Modern-day smog is often referred to as ‘photochemical’ smog. It is produced by complex, sunlight-stimulated chemical reactions among the components of automobile exhaust. It is responsible for much of today's air pollution in cities such as Los Angeles and Denver. Carbon monoxide from incomplete combustion of automobile engines, particulate matter and NOx all react to generate the noxious brown haze. An estimated 80% of smog today arises from vehicle exhausts. Not only does smog smell bad and obstructs vision, but both short-term and long-term exposure to it may be hazardous. Eye irritation develops upon short-term exposure. .
