Definition of air pollution

Each day we breathe about 15,000 litres of air, which makes it the foremost element required for life. The average composition of dry air is 78% nitrogen (N₂), 21% oxygen (O₂) and 1% of other gases, mainly Argon (Ar) followed by carbon dioxide (CO₂). There are many other constituents present in much lower concentrations. Among these constituents, an air pollutant can be defined as a substance present at a concentration sufficiently above its normal level to produce a measurable adverse effect on human, animal or plant life or on materials.

Air pollution may have natural (volcanic eruptions, vegetal emissions, humid zones, etc.) or anthropogenic origins, i.e. linked to human activity (fixed or mobile combustion sources, industrial emissions, etc.). Pollutants can also be divided into primary pollutants, which are emitted directly, and secondary pollutants, which are created by chemical transformations in the atmosphere.