By rights the air we breathe is a mixture of oxygen, nitrogen, and argon, with traces of helium, neon, krypton, and xenon, and presumably it has been approximately so since man began to breathe. Today, however, in numerous places on the earth, the air contains hundreds of substances which were never intended to be inhaled. Some of these make us uncomfortable or ill and at times hasten the end of human beings.
Extraneous substances such as dusts have been in the air for a very long time, and, while they protect us against the sun's actinic rays, they too have taken their toll. The more serious air pollution problem began well after the industrial revolution (1870), and it has increased year by year until the number of air pollutants is now almost as great as the number of human activities which produce them.
The chief air pollutants today are the
THE AIR WE BREATHE. JAMA. 1960;172(18):2085–2087. doi:10.1001/jama.1960.03020180095015