Air pollution is a broad term applied to all physical (particulate matter), chemical, and biological agents that modify the natural characteristics of the atmosphere in ways that are harmful to life. Air pollution has both natural and human causes. Air pollution issues include the following:

climate change - induced by greenhouse gas emissions into the atmosphere, most of which, like C02, do not harm humans directly.
smog - air pollution which decreases visibility and is harmful to human beings and other species, generally associated with cities.
acid rain - air pollution which causes chemical changes to rainfall which disrupts freshwater lakes and ecosystems.
second hand smoke - air pollution caused by tobacco smoking.
indoor air pollution - caused by emissions from household items and appliances harmful to human health.