Oxygen is actually a poison. It causes runaway chemical reactions that animals try to guard against with anti-oxidant processes. It promotes rust which is a sort of slow burning, and chemical explosions which are a super-fast burn. On the other hand, no human could live five minutes without it.
Fortunately, in air, oxygen is heavily diluted by nitrogen. Nitrogen, about 78% of the atmosphere, is a nearly inert element. It does not combine readily with other elements, and dilutes the more reactive oxygen to a concentration under 21%, so that it is safe to strike a match. Nitrogen is also colorless, odorless, and tasteless, like oxygen, leaving the senses free to operate through the air.
Air is the mixture of gases that makes life on the surface of the earth possible. It forms the atmosphere, which absorbs most of the dangerous ultraviolet radiation from the sun.
The atmosphere also insulates us from the cold of space, moderates the extreme temperatures of night and day, and burns up falling space debris with friction, while it holds the dangerous oxygen without which we could not live.
Other elements in air include argon, helium, neon, krypton, and xenon. These are all noble gases, retained from the primordial atmosphere at trace amounts, and each adds another element of non-reactivity to the air soup. Argon is the most common, at about 0.93% of the atmosphere, that is, nearly one cup of Argon in every one hundred cups of air. Like nitrogen it is colorless, odorless, tasteless, and nontoxic. The other noble gases are present in amounts which are vanishingly small.
Carbon dioxide is present in the atmosphere in small quantities as well, at 0.038%, varying by season and by location, but plants could not live without it. They use carbon dioxide in photosynthesis, to compound the simple sugars which are the basis of life on earth. As a byproduct, they release oxygen into the air. It was the cyanobacteria, feeding on carbon dioxide, which produced the first real quantity of oxygen in the atmosphere about 2.5 billion years ago. Then, once they evolved, true plants began maintaining an atmosphere in which animal life as we know it could come to be. Plants too depend upon carbon dioxide. But carbon dioxide is a greenhouse gas, as most people know, trapping heat that may warm the earth excessively. Many scientists are concerned about the excess carbon dioxide our industrial processes and our lifestyles have left in the air.
On average, water vapor makes up 1 to 4 percent of the air we breathe, depending on location. It is perpetually generated by evaporation, and removed by condensation and precipitation in an endless cycle. Water vapor in the atmosphere is obviously responsible for clouds, rain, and snow. Less obviously, it is the latent energy in water vapor which powers the most destructive storms. The energy that holds water in the vapor state is given up to the atmosphere when water changes state, and this heat energy increases the strength of storms. Water vapor is also a greenhouse gas, increasing the heat the air holds.
Pollutants are not a major component of air by volume. However, the damage done by pollutants can be major indeed. Chlorofluorocarbons emitted by old-style air-conditioners, refrigerators and spray cans are long-lived molecules that continue to impact the ozone layer of the atmosphere, which has protected the earth from excessive solar radiation for millennia. One consequence is an increase in skin cancer. Sulfur oxides, particularly sulfur dioxide, produced by the burning of some coals and certain grades of petroleum, are a building block of acid rain, which has widespread negative effects. At present, the greatest producer of sulfur dioxide in the world is China. Nitrogen oxides such as nitrogen dioxide are a basic component of smog. They are mostly produced by internal combustion engines, such as cars.
The air of Earth is not the only air. Venus and Mars have atmospheres composed mainly of carbon dioxide, one cold and thin, and one thick and insufferably hot. Heavy Saturn, Jupiter, Uranus, and Neptune have hydrogen-helium atmospheres. Moons Triton and Titan have mostly nitrogen atmospheres.
However, Earth's air is the only one Earth life is adapted to. If the air changes too much, life on earth will change. Our air is a delicate balance of elements, life-giving volatile oxygen, steadying nitrogen, warming carbon dioxide, and nourishing water. It is seasoned with pollen and spores, sea spray and volcanic gases, and the dust of meteorites from millions of miles away.