Healthy air If in the Middle Ages someone had said: “Believe me, I am like the air you breathe,” whomever had heard the phrase would have imagined a pure and white space, with a green landscape, fish in the river and birds chirping in the nest. Now, if that phrase would have been said today, the image would be that of a monster or a dangerous criminal. And yes, times have changed as cities, sounds, people and the air content have varied. The air, which we are aware of when we breathe badly or smell something bad, is composed of almost 79% nitrogen, 20% oxygen and the remainder — something like 2%— carbon dioxide, argon, neon, krypton, helium and water vapor. This is when it is in its clean state. But what created the air? It is said that 2,400 million years ago, there was a phenomenon called the Great Oxidation (others call it the Oxygen Revolution), and from there on the evolution of organic life occurred. Trees and plants needed air, as well as protozoans, fungi, invertebrate and vertebrate animals such as reptiles, amphibians, birds, fishes and mammals (among them, us).