"The base level does increase." Not quickly enough to make this a cause for worry in the next few millenia.

This is very interesting and I was not aware of this, but is it really about Earth Science? If your question is, "are CO₂ levels likely to reach level x by year y", then that would be an Earth Science question; but if it is "are CO₂ levels at level x impacting cognitive ability", then that is better for your question on neuroscience.

Why can't you just open the window a few times a day to lower the ppm in the room?

I can't emphasise enough how much I am concerned with this since I have well experienced the effects on this on myself as an office worker. The excess of CO2 (and lack of O2) brings a huge harm to concentration and cognitive abilities. Despite many people rising this question, I was suprised how ignorant most of the managers were to these claims. It was like they did not care at all, even hearing that it hurts the productivity. Probably because they had better workplaces. So, I am sure that this is a valid case to worry about, but I have found that many people just straight face ignore it!

wernersbacher - Oh, that's easy to answer: I'm not thinking about my brain, but about a couple of millions other brains. The level of CO2 is in the normal air we all share globally.

@noncom - A manager who ignores it does just plain not understand it. And I do not think it is as simple as it looks for us.

@noncom - Whether it is CO₂ or O₂ is interesting. One point is that the feeling to not get enough air is not the feeling of to low oxygen, but too much carbon dioxide. (That's easy to test in a pressure chamber used for training of divers or similar). I would very much expect that missing O₂ is the problem reducing cognitive ability. And that one feels the lack of CO₂ when suffocating does not mean that sleepiness is based on CO₂. But from what I have seen, it seems like CO₂ is the problem.

@noncom I asked this related question at psychology: How does intelligence or general cognitive ability vary with level of CO₂?

Also, something to think about is the case that the increase of the CO2 part in the gas mix can cause the decrease of the O2 part simply because the active emitters of this gas are in the room, and because it has to occupy some volume so it has to expell some other gas as long as it keeps being produced. And also because O2 gets utilized and directly exchanged for CO2 in the same volume so that the air in the room gets being "processed". Also CO2 is heavier so I think it helps its dominating properties. Need to check this with someone who knows the exact physics on this, though.

I'm physicist, so let's see: There is more oxygen than carbon dioxide in the air by two orders of magnitude. So we can ignore that issue. (~20% O₂ and 0.1% CO₂)

Oh, great! This part of the question is solved, then :)

How does the CO2 level actually vary with occupants? I would expect the ventilation system to mitigate some of this; since it should be constantly cycling air out and mixing in outside air.

@JMac - Sure, good ventilation helps. But in reality, that does not always happen. In a poorly ventilated classroom, 2000 ppm are easily possible. It varies as: base level * number of occupants * exhaled amount per person * time since last good ventilation - bad ventilation per time * time since last good ventilation. If you open the window, time since last good ventilation becomes 0 and stays 0 as long as it is open.

@VolkerSiegel Right, but in many workspaces, there are actual building codes and occupancy codes that require some amount of air changes in some amount of time. I guess my point was basically with modern building standards is the buildup actually noticeable, or is this already mitigated by common air-exchange practices?

I never heard and would never deduce such "fact". It is good that I read your question, and will take action right now. The best is to ask your coworkers to stop breathing. If they really need to, they can do it during lunch break and after work.

@JMac If building codes are completely followed, including activating air conditioning and and exchange as per user manual, in something like a classroom or an office: Low enough buildup to be not a problem, about 200 ppm or so over base level. But it's not realistic to assume that always happens even in modern western offices (Which often have well closing windows).

@BЈовић - No worries about my personal CO₂ levels, I am alone in the room and have windows that are about 110 years or so old. There is usual enough ventilation even with closed windows (10 glass panes, each not perfectly air tight). But I do not want to state something as fact that is not established as such, that's really important, and thanks for pointing it out. What specifically do you mean?

Lung air concentrations of CO2 are on the order of 100x atmospheric concentration. Modest increases in atmospheric CO2 (say, doubling or tripling) wouldn't be expected to substantially impact lung CO2.

@BryanKrause I see what you mean, but that does not work: It is about an offset. An example: We have an increasing level in a room, from 600 ppm to 1000 ppm over 4 hours. After 2 hours at 800 ppm some effect sets in. Now, we do the same, but have a base level of 700 ppm. We now reach 800 ppm after 1 hour, and the effect sets in. We end after 4 hours with 1100 ppm. In total The effect is present for one hour longer than before in.

@Volker Siegel: Would you care to apply your two orders of magnitude logic to other gasses? HCN, for instance: "A hydrogen cyanide concentration in the range of 100–200 ppm in air will kill a human within 10 to 60 minutes. (en.wikipedia.org/wiki/… )

@jamesqf - I'm happy to do the same estimation regarding volume of gasses, and would not object an experiment. It's of course advisable to not enter the experimental volume, but I have protective equipment at hand, I'm all in! We will find that the fraction of oxygen to breathe does not significantly change with adding an extra 200 ppm hydrogen cyanide. That is in part for the same reasons as for CO2, but also because oxygen becomes insignificant in itself with a cyanide poisoning. That the body no longer can use oxygen is often seen as the main problem of cyanide poisoning.

@VolkerSiegel What I mean is that the concentration of CO2 in the lungs is on the order of 40,000 ppm. Mixing 40,000 ppm with 400 ppm inhaled air vs 1000 ppm inhaled air isn't very different. CO2 is just an easily measured gas for monitoring overall air turnover. I am very skeptical of studies that show cognitive effects of low CO2 concentrations.

@BryanKrause Why low concentrations? But again, it is an offset. I see the point you make, and intuitively, it is very plausible. But it is not relevant what happens inside the lung, how it does what it does. I would really like that one of us convinces the other one - I'm happy to be the other one. A point regarding the concentration of 40,000 ppm (4%) and mixing it: I suspect the concentration is in the exhaled air. I assume the inhaled air is relevant here.

@BryanKrause I was wrong regarding inhaled and exhaled air: The exchange of new air is small, the gas mixture in the lung does not change much.

@VolkerSiegel By low concentrations I meant concentrations that may be elevated relative to normal atmosphere but low compared to the lung. My point was that adding a little CO2 to the air that is inhaled doesn't much impede the diffusion of CO2 from blood to (about to be exhaled) air. It's already quite an efficient process. Maybe you have to increase respiration rate by <1% to clear the same amount of CO2 - that seems implausible to have a measurable cognitive effect and would account for a tiny amount of respiration variance.

@BryanKrause - sounds like an argument from incredulity. Hypercapnia is from raised CO2 in blood and one cause is elevated CO2 levels in air. Drowsiness - mild cognitive impairment - at around 1000ppm is at the bottom of the scale for hypercapnia, but can be noticeable at lower levels than that. i.e. within the range we could get to with ongoing fossil fuel burning. I think it is a legitimate concern.

@KenFabian Do you have a citation for the 1000ppm level causing hypercapnia? And I disagree the argument is from incredulity, the argument is from the argument I just made: 400 vs 1000 is a small difference relative to 40,000.

(and ocean hypercapnia is entirely different and definitely an issue of concern with rising atmospheric CO2, because the ocean is not anywhere near the CO2 levels of human blood, where the source of CO2 is metabolism rather than the atmosphere)