The air is filled with many different elements but only four elements make up 99.998% of the air. Nitrogen is 78.084% oxygen is 20.747% argon is .934% and carbon dioxide takes up 0.033%. There are many elements that have parts per million and these elements are neon, helium, krypton, sulfur dioxide, methane, hydrogen, nitrous oxide, xenon, ozone, nitrogen dioxide, and iodine. There are also trace amounts of carbon monoxide and ammonia. People know carbon monoxide to be very deadly but since there are only trace amounts of this chemical combination in the air it has no affect on our immediate health. Even though someone might not think that these trace amounts matter they do. Saying these trace amounts don't matter is like saying that the tiny bugs outside your house don't play a very important role in the ecosystem, if you think they don't you are again wrong. These small amounts of elements play some vital roles in the grand scheme of things. Just like those tiny bugs outside, the tiny amounts react to something a little bigger than they do which effects something bigger and so on down the chain. This is why when we hurt our ecosystem by pouring in toxic fumes from cars we not only hurt the tiny bugs and the trace amount of elements but we also create a long chain of events that dramatically effects us. When you role a ball of snow outside after a heavy snow, at first it doesn't build that fast but after pushing and pushing you may get a ball bigger than your body which is exactly what happens. One element stops playing its part which causing a large chain of events that keeps getting bigger and bigger. The oxygen in our environment reacts with us and turns into carbon dioxide, which reacts with trees to turn back into oxygen, nitrogen doesn't get absorbed into our bodies but it is still very important for other beings that live with us on this planet. It is the amazing balance of every element combined with the perfect distance from the sun that makes the earth so spectacular. This is the reason we have not been able to find life on other planets, because of our composition of air.
To recap: there are many elements in the air but only four major ones which are nitrogen, oxygen, argon, and carbon dioxide. Although small, even trace amount elements have a major role to play in our health and well being so don't treat the ecosystem in bad ways and bike instead of drive when you are able to.