Question: Has all the air in the world been breathed before?

Phillip Wilkinson answered on 18 Mar 2013:

Very good questions!

It depends what you mean by air, if you mean Oxygen then as we breath this in and breath out Carbon Dioxide we can’t recycle this air. If you mean the individual atoms then maybe they have been breathed before.

There has been air breathing life on Earth for 3.9 billion years, so that is plenty of time for these atoms to have passed through at least one other creatures lungs and it only takes plants 2000 years to work through all the oxygen in the atmospher. However, in every lung full of air there are 3 x 10^26 atoms. So 3 with 26 zeroes after it. With numbers this big its very difficult to say for certain!