You mean air can make you sick? You mean that which you are breathing right now but are not getting sick from? Breathing oxygen can not make a person sick.
LOL Yes, it CAN.
In humans breathing 100 percent oxygen at normal pressure, here's what happens:
* Fluid accumulates in the lungs.
* Gas flow across the alveoli slows down, meaning that the person has to breathe more to get enough oxygen.
* Chest pains occur during deep breathing.
* The total volume of exchangeable air in the lung decreases by 17 percent.
* Mucus plugs local areas of collapsed alveoli -- a condition called atelectasis. The oxygen trapped in the plugged alveoli gets absorbed into the blood, no gas is left to keep the plugged alveoli inflated, and they collapse. Mucus plugs are normal, but they are cleared by coughing. If alveoli become plugged while breathing air, the nitrogen trapped in the alveoli keeps them inflated.
We don't breathe O2. We breathe Air which is a mixture of O2 and mostly Nitrogen with some trace amounts of other gases.
Breathing oxygen at depth can make you sick but this is conditional on a person being at depth. How many times a day do you breathe oxygen at depth? A month? A year? Sorry. The comparison falls flat on it's face because it is based on probability and preconditions.
Quite a bit, actually. For several weeks prior to moving back to WV, I breathed air at depth 2 -3 times a day. In about a month, I'll be breathing it at depth 3 - 4 times a day for a couple months, and after that I'll be breathing it at depth for a living.
Really doesn't matter though, because too much of anything is harmful. And people that overdo it hurt themselves. And when information about the dangers is out there, there is NO ONE to blame but themselves.