I am just going by wikipedia "It may also be caused by exposure to environments containing abnormally high concentrations of carbon dioxide, such as from volcanic or geothermal activity, or by rebreathing exhaled carbon dioxide." en.wikipedia.org/wiki/Hypercapnia

@KenFabian Yes, of course at high concentrations inhaling CO2 causes blood hypercapnia. I am asking about lower concentrations, like 1000 ppm

See the diagram showing main symptom of Carbon Dioxide Toxicity en.wikipedia.org/wiki/Hypercapnia

@KenFabian 1000ppm is 0.1%. I don't doubt that 10000 ppm (1%) has effects

@BryanKrause - You are correct, my mistake. Even so I think that evidence of drowsiness at 1000ppm is not really in question.

@KenFabian I strongly disagree that the evidence of drowsiness at 1000ppm is not in question.

There is a body of literature in an odd area of environmental building science that doesn't seem to be getting a lot of input from the broader neuroscience/physiology community, and I've read a couple of the studies that are either absolute trash in their methodology, or have results that are so remarkably strong that they are entirely implausible

They have no supporting biological mechanism

I don't feel competent to properly review the Allen et al study I linked to - which does appear to show cognitive effects at 950ppm. I am not convinced it is either "absolute trash" or "entirely implausible". More studies would be good but I don't think this one should be dismissed as wrong.

@VolkerSiegel I'm not at all sure that follows. As soon as you increase the concentration of CO2, that means that each breath you take will contain a little bit more CO2 and a little bit less O2. The difference in their relative abundancies is irrelevant. The missing datum is what is the minimum decrease of O2 concentration would have an effect. If we are hyper-sensitive to changes in O2 concentration, then even a tiny change could be detrimental.

The relative abundancies of the gasses won't change this.

@terdon No, a change in oxygen concentration of that magnitude would be even less impactful. I'll maybe try to dig up some related studies; I'd like to post an answer to the Psych&Neuro question but I want it to be more thorough than the responses here

@KenFabian I'll look at that one in more detail and try to find what the likely issues are. My skepticism mostly comes from the magnitude of the effects and their sample size. The rest-retest reliability on their scale makes differences very difficult to detect (and this is common for any measure of cognitive function). The fact that their groups shw such ridiculously strong effects makes me very very suspicious that they are either missing a huge confounding effect or made an error in analysis

For example, the effects they report are much larger than for 4 alcoholic drink equivalents for a study using the same scale: Streufert, S., Pogash, R. M., Gingrich, D., Kantner, A., Lonardi, L., Severs, W., ... & Roache, J. (1993). Alcohol and Complex Functioning 1. Journal of Applied Social Psychology, 23(11), 847-866.

@BryanKrause So you think there's actually a physiological function carried out by excess CO2? That would be very interesting. Please let me know if you do end up posting an answer.

@terdon High enough excess CO2, definitely; CO2 is the main sensor mechanism for respiration and high CO2 definitely has physiological effects. It's just that the difference between 0.04% and 0.1% is tiny compared to the ~5% CO2 partial pressure in the blood/lungs.

You would expect that change to produce, on average, about a 1% increase in respiration rate to keep blood CO2 constant. A 1% increase in respiration rate would be a tiny fraction of the overall variance in respiration rate

@BryanKrause Ooooh! So you're saying that high CO2 is what triggers the breathing reflex and not low O2?

@terdon That's right. O2 matters too, and respiration will still be affected by O2 especially in situations lacking hypercapnia, like at altitude, but CO2 is the primary central mechanism. This is one reason why nitrogen gas is dangerous indoors: people don't tend to respond quickly enough to the low-oxygen environment, because blood CO2 stays normal

You can pass out before it registers that you aren't getting enough oxygen