An air pollutant is a substance in air that could, at high enough concentrations, harm human beings, animals, vegetation or material. Air pollutants may thus include forms of matter of almost any natural or artificial composition capable of being airborne. They may consist of solid particles, liquid droplets, or gases, or combinations of these forms.

 

Among the commonest pollutants and widespread health threats in the air are:

 

(Ground-level) ozone

Particulates

Carbon monoxide

Nitrogen oxides

Sulfur dioxide

Lead