The main change in the atmospheric composition is primarily
due to the combustion of fossil fuels, used for the generation
of energy and transportation. Variant air pollutants have
been reported, differing in their chemical composition, reaction
properties, emission, persistence in the environment, ability
to be transported in long or short distances and their
eventual impacts on human and/or animal health. However,
they share some similarities and they can be grouped to four
categories: