It has long baffled me that a microscopic part of our atmosphere should be condemned as the greatest human evil of our time. Carbon dioxide the source of life on this planet, has been stigmatized while only occupying .04% of our atmosphere. In English: four one hundredths of one percent.
I know that oxygen is not measured by any animal sensory system, instead we detect an insufficiency oxygen by measuring the high proportion of CO2. When we start to feel we are out of oxygen it is because we detect too much CO2.
The contemporary way to commit suicide is to put a bag over your head and shoulders, and fill it with helium. As long as the CO2 is escaping and the helium is replacing oxygen you go to sleep and die because your brain is not getting oxygen and the CO2 mechanism to warn you doesn't have enough exhaled CO2 to be alarmed.
So how much CO2 can we detect? The answer is: we can't detect CO2 until it is 100 times greater than its concentration in the atmosphere. Here is the study that shows that we can't detect what we exhale at the level of 4%. That is possibly the point at which we know we should exhale. There is probably some point above that where we are forced to breathe. I'm sure people who do free-diving, know what that level of CO2 must be.
In any case, we are not very sensitive to CO2 is in our environment and can’t personally detect when it is at a dangerous level. That answers my question.