*Exhaled air consists of 78 percent nitrogen, 16 percent oxygen, 4 percent carbo...

a-saleh · on Jan 17, 2019

The minimum oxygen concentration in the air required for human breathing is 19.5 percent. ... hen oxygen concentrations drop from 19.5 to 16 percent, and you engage in physical activity, your cells fail to receive the oxygen needed to function correctly. Mental functions become impaired and respiration intermittent at oxygen concentrations that drop from 10 to 14 percent; at these levels with any amount of physical activity, the body becomes exhausted. Humans won't survive with levels at 6 percent or lower. https://sciencing.com/minimum-oxygen-concentration-human-bre...

dredmorbius · on Jan 23, 2019

You're not going to thrive or compete athletically on second-hand air.

But if you're not breathing on your own, it can save your life. That's what rescue breathing (what used to be called "artificial respiration") is.

There is more than enough oxygen in exhaled air to support life. At least until medical care arrives.