The gases that humans breathe in are nitrogen, oxygen, argon and carbon dioxide, with nitrogen in the largest proportion and carbon dioxide in the smallest proportion. The gases that humans breathe out are the same, although more carbon dioxide and less oxygen is present.
Earth's atmosphere at sea level is approximately 78 percent nitrogen gas by volume. The second most common gas is oxygen, which makes up 20 to 21 percent of the atmosphere by volume. Other gases are present in much lower proportions, with argon making up around 1 percent and carbon dioxide around 0.5 percent. Trace amounts of hydrogen, ozone, water vapor and noble gases such as helium and neon are also present.
The exact proportions of gases vary according to elevation, location and humidity. Areas with higher humidity have a greater proportion of water vapor. The proportion of oxygen decreases with elevation, and regions with a large industrial presence tend to have higher proportions of carbon dioxide and various pollutants, such as sulfur dioxide. The only gases significantly taken in through human respiration are oxygen and carbon dioxide. When gases are inhaled, oxygen is absorbed to the bloodstream; when gases are exhaled, carbon dioxide is removed from the body.