The pollutant routinely forms in urban areas because of a combination of air temperature conditions and air pollution from local factories and motor vehicles. Smog causes respiratory problems in humans, lowers visibility, and damages vegetation. Chemical reactions in the troposphere convert some of the NO x gases into ozone. Although ozone is an important gas in screening out ultraviolet radiation in the stratosphere, it is an irritating air pollutant in the lower atmosphere. Ozone in the troposphere causes respiratory problems, eye irritation, and possibly lung cancer in humans.
Ozone also damages trees and crops near urban areas.
The Arctic haze results from air pollutants, chiefly originating from Asia and eastern Europe. In the spring, the haze absorbs a lot of heat that normally would reflect off the ice and snow and back into outer space. The haze is probably contributing to global warming. Like methylmercury, lead tends to accumulate in the human body and is not easily excreted. In ancient Rome, wealthy individuals who were unaware of the health risks often used lead as a seasoning in wine and commonly drank from lead-glazed pottery.