Air is a gaseous substance that is composed primarily of nitrogen, oxygen and argon. The air in the atmosphere that surrounds the Earth is approximately 78 percent nitrogen, 21 percent oxygen and 1 percent argon, with the remainder made up of various other gases including neon, helium and hydrogen.
Although the proportion of gases found in air does not change as one moves further from the Earth's surface, the density of the air does. This means that there are fewer molecules of nitrogen, oxygen, argon and other gases a mile above the Earth's surface than an inch above it, but the same ratio of gases.
Some natural and unnatural non-gaseous particles are also found in the atmosphere, especially in the portions closest to the Earth's surface. These include volcanic ash, aerosol pollutants such as chlorine and mercury, pollen, and mold spores. The levels of these substances in the air tend to vary by location and with the seasons.
Air pollution is defined as the introduction of chemicals, particulate or biological matter into the air. This is typically considered a problem because it reduces the levels of ozone, an oxygen species, in the stratosphere. With less ozone in the atmosphere, the sun's harmful UV rays can more easily reach the Earth's surface, contributing to problems such as skin cancer and global warming.