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係Volatile organic compound
Volatile organic compounds (VOCs) are organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere. (The term VOC is also occasionally used as an abbreviation, especially in biological contexts, for "volatile organic carbon".) A wide range of carbon-based molecules, such as aldehydes, ketones, and hydrocarbons are VOC's. The term often is used in a legal or regulatory context and in such cases the precise definition is a matter of law. These definitions can be contradictory and may contain "loopholes"; e.g. exceptions, exemptions, and exclusions. Others believe the concept that a volatile organic compound is any organic that participates in a photoreaction, as found in the EPA's definition, is very broad and vague. Organics that are not volatile, as described above, can fall into that definition. The term may refer both to well characterized organic compounds and to mixtures of variable composition. Most often the definition used is one from the United States Environmental Protection Agency (see below).
Sources of VOCs
Common artificial sources of VOCs include paint thinners, dry cleaning solvents, and some constituents of petroleum fuels (eg. gasoline and natural gas). Trees are also an important biological source of VOC. It is also known that trees emit large amounts of VOCs especially isoprene and terpenes. Significant biological sources of methane are termites, cows (ruminants) and cultivation (estimated emissions 15, 75 and 100 million tons per year respectively).
Another significant source of VOC emission is crude oil tanking. Both during offloading and loading of crude oil tankers VOC are released to the atmosphere. Lately there has been an environmental focus on this issue resulting in both VOC handling on newer tankers and also crude oil loading terminals.
Also many VOCs are found in brownfield sites.
VOCs are sometimes accidentally released into the environment, where they can damage soil and groundwater contaminants. Vapours of VOCs escaping into the air contribute to air pollution.
VOCs are an important outdoor air pollutant. In this field they are often divided up into the separate categories of methane (CH4) and non-methane (NMVOCs). Methane is an extremely efficient greenhouse gas which contributes to enhanced global warming. Other hydrocarbon VOCs are also significant greenhouse gases via their role in creating ozone and in prolonging the life of methane in the atmosphere, although the effect varies depending on local air quality. Within the NMVOCs, the aromatic compounds benzene, toluene and xylene are suspected carcinogens and may lead to leukaemia through prolonged exposure. 1,3-butadiene is another dangerous compound which is often associated with industrial uses.
Some VOCs also react with nitrogen oxides in the air in the presence of sunlight to form ozone. Although ozone is beneficial in the upper atmosphere because it absorbs UV thus protecting humans, plants, and animals from exposure to dangerous solar radiation, it poses a health threat in the lower atmosphere by causing respiratory problems. In addition high concentrations of low level ozone can damage crops and buildings.資料來源： wikipedia