As a species, humans require air to survive; a need it shares with most other members of the Kingdom Animalia. After a human breathes in Earth's air (roughly 78 percent nitrogen and 21 percent oxygen), he or she exhales a mixture of compounds similar to the air inhaled: 78 percent nitrogen, 16 percent oxygen, 0.09 percent argon, and four percent carbon dioxide. Some scientists suggest that exhaled air contains as many as 3,500 compounds, most of which are in microscopic amounts. There is some variability in this, however. Air quality can affect both the content of what humans both breathe in and breathe out, a concern some conservationists worry about when it comes to industries and automobiles releasing potentially harmful gases. Similarly, some doctors suggest that monitoring the chemical content of an air a human exhales can be a useful diagnostic tool in catching respiratory ailments.
TL;DR (Too Long; Didn't Read)
Humans, and many other species, need air to live. They breathe in the combination of elements and compounds and exhale a similar set with different proportions. Exhaled air consists of 78 percent nitrogen, 16 percent oxygen, 4 percent carbon dioxide and potentially thousands of other compounds.
Breathing at a Glance
Humans, along with many other animals, breathe in air through their mouth, into their lungs. The chest cavity containing the lungs expands and contracts with the diaphragm as it moves down and up. Inside the lungs, small sacs called the alveoli fill with the air. From there the oxygen in the air transfers through the thin walls of the alveoli into the blood, where it is used in aerobic respiration, the process by which cells turn oxygen and sugar into chemical energy, carbon dioxide and water. The blood then carries the leftover carbon dioxide back to the lungs and the human exhales it, along with the other parts of the air not necessary for human life, like nitrogen. On average, humans use and absorb around 4 percent of the oxygen they take in from air.
What's in a Breath?
Nitrogen makes up the bulk (78 percent) of the air that humans breathe in and out, considering human bodies have no use for it. Second place belongs to oxygen (21 percent in, 16 percent out) and at a distant third carbon dioxide (0.04 percent in, four percent out). Other trace elements exist in exhaled air, such as argon (0.09 percent both ways, again because humans don't use it). Human beings also exhale water vapor, a byproduct of cellular respiration, at a rate that varies depending on the person, their health and other factors.
Other chemicals can exist in the air that humans inhale and exhale, some of which can be damaging to a human's health. Particulate matter from industries, smoke from cigarettes and other chemicals like sulphur and nitrogen oxides can cause harm to the lungs. Some forms of dangerous matter, like germs and particulates, get caught by the hair-like growths that line the passage into a person's throat. Called cilia, they help protect people from these elements in Earth's air, but it's not a perfect system and sometimes things can reach the rest of the lungs and get caught in the alveoli. Germs, for example, can potentially cause infections.