Air is a mixture of gases that fill the atmosphere , it is made of up of mostly nitrogen (78%) and oxygen (21%) with the remainder a combination of other other gases such as carbon dioxide and argon. Together this lets our planet breathe and there are no ill effects to this known.
Air pollution then can be defined as a gas that is released in a big enough quantity to harm to health of people or animals, kills or stop plants from growing, damage or disrupt the environment or cause other Kinds of nuisance such as reduced visibility or unpleasant odours.
Similarly to water pollution or land contamination it all comes to down to the quantity of certain gases or chemicals that makes the difference between harmless and pollution , for example Carbon Dioxide is in our air at a concentration of less than 0.05% and we can breathe it in all day without it having an effect on us (we also breathe it out) however air with a high percentage of this gas ( eg between 5-10%) is toxic and can kill you within a matter of mintutes.
Because Earth's atmosphere is very turbulent air pollution tends to disperse quite quickly. In less enlightened times factory operators assumed if they built the smokestacks high enough that the wind would just blow their smoke away diluting it into the air so it wouldn't be a problem however the Earth is a lot smaller that we thought and pollution doesn't just disappear.
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