Smog in California Characteristic coloration for smog in California, in the beige cloud bank behind Golden Gate Bridge
Smog is a kind of heavy air pollution. Under special weather conditions, it can stay for an extended period of time over densely populated cities, such as London, Los Angeles, Athens or the Ruhr area. The word "smog" is a portmanteau constructed from the words "smoke" and "fog", although modern usage of the term does not require either a smoke or fog component and "smog" is often used as shorthand for "air pollution".
Photochemical smog is caused when two kinds of air pollution combine in the presence of sunlight. The first kind is the particulates and nitric oxides from the exhaust of fossil fuel-burning engines in cars, trucks, coal power plants, and industrial manufacturing factories. The second kind is the emissions of volatile organic compounds from paints, solvents, pesticides, and other chemicals. According to , gasoline and other petroleum-based chemicals and solvents often vaporize directly into the atmosphere, contributing to ozone. Two major residential sources are gasoline-powered lawnmowers and the starter fluid for backyard grills.
Smog can form in almost every climate, but is far worse during periods of warmer, sunnier weather when the upper air is warm enough to inhibit vertical circulation. It is especially prevalent in geologic basins encircled by hills or mountains.
Episodes of smog became common in London in the late 19th century and were nicknamed as "pea-soupers". The Great Smog of 1952 darkened the skies over London and killed approximately 12,000 people. The British government first blamed a flu epidemic, reluctant to admit that coal smoke was to blame. In 1956 the Clean Air Act introduced smokeless zones to the capital. Only smokeless fuels could be used in these areas. Consequently, reduced sulphur dioxide levels made the intense and persistent London smog a thing of the past. Smog caused by traffic pollution, however, does occur in modern London.
Smog is a problem in a number of cities and continues to harm life. According to the U.S. EPA, air is unhealthy because of smog if it contains more than 80 parts per billion (ppb) of ozone (the primary component of smog) , more than 53 ppb of nitrogen dioxide or 80 ppb particulates. High levels of smog aggravate and even cause human respiratory problems, including emphysema, bronchitis, and asthma. Hospital admissions and respiratory deaths often increase during periods when ozone levels are high .
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