjects were allowed to acclimate to CO2 in a human rated chamber for 15 minutes before early effects of CO2 were assessed through administration of the Cognition battery (~20 min) via Apple iPad. After completion of initial testing and another 15 minute rest period (1 hour total post chamber entry), decision-making competencies were assessed by the Strategic Management Simulations (SMS), which were administered by laptop computer. The SMS lasted for approximately 80 minutes. After a final 15 minute rest period, subjects completed a second Cognition battery. These in-chamber data were compared to Cognition testing pre- and post-exposure results. Data were analyzed using linear mixed effect models with random subject intercept to account for the repeated measure design.

    Performance across the Cognition battery and on most measures of the SMS were lower at 1200 ppm than at 600 ppm; however, at higher concentrations of CO2 performance was similar to, or improved, relative to baseline for most measures. The complex decision making performance of astronaut-like test subjects in the present study and in naval officers in a separate study was found not to be as severely impacted by CO2 as it was in younger adults tested previously by Satish et al. (2012). It is possible that the decision making processes of young adults differ from those of older, more experienced, highly motivated and achieving adults; however, similar susceptibility of “professional grade employees” in a more recent study by Allen et al. (2016) suggests that particular experiences rather than age per se may correlate better with differences in decision making performances as measured by the SMS in ground-based studies. While astronaut-like test subjects in our study and naval officers are not affected by these CO2 levels, studies should be conducted to assess other performance degradation stressors on astronauts, like sleep deprivation and microgravity, together with CO2 exposures.

    So while they did seem to replicate the effect of a decrease in performance going from 600 ppm to 1200 ppm of CO2, they did not find any impaired performance at higher levels. Combining the earlier press release comment that the unblinded data “suggest differences” with the comment that “performance was similar to, or improved” one might even suspect that they found evidence of a paradoxical significant effect at at least one of the higher levels of CO2, and chose not to publish. Should we interpret this as a failed replication followed by a strong “desk drawer effect”?

    • albatross11 says:

      I wonder if physical condition is correlated with tolerance for slightly elevated CO2 levels. Naval officers and astronauts are likely to be healthy and in pretty good shape, after all.

  18. Corey says:

    Opening a window is great advice for the bay area, but what can you do in winter? I live in New England and opening a window in the wintertime is suicide.

  19. baconbits9 says:

    Some things that might help a few of the concerns about light and noise.

    1. Exchange fans. My wife is sensitive to light and noise but can sleep well with the window fan in. Because it is about 10 inches high we can pull the shade down to the top of it and block most light, and the fan itself let’s little through. The ambient noise of it running also covers up the road noise.

    2. If you have and attic space with a fan in you can get quite good whole house ventilation at night by blowing the hot air out of the attic which will draw air up in a mild chimney effect.

  20. Plumber says:

    Ventilation was such a concern at the turn of the last century that 
    heating radiators were sized to warm bedrooms when with open windows 


    “sleeping porches” were common.

    Anyway my main reaction is to:

    “…Three people share one of the bedrooms at my group house (look, the Bay Area is really bad)….

    ….buildings where many people are crammed into small spaces in close proximity (hello, Bay Area readers!)…”

    While in the half-century of my lifetime the most time I’ve every spent outside of the Bay Area was a few months living in Seattle in 1999, and working in Hollister in 2006, and I really don’t know what life is like elsewhere, but I read that 
    tent encampments are growing in cities outside of the Bay Area in the last decade as well (the R.V.’s were already here before this decade), and I’m bewildered that
    housing becoming less and less affordable isn’t the main topic all the time.

    Why not?

    • vaniver says:

      I’m bewildered that housing becoming less and less affordable isn’t the main topic all the time.

      I mean, it’s like Trump becoming president, or horrible weather, or something; people complain a lot and then get used to it. Unless you’re looking for housing, you’ll have a year-long lease and be able to mostly not think about it until you maybe need to find a new place.

      This issue is also directly adversarial; my “agh rent prices are ridiculous” is my neighbor’s “oh man, my house is worth so much money! Isn’t that great?”, and the homeowners tend to be the ones more invested in local politics than the renters.

      • Plumber says:

        “This issue is also directly adversarial; my “agh rent prices are ridiculous” is my neighbor’s “oh man, my house is worth so much money! Isn’t that great?”, and the homeowners tend to be the ones more invested in local politics than the renters”

        Well I’ve been a homeowner (for seven years now) but I guess I still think like a renter.

        Besides not wanting to cash in and move to the frontier, I’d like my sons to be able to live in houses that aren’t hours (or days) away, and the increasing numbers of the visibly homeless is worrying.

        Judging by news stories from Los Angeles and Seattle, plus the national rent prices charts I don’t think this is just a Bay Area problem.

    • jefftk says:

      I thought sleeping porches were more about cooling ventilation than freshness ventilation?

      • Plumber says:

        “I thought sleeping porches were more about cooling ventilation than freshness ventilation?”

        jefftk, Sleeping porches were made for pre-airconditioning times, but the Victorians were fearful of bad air “miasma’s” hence the oversized radiators (something I learned from “They Lost Art of Steam Heating” and other books.

    • tcheasdfjkl says:

      housing is absolutely the main topic all the time in local politics, though usually not in a very constructive way, unfortunately

    • Mary says:

      Besides the other reasons listed above, lack of affordable housing keeps the riffraff out without your needing to be obvious about it.

    • toastengineer says:

      Thinking of “sleeping porches,” having to sleep outside always felt kinda nice in a vague way, and maybe ventilation is why.

  21. konshtok says:

    you need scientific studies to tell you that stale air is not good?

  22. Yakimi says:

    I can’t find it now, but I remember reading a study which found that even normal atmospheric concentrations of CO2 have a deleterious effect on cognitive ability compared to a more oxygen-rich mixture of gases (I think the effect was 10% or so?).

    Does anyone know what I’m talking about?

    • bean says:

      That’s mixing up two different things. Atmospheric CO2 is about .4%. Atmospheric oxygen is about 21%. I suspect that you’d get more benefit out of a 30% O2/.4% CO2 mixture than you would out of a 21% O2/0%CO2 mixture.

      • [Thing] says:

        Wait, so if I understand those tweets correctly, they’re saying that breathing a helium-oxygen mixture instead of the naturally occurring nitrogen-oxygen mixture we’re used to (and I guess minimizing CO2 while you’re at it) is an effective nootropic? How practical would it be to try and self-expirament with this? I imagine one would need to use a face mask, but I already do that at night for sleep apnea.

        • vaniver says:

          When I looked into this, it seemed like it would be a few hundred dollars for a few hours of heliox, counting a bunch of fixed costs. Supposedly it’s about $30-$70/hr for long-term consumption, which I expect is likely more than it’s worth.

          • [Thing] says:

            That’s too bad. It’s still interesting that it works at all (if it actually does), given how rare it is for interventions to improve cognitive function beyond the baseline of a healthy person in a normal environment. Stimulants suck compared to actually getting good sleep, IMO, and I always build up a tolerance to them pretty quickly anyway. Good sleep, diet, and exercise require a lot of time and self-discipline. So the idea of getting smarter/more energetic by just breathing from a tank sounds almost magical.

          • Douglas Knight says:

            That’s treating helium as a consumable, right? Recycling helium has to be cheap at scale.

            An airtight building could use helium and recycle it. Who has airtight buildings? Spaceships and submarines. Biohazard labs probably don’t want unnatural atmospheres for experiments.

            For an individual, it would be difficult to capture the exhalation. But if you’re doing this sitting at a desk, probably not that difficult.

  23. helloo says:

    Another thing to consider regarding plants – most of them work to remove CO2 ONLY during the day with sunlight and such.
    At night, plants tend to absorb O2 and release CO2 like animals. There are some that still release O2 even at night but they are few in number and I have no idea how they work.

    Regarding windows – I wonder how much they will help do to airflow, rather than ventilation. Remember that one way to resolve hyperventilation is to just breathe in and out of a bag. Your exhales, if they stick to you probably contain a lot more CO2 than your environment.

  24. mcpalenik says:

    When I heard about this, my first question was: didn’t any of these people notice they only felt bad at home? Shouldn’t it have been a big red flag when they went to the office, or went for a walk, and all their problems disappeared?

    When I was in graduate school, I used to occasionally have days where I didn’t leave my apartment and I would always end up with a headache and feel kind of foggy, even if I did end up going out in the evening. I’m still not convinced this was due to CO2, but it certainly never would have even crossed my mind at the time.

  25. Douglas Knight says:

    I think it’s too early to fixate on CO₂. Most of the studies are about ventilation, which clears all kinds of pollutants, not just CO₂. But, yes, nighttime and crowded conditions increase CO₂ but not other pollutants.

  26. gmaxwell says:

    Some comments above are suggesting some mildly rocketsciency tools like CO2 scrubbers or oxygen concentrators.

    The standard tool for improving indoor ventilation where climate or other considerations preclude just opening a window is an Energy Recovery Ventilator ( e.g. ) which pumps inside air out and outside air in while passing both streams through a rotating heat exchanger which allows it to transfer the outgoing air’s temperature and humidity properties to the incoming air.

    You’d normally set it on a timer so that it runs runs when you’d expect CO2 levels to be highest… but you could get all fancy and use indoor and outdoor air quality sensors to trigger one based on its usefulness.

    This is a fairly standard appliance, and even in a mild climate where they aren’t critically important — e.g. the bay area — I found that competent HVAC people knew about them when I asked.

    Another thing to keep in mind is that many inexpensive CO2 sensors aren’t. CO2 is apparently somewhat costly to detect so there are many devices that have a VOC sensor and report CO2 numbers based on some model that equates VOC levels to CO2 levels for ‘typical’ indoor air. You don’t want to breath air with high levels of VOCs either… but if you’re testing CO2 specific mitigations and not finding them effective, make sure you’re actually measuring CO2.

    • Paul Brinkley says:

      Another thing to keep in mind is that many inexpensive CO2 sensors aren’t. CO2 is apparently somewhat costly to detect so there are many devices that have a VOC sensor and report CO2 numbers based on some model that equates VOC levels to CO2 levels for ‘typical’ indoor air.

      +1 This was a concern I had when I read about Gwern’s experiment being done with a NetAtmo. It’s not clear to me how reliable their CO2 meter is. Gwern uses another device to double check the NetAtmo’s measurements… of temp and humidity, not of CO2.

      Thanks to your comment, I now have a plausible explanation of a potential failure mode. Are there common phenomena that could alter VOCs without altering CO2 (as much)?

      • gwern says:

        Netatmo swears that their CO2 sensor is a real CO2 sensor; I kept hoping to find something cheaper but everything cheaper is either using a fake CO2 censor (often buried in the fineprint) or has some other fatal drawback.

      • gmaxwell says:

        > Are there common phenomena that could alter VOCs without altering CO2 (as much)?

        Absolutely! Lots of things emit chemicals that VOCs detectors detect… people, cleaning products, cooking, detergents, fragrances, … plants… Some plants also remove some VOC.

        I believe that fake CO2 meters work by equating VOC to CO2 levels in typical office environments where the primary CO2 and VOC sources are people respiration… so if you have VOC producers which aren’t reflected in that model, your results will be who knows what.

        I would assume that any CO2 mitigation strategy that involves anything except cycling out air or reducing the number of people will have its effects not reflected in a fake CO2 meter’s output.

        As far as how to tell what a meter is measuring– if it’s cheap it’s probably not measuring CO2. I speculate one could try to test by using CO2 (e.g. MIG welding gas, or baking soda + vinegar) and a Voc source like acetone (nail polish remover) or ethanol…

        But personally, I think specifically targeting CO2 only is probably not a great idea. If your air is high in CO2 it’s probably also high in other harmful chemicals, potentially including ones we haven’t identified. Circulating filtered outdoor air is a universal fix– unless the outside air quality too poor. But if the outside air quality is that poor there is likely no economically efficient way to deal with that, short of just moving someplace else.

        It’s even plausible to me that the correlational studies showing reduced cognitive performance in high CO2 environments could be due to other chemicals and that CO2 is only correlated because it predicts the respiration vs air replacement rate.

    • rm0 says:

      I’ve tried to do some research on this just now. Anyone who actually knows what they’re talking about, feel free to step in and correct me.

      According to this pdf, “Review of Sensor Technologies Used in Portable Gas Monitors”, the type of sensor to avoid looks like “Photoionization Detector[s] (PID)”.

      PID’s are commonly used for detecting Volatile Organic Compounds (VOC’s) … PID’s suffer from sensor drift and humidity effects, making calibration requirements more demanding than other common gas detectors. PID technology is most commonly found in portable instrumentation.

      The type I think you want is an NDIR/”Non-Dispersive Infrared Absorption”/Infrared sensor:

      … can be made very specific to a particular target gas. The limitation of NDIR technology for gas detection is dependent on the uniqueness of the absorption spectrum of a particular gas. NDIR sensors are also extremely stable, quick to respond to gas and can tolerate long calibration intervals. Infrared sensors are commonly used to detect methane, carbon dioxide and nitric oxides in both portable and fixed gas detection instrumentation.

      (emphasis mine)

      Wikipedia is not particularly helpful wrt discussion of VOCs, as it does not mention them. It says:

      The most common principles for CO2 sensors are infrared gas sensors (NDIR) and chemical gas sensors.

      NDIR sensors are most often used for measuring carbon dioxide. The best of these have sensitivities of 20–50 PPM. Typical NDIR sensors cost in the (US) $100 to $1000 range.

      Chemical CO2 gas sensors … On the downside, short- and long term drift effects as well as a rather low overall lifetime are major obstacles when compared with the NDIR measurement principle. Over time, the zero point of the sensor needs to be calibrated to maintain the long term stability of the sensor.

      I do not know if a PID is the same as a “chemical gas sensor”.

      Wikipedia’s page for NDIRs states (with a “citation needed”) that

      many measurements in the IR area are cross sensitive to H2O so gases like CO2, SO2 and NO2 often initiate cross sensitivity in low concentrations

      I am not sure how low, or if this is important.

      I found another article titled “CO2 Sensors vs. VOC Sensors for IAQ – What’s the Difference?“. This article is from a website selling CO2 sensors, and I wasn’t able to find this information elsewhere. Relevant information:

      CO2 sensing technology by IR (infrared) is stable and is not subject to the short-term, random drift found in air quality sensors. Most IR carbon dioxide sensors only measure CO2.

      Some manufacturers of air quality sensors are now providing an output in “CO2 equivalent units.” This measure is considered misleading and may confuse many new to the indoor air quality industry.

      In summary: look for NDIR sensors, not PID, not anything to do with VOC. Looking at the calibration interval may provide a proxy for the type of sensor. Most sensors are NDIR, apparently.

    • ahenobarbi says:

      Those should have filters inside, if you pick one with HEPA filter you don’t really need to worry about quality of air outside (if the air outside is bad even after removing 99.97% of particles of size <=0.3 µm you probably should relocate).

  27. Douglas Knight says:

    I find it hard to believe that breathing in CO₂ could affect cognitive function. The air we breath out has 100x the CO₂ concentration of the air we breath in. Assuming that the lungs are less than 99% efficient at gas exchange, the equilibrium concentration of CO₂ in the blood won’t depend on the CO₂ in the air. (In some of these bad conditions, the air we breath in has CO₂ elevated 10x. But lungs probably aren’t even 90% efficient.)

    We do have a panic response to high levels of CO₂, but I think that is generally believed to be driven by blood levels, not special detection in the lungs.

    • mcpalenik says:

      I don’t really know anatomy, but if it’s just diffusion, then you would expect blood equilibrium concentrations to roughly match the air plus a little bit because you have a CO2 “source” inside you. I kind of doubt your lungs have any kind of active mechanism for pushing CO2 in one direction. You’re right though, it’s a tiny change in concentration proportionally, but sitting here, typing on my phone without thinking about it too carefully, it seems plausible that it could have a noticeable impact. I doubt your blood CO2 level has to get all that high before you feel like you’re suffocating, and we’re talking about a small fraction of that.

      I also agree that it sounds a little weird and vaguely suspicious.

      • Douglas Knight says:

        It’s not “plus a little bit”; it’s a hell of a lot. Exhalation concentration is 100x inhalation concentration.

        • mcpalenik says:

          I would assume the “cleaned” blood is about the concentration of the external environment. It has to be if it’s based on diffusion. I was assuming that the body is reasonably good at keeping most of your blood cleaned and that the “dirty” co2 levels don’t represent the average in your body. Or at least not the average of what you need to send to the brain. Of course, I haven’t actually bothered to look into this at all.

          • mcpalenik says:

            And anyway, forget about averages, since that was just me being kind of lazy initially. What we really care about is the CO2 concentration in blood after it’s cleaned (since this cleaned blood is what gets sent around your body to be used), which is limited as a lower bound by the CO2 concentration in the atmosphere.

    • Cerastes says:

      Actually, your blood has WAY more CO2 than the air, almost all in the form of dissolved bicarbonate ions which serve to buffer blood pH fluctuations. That’s what’s *really* happening at your lungs (and, incidentally, your kidneys) – your body is monitoring blood pH, and dumps or retains bicarbonate to regulate that, because a 0.5 pH swing in either direction will kill you. This is why lactic acidosis, even though it’s a totally different acid, causes rapid breathing – your body is dumping CO2 to try to restore the blood pH balance until other systems can deal with the lactic acid (ditto for vomiting – dump hydrogen ions into the stomach then expel them). Conversions to and from bicarbonate (although they happen in any aqueous solution) are both mediated by enzymes, so it’s an active process.

      Interestingly, it’s actually a key issue in the vertebrate invasion of land. CO2 dissolves well in water even without enzymatic help, but the other direction, not so much. When you live in water, you can just dump bicarbonate straight out through your gills, but when you start breathing air, you need to force the reaction in the opposite direction. Add in strenuous movement (e.g. walking on land for the first time ever) and consequent lactic acid release, and if you want to keep your blood pH balanced, you’re gonna have a bad time. There’s even suggestion that the dermal armor of some early tetrapods served as a calcium reserve to buffer these changes in pH, as happens in modern crocodilians.

      TL;DR – it’s not a simple diffusion system, but an active system primarily concerned with regulating blood pH.

      • mcpalenik says:

        So i’m still unclear as to how co2 gets from your blood into the air in your alveoli. It seems like diffusion must play a role. So that would still mean you’dgave to breath more deeply or frequently or something to maintain the right co2 levels because the post “cleaning” co2 levels would still be higher than they were before (maybe only slightly though, I guess) if any part of process is diffusion limited. I could still see where it might trigger some mild suffocation reflex or discomfort.

        Anyway again, I do find this whole thing a little suspicious, I just don’t think is necessarily easily discounted. Edit i’m walking somewhere as I write this, so I can’t figure out what would be a significant change in co2 levels given that a relatively small ph change is deadly. I’m not sure if a change in a factor of 10 in air co2 levels would be insignificant or not but you could definitely calculate it.

        • Cerastes says:

          Think of it as assisted diffusion, but where the assistance level is regulated by the body. Normally, CO2 will dissolve in water as carbonic acid, and the process is reversible and concentration dependent. In the body, the reaction rates are increased (in both directions) by the enzyme carbonic anhydrase, and the body can choose to upregulate or downregulate production of this enzyme as needed.

          I highly doubt that blood pH is changing in response to CO2 concentrations; a 0.5 pH swing is lethal, but a 0.1 pH swing is extremely noticeable and bad (as in “requires immediate medical emergency care” bad). I suspect instead that the problem comes with too much bicarbonate and not enough ability to dump CO2 from the various tissues, like an over-congested freeway. However, it’s not an area I’m familiar with in more detail, as my favored tissue of study (muscle) is pretty resistant to these problems, especially in ectotherms (whose blood pH can swing a full point with no apparent ill effects).

          • mcpalenik says:

            So, taking a minute to look at this a little more carefully, it sounds like your body can increase the rate of H+ + CO3- CO2 + H2O, which should make more CO2 available to diffuse through the alveoli into the air.

            Just to check that, I wrote out a differential equation for diffusion (through alveoli into air) and for the reaction rates (I assumed linearity in the reaction rates, because I didn’t want to turn this into a complicated project). It turns out even with linear reaction rates, the resulting algebraic equation from the 3 coupled differential equations (1 for diffusion+reaction, 1 for reaction, 1 for diffusion) is basically impossible to solve. But I could take the limit in that the reaction rate is extremely fast and extremely slow (using 2 different Taylor series in omega and 1/omega).

            In the end, the only result I actually got was that when the reaction happens quickly, it’s limited both by the reaction rate constant and by diffusion (linearly), and when it happens slowly, it’s limited mainly by the rate constant.

            In the fast limit, omega = -(1478D/LV + 1479K), where D is the diffusion constant and K is the rate constant. In the slow limit, it’s proportional to K. So, this does sort of lend credence to the idea that the external CO2 concentration shouldn’t limit your body’s ability to expel CO2 if it can upregulate K, which should be more important than D.

          • mcpalenik says:

            I have to stop doing this without spending enough time thinking. I tried to post after work but before leaving and I realized on the way home that if I did what I thought I did, which was to linearize everything (diffusion and reaction rates) then the relative effects of D and K should have been equivalent. So I have no idea what I actually did. I’ll have to check my notes tomorrow. But it’s almost definitely wrong.

          • Cerastes says:

            Well, there’s another wrinkle – tidal ventilation, which is mostly controlled by CO2 concentration in the lungs.

            Basically, we take in atmospheric air (20% O2, minimal CO2), then hold that gas as we drain out the O2 and dump in CO2. In mammals (maybe birds? I’d have to check), exhalation is triggered by 5% CO2, which is why you *always* get 5% CO2 even during exercise – as CO2 production ramps up, we just breathe faster and deeper to get more air at once and changeover more often.

    • Robert Jones says:

      My thought exactly. I just don’t see how atmospheric CO2 could have the effects indicated at those concentrations.

  28. Neutrino says:

    With an aging boomer cohort among your prospective patients, do you notice any CO2 negative impact from use of C-PAP machines?

  29. Armand says:

    I wonder if this is an issue for people who live in their cars or the vanlife crowd.

  30. Aceso Under Glass says:

    Anecdata: I got a CO2 monitor, and the science-specified zone of concern exactly matched my perception of when a room felt stuffy.

  31. lifetilt says:

    I had no idea this wasn’t settled science. I work in life support systems for space flight and it’s generally accepted that serious cognitive issues occur at high CO2 concentrations. Crew members are trained to detect symptoms. However, in the safety community, there’s currently some controversy over whether crew member symptom detection is a reliable means of detection, in part because of the cognitive effects which may cause an affected crew member to fail to report, even if they know exactly what to look for.

  32. captbackslap says:

    Married people normally get significantly worse sleep (something like 40 minutes of actual sleep time per night) when they sleep with their spouses than when they sleep in separate rooms. This has generally been attributed to partner movement, but now I’m wondering if the added CO2 isn’t a big factor as well.

    • vaniver says:

      I have also noticed that sleeping in a room with my boyfriend makes a big difference heat-wise; I didn’t notice this before moving to the Bay Area because everywhere else had functional central air. ><

    • mcpalenik says:

      Married people normally get significantly worse sleep (something like 40 minutes of actual sleep time per night)

      What? I would look this up but it sounds way too far off to even bother (a cursory google search didn’t get me to the right information and I don’t actually care enough to put more time into it). Show me anyone who can function while regularly getting only 40 minutes of actual sleep per night. If that were the case, it would be worth it for my wife and I to sleep in separate beds, even if it had to be for an hour and a half (assuming that even alone, we could only get actual sleep just under 50% of the time), to save the remaining wasted hours when we could be doing other things.

      To be fair, I know you are capable of getting some of the benefits of sleep just by lying in bed quietly without actually being asleep, but 40 minutes seems way too short to be functional in the long term.

      • yodelyak says:

        You are misreading the parent post. The claim is that married couples get ~40 minutes of sleep per night less than others, not that they get 40 minutes of sleep per night total.

        • mcpalenik says:

          I guess that’s the only reading that makes any sense, but in that case, I would have expected it to be phrased more like “something like 40 minutes less of actual sleep per night” or even “by 40 minutes less per night”. Anyway, thanks for pointing out that interpretation because even after reading it several times, the only other possibility that came to mind was that he meant 40 minutes on the hour and had made a few mistakes.

  33. Nate the Albatross says:

    There is an odd tradition in Bretagne France where people (who live in stone houses in mild weather) open their windows and hang their bed coverings out the window every morning. My father-in-law does this and I see plenty of quilts when I walk around. I’m not convinced that the practice accomplishes anything, but it does point in the direction of people feeling suffocated at night and trying to remedy it somehow. CO2 probably also explains why some people are so refreshed by time outdoors.

  34. vaniver says:

    I want to note that “industrial-grade quantities” means “industrial-grade quantities.” When my office looked into doing this, I think we estimated that we would need to turn about a quarter of the office volume into plant space, and it would require a potentially serious amount of maintenance. (Plant leaves get dusty as part of their normal operation, since they’re sucking the air towards them; this then reduces their effectiveness until you wipe the leaves down.)

    The idea I had for Dragon Army, which we didn’t end up doing, is shelving around the top of the room, with succulents covering the whole surface of the shelves; it seemed like we could also rig up some sort of sprinkler system to automatically water them. But this was definitely a “several engineers spend weekends making this exist and ironing out the bugs” style project and not a “oh yeah, everyone should definitely do this” project. [But note to underemployed engineer-style people in Berkeley: this is possibly something group houses would be interested in paying you to do.]

  35. marshwiggle says:

    If opening a window is a problem, couldn’t one do a little physics and figure out where carbon dioxide concentrations would be lower? That is probably a place that already has decent contact with the outside. Then run a quiet low powered fan (like they make for computer cooling), probably with a longish tube to direct the flow, to get circulation between where people sleep and where there is already lower carbon dioxide. That’s only .001 liters per second or something of airflow, but that’s like 300 liters in a night. At 2000 ppm, that’s only .6 L of carbon dioxide removed per night. Ok, you’d need a more powerful fan then. That shouldn’t be too hard, though at that point it’s only worth it if you can’t use the ceiling fan on reverse + open bedroom door type suggestions.

  36. ex-PFC Wintergreen says:

    I had similar suspicions a couple years ago about air quality because I was constantly feeling tired, no matter how much I slept. I bought an Awair air quality monitor (it broke after about a year, although I did like it while it lasted) and the CO2 levels went way up in my tiny bedroom when I slept. I started leaving the door open and my fatigue levels improved, although I didn’t quantify it.

  37. Jo says:

    Before everybody runs off buying plants: If you are allergic against house dust mites (rather, their poo-poo), keep in mind that you should NOT put so many dust-catching plants into the room in which you sleep.

    • Plumber says:

      “Before everybody runs off buying plants: If you are allergic against house dust mites (rather, their poo-poo), keep in mind that you should NOT put so many dust-catching plants into the room in which you sleep”

      Thanks for the tip @Jo, both me and my son have that allergy.

  38. Robrace says:

    I wonder how this compares to the effect of high altitude which, if I understand correctly, also results in a buildup of CO2 in the body? Having experienced the effects of high altitude at 18,000 feet in Peru, I can attest to the feeling of “fuzzy thinking” prior to acclimation.

    • Squirrel of Doom says:

      The problem with being at 18k feet is mainly that the loss of almost half the oxygen at sea level.

      If CO₂ is part of the problem, it must be secondary.

      • albatross11 says:

        At lower pressure, the partial pressure of CO2 should be lower, too. So it seems like this would be less of a problem at higher altitude. (Am I missing something?)

  39. CatCube says:

    If you’re concerned about this, the best solution is to open a window or an internal door in your bedroom at night.

    Note that this advice is exactly opposite current Underwriter’s Laboratories and fire department advice to always close your bedroom door for fire safety.

    I’m probably going to do some effort posts on structural engineering, with some discussion on building codes. One thing of interest for this discussion is that building codes have been getting more aggressive with requirements for closers on doors in dormitories to prevent the occupants from keeping their doors open, to prevent the spread of fire.

    • yodelyak says:

      I remember thinking the main reason to keep your door closed is to protect against smoke inhalation before your fire alarm wakes you up. Maybe the solution is dutch doors, and close only the top half?

  40. Anatoly says:

    Any recommendations on which CO2 monitor to buy, focusing on accuracy and sub-$200? Gwern used Netatmo 2 years ago, is is still a good choice? (not delivered to my country, so even if it is, would appreciate other rec’s). I especially want to measure some different areas at my workplace now, to see if I can put a number to a vague feeling I’ve been having.

  41. onyomi says:

    Anyone tested with AC running versus no AC running in a closed space (or compared to windows open and no fan or windows open+fan)?

    I’d have to buy a monitor myself to be sure, but I’d guess:

    door and windows closed, no AC, no fan<door and windows open, no AC, no fan<door and windows closed, AC<door and windows open+fan?

  42. Loriot says:

    I already sleep with my bedroom door open, but I signed up for the study anyway. I figure I can try opening my windows too to see how that changes things. I’m kind of curious what the CO2 levels in my apartment are like, but I’m not sure where to get a sensor. Or I could just be a control.

  43. tatarerike says:

    It looks like there’s some confusion about net vs nightly CO2 assimilation. Plants grab CO2 from the air and use energy from light to make sugars, essentially. This is photosynthesis. Most plants fix CO2 during the day when there’s light, but as mentioned above, CAM plants fix their CO2 at night.

    However, in addition to this, plants also respire (like animals, and all living things). This means breaking sugars apart to release energy and CO2. Net CO2 assimilation is the amount of CO2 fixed during photosynthesis minus the amount respired.

    For the purpose of reducing CO2 concentration during sleep, we care about the net amount of CO2 assimilated at night, not the overall net assimilation. Fortunately, this paper gives values! Unfortunately, they’re very low.

    From eyeballing the graphs it looks like the highest average CO2 fixation rate during the night was ~8 micromol per second per kg plant material, found in Crassula ‘Himakuri’. This works out to 0.2304mol per kg plant per night. Using the first number I found for ‘kg CO2 human breath’ (an uncited ‘2.3 lb’), I estimated one person produces approx 8 mol CO2/night.

    Of course the real question is about ppm, not moles, but I’ve reached my limit of unit conversion for the moment.

    The other real question is whether Crassula ‘Himakuri’ is really the fastest-growing CAM plant. I would bet it isn’t. Most CAM plants grow pretty slowly (including most trendy ornamental succulents I see for sale), but there are exceptions. I was going to recommend mother of thousands (Bryophyllum spp.) but apparently CAM photosynthesis is induced by short day conditions? Hmm. More research needed.

    • toastengineer says:

      Can’t you just get some regular ol’ plants and shine a grow light on them all night?

    • raj says:

      Plants need to add new biomass to fix c02, right? But the numbers in the study seem to be by total plant biomass, so I’m sort of confused.

      Elsewhere in the thread people are suggesting that plants can’t make a dent in c02 levels compared to a human, but your napkin math suggests that while you’d need a lot of plants, it might be doable? What does 10s of kgs of plant look like?

  44. pontifex says:

    This is good advice, but the timing is really poor for folks in the Bay Area.

    The wildfires have put tons of particulates into the air. The Bay Area Air Quality Management District [BAAQMD] issued an advisory late this afternoon:

    Avoid physical outside activities and don’t let your children play outdoors. […] If you are advised to stay indoors, keep indoor air as clean as possible. Keep your windows and doors closed unless it’s extremely hot outside. If you don’t have an air conditioner, staying inside with the windows closed may be dangerous in extremely hot weather. In these cases, seek alternative shelter.

    The map at is the worst I’ve ever seen it. There is a lot of particulate matter in the air right now. Nothing will keep it out except a HEPA filter or a P95 or P99 mask (the kind they use for removing asbestos). Dust masks will do nothing unless they’re rated for P95.

  45. J says:

    Silly rationalist, one does not simply make wild health claims like “fresh air is good for you” all willy-nilly. As I type this, my DC1100 Pro particulate meter tells me that my indoor PM0.5 levels are around 5000, while its air quality chart tells me that anything over 3000 is “VERY POOR”. Fortunately, I’ve been running my central furnace fan with a MERV 13 filter all day, because the outdoor levels are over 10,000 (probably thanks to all the wildfires — hi, Bay Area readers!), which is the kind of big number I don’t normally get unless I’m running a sander indoors. So opening a window is a terrible idea right now!

    A more common outdoor PM0.5 level where I live is around 2000, which is merely “POOR”, and I’ve been amazed at the range of readings I’ve gotten in different homes I’ve visited — 2000 (which went down to 600 after vacuuming) in the basement, but around 100 upstairs (“VERY GOOD”). And the higher MERV rated filter seems to be a big part of it.

    The weird thing is that fine particulates seem to be pretty much completely undetectable to me; I would never have guessed things were so bad tonight. It’s the larger PM10 stuff that aggravates allergies.

    But apparently what happens is that the tiny particulates get down deep into your lungs and get absorbed right into the blood stream. Which is why they seem to be pretty strongly associated with cardiovascular disease (hi, anxiety-prone readers!).

    The vague sense I get is that the research is pretty clear on fine particulates, but I’ve been hoping someone like Scott would do a “more than you wanted to know” about it so I can continue to be lazy about it. I might even be persuaded to bribe Scott or Gwern with a monitor like mine ($200-$260) to write such a thing.

    • pontifex says:

      It was terrible yesterday. The weather did something weird where the all the smoke got redirected here. (warning: autoplaying video).

      I need to get a MERV 13 filter, if they’re not already sold out.

      People don’t seem to be freaking out about this like they did when the north bay was burning. I remember back then, all the dust masks were sold out at the hardware store– even the ones that would do nothing to help you. I guess in that case you could literally smell the smoke, though.

      The vague sense I get is that the research is pretty clear on fine particulates, but I’ve been hoping someone like Scott would do a “more than you wanted to know” about it so I can continue to be lazy about it.

      Yeah, that would be good. It’s not very controversial, though, is it? Everyone more or less accepts that particulates cause heart disease. They even have a pretty plausible physical mechanism lined up. If there is a mainstream researcher saying “no, actually PM0.5 doesn’t cause heart disease” then I haven’t seen them. (To be fair, I haven’t looked too hard either).

  46. deciusbrutus says:

    One anecdotal point of evidence: I noticed that I performed better on tests that I took ashore compared to tests I took aboard the SSBN, and while submerged I experienced intrusive hallucinations that I was successfully pressured to completely deny.

    I don’t have any evidence that the higher CO2 levels cause such hallucinations in anyone, but I would not be surprised at all if the Navy simply accepted somewhat reduced performance on “abstract tests”.

    Also of note is that I expect terrestrial (O2+CO2) levels to be constant, while submarines have separate O2 generation and CO2 scrubbing, meaning that O2 and CO2 vary independently. Raising the O2 levels by a few percent does provide a significant boost in energy, and even a 1% change in “percent of air that is O2” is a change of 10,000 PPM.

    Given that the only plausible way for atmosphere to affect cognition is through the lungs and blood, what are the results of cognitive tests with various blood O2 and blood CO2 levels?

  47. Radu Floricica says:

    My city is getting a bit too polluted, so I did similar research a couple of weeks ago. By far the best solution, both to pollution and CO2, is an AC with a good (even HEPA) filter and real ventilation – i.e. takes air from the outside, filters it, and warms/cools it. Problem is… I don’t think I found any. I found plenty of (really cheap) AC that offer “ventilation”, but as far as I can tell it just means they work as a fan.

    The second best solution is to separate the three components: AC for temperature, air purifier for filtration, and heat exchange for ventilation. Last one can be bought and installed separately – though I can’t find a link right now (I did a couple of weeks ago, though, so I know they’re out there).

    • Kibber Shuriq says:

      Disclaimer: not a HVAC specialist, so take it with a grain of salt: The ACs that allow outside air intake are of the “central AC (ducted)” type. Installing them typically involves running a lot of air ducts inside the house, including a duct for the outside air intake, which is why they’re usually installed in new buildings specifically designed for that or (less often) during complete renovation. The wall type ACs you usually see advertised are much easier to install, but indeed do not offer outside air intake capability (which would require running a duct!).

      • Radu Floricica says:

        Yeah, that’s my conclusion as well. But I don’t think it holds water from a purely engineering PoV. Kitchen walls can get large enough hood holes made for under $50, and that’s at the upper end of what’s needed.

  48. octopus says:

    Scott, maybe some of your roommates are CO2 retainers? Has been noted in scuba divers.

    Possibly interesting secondary source which references unknown primary source:

  49. peterj says:

    I wonder if the effects in the studies and those mentioned in the comments are due to low oxygen concentrations instead of high CO2. There would normally be a strong inverse relationship between the concentrations of the two gases. I would assume the studies on CO2 controlled for O2 but per perhaps not.

    • Lambert says:

      Oxygen is a whole different ball game. If you respired enough oxygen to put a dent in its concentration, the CO2 would already be affecting you quite badly.
      CO2 has effects at fractions of a percent, whereas O2 concentration has to fall several percent before it becomes an issue.

  50. Thegnskald says:

    My takeaway on the sum of the evidence presented here, and in the comments:

    Carbon dioxide probably isn’t directly harmful – the body acclimitizes relatively quickly (on the order of days) – but changes in carbon dioxide probably are harmful, possibly because they trigger some sort of alert in the body that something is amiss.

    Tentatively, I suspect that the body has a mechanism that goes “Oh shit danger danger danger” when air CO2 levels rise dramatically in a relatively short timescale, we are just trained to ignore them over years, so instead some people just get really shitty sleep as they only mostly successfully ignore their body yelling at them to get out as they are going to suffocate to death if they stay put.

    (Thought: Are children’s night terrors, such as the monster under the bed, associated with CO2 levels? And that phase of their life is training to ignore their alarm systems?)

    • mcpalenik says:

      (Thought: Are children’s night terrors, such as the monster under the bed, associated with CO2 levels? And that phase of their life is training to ignore their alarm systems?)

      I doubt it, given that for me (and for other kids as I understand it), hiding under the covers was helpful in alleviating the fear, which doesn’t exactly reduce the amount of CO2 you’re breathing in.

  51. lflick says:

    For two years, I’ve slept outdoors almost every night. If I sleep indoors, I wake up with a headache and body aches.

    I don’t remember having these symptoms during the many previous years when I slept indoors almost every night!

  52. DNM says:

    The current line in the building world is “build tight, ventilate right”. A tightly – sealed medium to large apartment building has dedicated outdoor air and is not a problem. An older house that has a kitchen fume hood or bathroom exhaust fan that vents outside is replacing that air somehow – probably by air pulled through the walls, bringing all kind of mold and ick with it.

    “Modern” housing is hit and miss. Some areas are now required to use ERVs in single family homes. Even the Bay Area now has ventilation requirements in ASHRAE 62.2, because even when the weather is acceptable people are doing a terrible job of opening windows enough to maintain air quality. So super new houses should be fine. Houses from 10-20 years ago may require you to physically open the windows to get any meaningful ventilation.

    The above is rough and I didn’t look up specific climate zones or years.

  53. John Cody says:

    Useless personal anecdote: I read Charlie Stross’s Accelerando and half of Robin Hanson’s The Age of Em inside a hyperbaric chamber filled with >two bar of pure O2. It was an extraordinarily vivid experience.

    No, I was not using oxygen as a nootropic. I was recuperating from an unearned DCS hit that seems to have been caused or exacerbated by a large patent foramen ovale.

  54. justmyfault says:

    I don’t have time to carefully check the numbers, but I’ve heard about algae-based CO2 scrubbers, like this DIY one
    Just commenting in case someone is interested to play around with this idea.

  55. Thegnskald says:

    Another question, thinking about it:

    How do these super-well-insulated houses in California deal with radon, which naturally filters up from the ground and has a tendency to fill up houses with insufficient ventilation?

    • The Nybbler says:

      Radon abatement systems usually take air from outside the conditioned envelope, so they wouldn’t need to change. Most of California isn’t a high-radon area though. And if the radon levels are borderline you could just oversize your heat exchange ventilator somewhat.

  56. Tinman says:

    I have heard (anecdotally) that US submarines, in addition to having a higher concentration of CO2, have lowered oxygen levels when submerged, (to suppress damage in case of flames) and that it leads to the crew being much more prone to anger. I haven’t been able to back up the claims of differing oxygen levels properly, but seeing as submarines are deep-sea levels it is prudent to use different atmospheric mixtures at depth.

    Additionally, it is very possible that humans can adapt long-term to higher concentrations of CO2. Mammals such as the previously mentioned and notably cold blooded naked mole rat can adapt to very high concentrations of CO2 long-term but cannot survive short-term spikes.

  57. A potential solution is to live inside a “breathable” envelope i.e. walls that are not 100% vapor proof and allow air to move through. I know a little bit about this because I left Goldman Sachs and a previous SF start-up to co-found a hempcrete construction company with a high school classmate. Besides being carbon negative during the hemp plant’s growth cycle, the hempcrete (hemp stalk + limestone binder + water) also gradually removes carbon from the surrounding atmosphere as it cures and calcifies.

    My cofounder and I lived in a hempcrete tiny home all summer in Bellingham, Washington while we built the largest hempcrete house in America (3k+ square ft). Anecdotally, I did feel like the air quality was better but am now inspired to start measuring CO2 levels.

    Scott, I’ve been reading this blog for a few years now and regret not meeting you after you moved to Berkeley and I was in San Francisco. Hempitecture will be in the SF for Carbon Smart Building Day on September 11th and plan to meet with our YC Start-up School advisor on the 10th. We would love to connect while in the Bay Area.

  58. heepsheap says:

    I spent a few years in the radon/ indoor air pollution business. Indoor air pollution tends to be more complicated than just one number. There are a lot of pollutants and a lot of causes of them. Buildings with high levels of CO2 might also have high levels of other nasties as well (CO, VOCs and particulates). if you are only measuring CO2 levels it would be easy to attribute bad effects to the CO2 when they are in fact caused by something else. This would account for the variation in whether a study finds problems with high CO2 levels i.e. high CO2 and no other nasties vs. high CO2 and lots of other nasties. Ventilating the building would in most, but not all cases reduce all contaminates and thus relieve symptoms.

  59. mdahlhausen says:

    So as building mechanical design engineer, I can weigh in on the relevant knowledge here.

    Commercial buildings have required balanced ventilation for decades, mandated in building code through ASHRAE Standard 62.1 – Ventilation for Acceptable Indoor Air Quality. Residential buildings follow ASHRAE Standard 62.2, the residential version, with some slight differences.

    Commercial buildings have ducted supply and return, and the ventilation rate procedure method requires that each space in the building get at least the minimum ventilation rate, generally around 15 cfm/person.

    Residential buildings have no such requirement. In most jurisdictions, the code requires ventilation rated at (1 cfm)*(floor area / 100 sf) + (number of bedrooms + 1)* 7.5 cfm, usually met by a bathroom exhaust fan. This is just exhaust, so the make-up air comes in through cracks in the building envelope, around windows/doors, etc. There was considerable debate in the 62.2 committee on whether this was adequate, especially since this was a whole-house rate, and didn’t guarantee fresh air in rooms like bedrooms. More on that debate. Most homes were built before there were any ventilation standards at all, so your home may not even have that.

    The next version of 62.2 will give credit to having a balanced ventilation system – a supply duct and an exhaust duct, possibly ducted through an energy recovery ventilator. This will greatly improve air quality despite a lower ventilation rate because the system can be balanced to make sure each room gets fresh air.

    As for the CO2 question, it’s not clear whether CO2 is the culprit, or whether CO2 is just a good proxy for all the other crap in the air like volatile organic compounds, mold, or particulates that make people sick and are harder to measure.

    The Indoor Environment Group at Lawrence Berkeley National Lab has a large research program on this. (See a good intro paper “Why We Ventilate” here.

    They’ve found that many California homes exceed the NOx, CO, and formaldehyde 1-hr exposure standards quite frequently, with unvented natural gas cooking ranges being the primary source of many pollutants.

    For an existing home, if you don’t have the money to install a balanced ventilation system, it’s worth at least installing a kitchen range exhaust hood that vents directly to the exterior that you use when cooking. LBNL is coming up with ‘capture efficiency’ standards for range hoods, but the bigger issue is that people don’t have range hoods or don’t use them while cooking. San Francisco just mandated direct-exhaust kitchen range hoods in their building cook – a step in the right direction.
    Other things help too – using low VOC paint, getting rid of carpet and opting for old instead of new furniture to avoid the off-gassing from new glues and fabric. And obviously, no smoking or burning things indoors (cigarettes, marijuana, incense, candles).

  60. Susmit says:

    Re. crassulacean acid metabolism (CAM) plants, it may be worth noting that they don’t evolve oxygen at night, and only absorb carbon dioxide. Since they have finite capacity to absorb and store carbon dioxide in this way, it’s important to let them catch some sun every day so that they can consume the absorbed carbon dioxide and be ready to absorb more.

    I use sansevierias (specifically, a long-leaved variety of S. trifasciata) to manage indoor air quality. I’ve found them to be more hardy and forgiving to first time gardeners, and unlike most other CAM plants, they show vigorous growth once established. For what it’s worth, a NASA study done in 1989 notes that sansevieria can absorb some airborne toxins.

  61. Oferet says:

    In Judaism there’s the concept of “yeridat hadorot” – “descent of the generations” – according to which the physical and spiritual level of one generation is lesser than the precedent generation’s. Maybe they knew 😉

  62. dfranke says:

    I suspect that cardiovascular fitness is a major factor in individual variation here, with fitter individuals having fewer negative effects from poor air quality. This would help explain why the Navy hasn’t noticed any problems from submarine crew. Consider asking your experiment participants to report their estimated VO2 max using one of the methods from

  63. MathiasBonde says:

    Back when I was in high school, I would often come home with really bad migraines. It took me two years to figure out that it was the lack of fresh air in our high school buildings that was the cause. I started combating this by going outside and getting fresh air every single time there was a break. After making these adjustments I did not get any headaches.

    Nowadays I’m hyper aware of how important fresh air is to my well-being and I keep my windows open 24/7.

  64. arunbharatula says:

    Those tables are for open windows and ventilation.

    How many succulents would you need to achieve the same effect?

    Like someone else has already said, multiple gasses contribute to indoor air pollution. Conceivably, multiple air pollutants are detrimental. In areas with low levels of outdoor air pollution, open windows may take care of multiple pollutants. Succulents may or may not.

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