…, a man is known to breathe out 400 cc of air at each breath, so that a single breath of air must contain about 1022 molecules. The whole atmosphere of the earth consists of about 1044 molecules. Thus one molecule bears the same relation to a breath of air as the latter does to the whole atmosphere of the earth. If we assume that the last breath of, say, Julius Caesar has by now become thoroughly scattered through the atmosphere, then the chances are that each of us inhales one molecule of it with every breath we take. A man’s lungs hold about 2000 cc of air, so that the chances are that in the lungs of each of us there are about five molecules from the last breath of Julius Caesar.
PS: A quotation from: Jeans. J., 1940, An Introduction to the Kinetic Theory of Gases, Cambridge University Press.
PPS: Lately, I am reading Introduction to Physical Gas Dynamics, by Vincenti and Kruger. I am finding it good to get back to reading (studying?).