(I) The air we breathe is often dirty and contaminated with many pollutants, particularly in urban areas. (II) As a matter of fact, air pollution consists of gases, liquids, or solids present in the atmosphere at levels high enough to harm living beings as well as materials. (III) Although air pollutants can come from natural sources, human activities also make a major contribution. (IV) Most industrial countries have taken measures to reduce carbon emissions. (V) For instance, motor vehicles and industry are the two main human sources of air pollution.