Air is a mixture of several gases that is a precious resource and a necessity of life. Pure air is completely invisible and odourless, however pollutants in the air can cause it to have a coloured tinge or odour. The importance of clean air as a resource should not be underestimated as humans use an extremely large amount of air simply to stay alive. Adults breathe in approximately 10-20 cubic metres of air daily - the equivalent of about 20 000 breaths.
Air makes up a large portion of the atmosphere of the Earth. The atmosphere is around 1000 kilometers thick, but the bottom two kilometers is where most of the air is concentrated - along with a large amount of the air pollutants. Although the composition of air doesn't change much as you travel through the layers of the atmosphere, the concentration of air decreases drastically as you go up in the atmosphere. Pure, dry air is composed of 78% nitrogen, 21% oxygen, and less than 1% of argon, carbon dioxide, and other gases including varying amounts of water vapour.
Since air is such an important resource, air pollution is a significant concern. Issues surrounding air quality - namely urban smog, acid rain, use and production of different fuels (namely the use of fossil fuels), and global warming - have become more prominent in recent years. Although air pollution is a significant issue in places around the world, with places like China issuing air quality warnings that advise people to stay indoors due to poor air quality, there have been advances in air pollution reduction technology that have led to declines in emissions of different pollutants. The quality of the air we breathe depends on how quickly pollutants are emitted and how well the atmosphere can disperse these pollutants. This dispersion is controlled by the wind as well as weather and the temperature.