Air pollution is caused by a wide variety of things. The earth is great at cleaning the air on its own. However, air pollution has grown so much, the earth can no longer clean all of it. This is starting to have adverse effects on the environment such as causing acid rain, smog and a wide variety of health problems.
Cars, trucks, jet airplanes and other combustion engine vehicles cause air pollution. The exhaust from these contains carbon monoxide, nitrous oxide and gaseous oxide. This type of air pollution creates smog (as seen in Los Angeles) which causes respiratory health problems and holes in the ozone layer, which increases the exposure to the sun