With depletion of the ozone layer, solar UV-B radiation passes deeper into the
atmosphere, closer to earth. Here it encounters a variety of chemicals
emitted by human activities into a chemical mixture that is thickest around
the major cities of the world.
In the lower atmosphere, UV-B is a catalyst in the production of some very
reactive chemicals (such as hydroxyl, hydrogen peroxide and ozone). These
compounds help the solar UV radiation to cook the mixture of chemical
ingredients into a photochemical soup hazardous to human health, plants and
materials. Included in this soup are a variety of acidic compounds formed from
human emissions of sulphur and nitrogen oxides which eventually fall to earth
as acid deposition or acid precipitation and alter soil, freshwater and plant
life.
This soup, known as photochemical smog, has reached unhealthy proportions in
cities around the world: Los Angeles, Denver, Chicago, New York, Mexico City,
Manila, Vancouver, Toronto, Berlin, Taipei and Tokyo. Drifting ozone smog
also endangers areas outside of large population centres, where due to the
absence of other neutralizing pollutants in the air, ozone levels tend to be
higher and last longer than over cities.
Photochemical contains chemical compounds hazardous to human health, causing
and enhancing respiratory diseases and increasing the risk of respiratory or
heart failure. Its constituents are hazardous to plant life including
agricultural varieties. Acidic components such as sulphuric acid and strong
oxidizers such as ozone reduce the useful lifetime of materials including
plastics, metals, and limestone.
