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You say, "Our current understanding is that no amount of CO2 is dangerous as long as everything else is fine," but I don't think that is correct. Like any other non-oxygen gas, carbon dioxide is an asphyxiant, and it is routinely used as such to kill rats, but it has significant toxicity even at much lower levels. NIOSH says:

> Signs of intoxication have been produced by a 30-minute exposure at 50,000 ppm [Aero 1953], and a few minutes exposure at 70,000 to 100,000 ppm produces unconsciousness [Flury and Zernik 1931]. It has been reported that submarine personnel exposed continuously at 30,000 ppm were only slightly affected, provided the oxygen content of the air was maintained at normal concentrations [Schaefer 1951]. It has been reported that 100,000 ppm is the atmospheric concentration immediately dangerous to life [AIHA 1971] and that exposure to 100,000 ppm for only a few minutes can cause loss of consciousness [Hunter 1975].

(https://www.cdc.gov/niosh/idlh/124389.html)

100,000 ppm is 10%, so at that point the carbon dioxide has reduced the oxygen in your air from 21% to 19%, far from asphyxiation conditions.

Even at much lower levels, carbon dioxide can produce drowsiness and mental impairment.

On the other hand, reaching 5% or 10% carbon dioxide by oxidizing carbon with oxygen from the air, for example by breathing or having a fire, will reduce the oxygen content of the air to an extent that is more dangerous than the carbon monoxide. So carbon dioxide toxicity is generally not the thing to worry about with respect to indoor air safety. But that doesn't mean it's not real.

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a1371 3 days ago | parent | next [-]

It's good to keep in mind that in your home the CO2 usually ranges from 400ppm to say 2,000 ppm. Getting to something like 100,000 ppm is not really plausible for a typical home. I get how in a submarine the conditions are different.

From 9:50 onwards of this video explains what I meant: https://youtu.be/CkGDN85I29U?t=590

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samus 3 days ago | parent | prev [-]

The point is that in the scenarios you described CO2 increases at the cost of oxygen, which is obviously not good, and that's what GP was referring to as "everything else is fine". It would be interesting to see what happens if instead some of the nitrogen in the air is replaced by CO2. But that's obviously not what usually happens. And there are far more damaging air contents to worry about. Like mold spores.

A crucial difference to CO is that CO2 doesn't cause permanent damage as long as oxygen supply is restored in time. Compared to that, hemoglobine touched by CO becomes essentially useless for the body since CO has a similarly high binding affinity to hemoglobin as oxygen. Recovering from that pretty much requires replacing the affected red blood cells.

	▲	kragen 3 days ago \| parent [-]
		> It would be interesting to see what happens if instead some of the nitrogen in the air is replaced by [CO₂]. Well, if you think it would be interesting, click the NIOSH link I provided and read the references, because that's pretty much what they're talking about. > A crucial difference to CO is that [CO₂] doesn't cause permanent damage as long as oxygen supply is restored in time. This is not correct; CO₂ poisoning can cause permanent injuries, including death, even when oxygen supply is never cut off, much less when it is restored in time. The comment you are replying to explained this in some detail and provided (abbreviated) references. > [hemoglobin] touched by CO becomes essentially useless for the body since CO has a similarly high binding affinity to hemoglobin as oxygen. Recovering from that pretty much requires replacing the affected red blood cells. This contains two major errors. First, CO binds much more strongly to hemoglobin than oxygen, about 240× as strongly; if it didn't, CO levels would have to be almost as high as oxygen levels to have an effect, but in fact 0.4% CO in the atmosphere is enough to kill you in half an hour. The second error, contradicting the first, is your claim that recovering from CO poisoning requires replacing the affected red blood cells. While CO binds to hemoglobin more strongly than oxygen, it isn't that strong; the carboxyhemoglobin thus produced can in fact release its CO and become functional hemoglobin again, with a half-life of about 5 hours. If replacing the affected red blood cells were required, it would be eliminated in about 30 days rather than about 5 hours. If, on the other hand, CO had a similarly high binding affinity to hemoglobin as oxygen, as you said it did, then it would be eliminated in about a minute rather than 5 hours.