A combination of atmospheric chemicals, typically pollutants, interacting with sunlight causes smog. Smog, most common in sunny cities, forms ground-level ozone, which can cause respiratory problems.
Smog, a combination of the words smoke and fog, is a major pollutant of large, sunny cities. The chemicals in car exhaust, particularly nitrogen oxides and hydrocarbons, are the primary components of smog. These pollutants combine with energy from the sun to form ozone as well as various nitrates and acids, most of which are biological irritants. Ozone is an important component of Earth's atmosphere, providing a barrier against harmful ultraviolet radiation. However, ground-level ozone causes an assortment of health problems for people. These can be as mild as wheezing and eye irritation or as serious as pneumonia, heart attack and lung cancer. Children, the elderly and individuals with existing respiratory problems are especially at risk.
The type of smog most common in this century is the photochemical smog that results from pollution and sunlight. However, other factors also cause smog or smog-like substances. Coal fires cause smog, and there are reports of this from as early as the Middle Ages. The famous pea-soup fog of London is a result of coal. The sulphur dioxide that results from a volcanic eruption creates a thick, smog-like cloud, or vog.