September 26, 2003
A: FROM MENTOR JOAN LUSK IN RI
Exhaled air contains 4% - 5% CO2, which is a lot more than the 0.033%
in (dry) air. The air we inhale has about 21% oxygen. These
percentages are by volume, so they are proportional to the number of
molecules of oxygen and CO2 per volume. If we think of respiration
as the reaction of oxygen to make CO2, then the CO2 we exhale would
still leave 16% - 17% oxygen (21% minus 4 or 5%) in the air for the
CPR victim. In other words, we don't use up all the oxygen in each
breath. Probably there would be even more oxygen available, if the
CPR-doer takes a quick breath and exhales - some of the air going to
the victim would not even have reached the lungs of the CPR-doer, but
just have been in the mouth and trachea.

You might try exhaling into a bell jar and comparing how long a
candle can burn on your exhalation compared to burning in air. If
I'm right, the exhaled air won't immediately extinguish a candle; it
should burn about 3/4 as long.