Air pollution occurs when harmful gases, dust, fumes, or odors are introduced into the air in amounts that can negatively impact human health, animals, plants, or materials. Pollutants can be primary, emitted directly from sources like vehicle exhaust or industrial smokestacks, or secondary, formed through chemical reactions between primary pollutants in the air like photochemical smog. As urbanization and industrialization increased, air pollution became more concentrated in cities due to emissions from industrial, commercial, and domestic sources, endangering public health as evidenced by thousands of deaths in London in 1952 due to particle pollution. Both natural sources and human activities like transportation and industry contribute to air pollutants in the atmosphere.