Curbing outdoor and indoor air pollution would improve health substantially. Indoor air pollution is due to burning wood, animal dung, crop residues, and coal for cooking and heating. In cities that lack pollution controls, millions of people are at risk from outdoor pollution. Densely populated and rapidly growing cities such as Bangkok, Manila, Mexico City, and New Delhi are often entombed in a pall of pollution from trucks and cars and from uncontrolled industrial emissions. Ozone is a powerful secondary pollutant formed when oxides of nitrogen and unburned volatile organic hydrocarbons, mostly from vehicle exhausts, combine with oxygen under the action of sunlight.
Ozone is a main component of smog.
Air pollution is not only a health hazard but also reduces food production and timber harvests, because high levels of pollution impair photosynthesis.