Claude Monet's painting of 19th century London smog. It was caused mainly by burning coal, fires.

Air pollution is a type of environmental pollution that affects the air and is usually caused by smoke or other harmful gases, mainly oxides of carbon, sulphur and nitrogen.

In other words, air pollution is the contamination of air due to the presence or introduction of a substance which has a poisonous effect.

Many of the world's large cities today have polluted air or low air and complain about its daily quality.