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1A useful definition of an air pollutantis a compound added directly or indirectly by humans to the atmospherein
2suchquantities as to affect humans, animals, vegetation, or material adversely (有害地.. Air pollution requires avery
3flexibledefinition that permits continuous change. When the first air pollution lawswere established inEngland in the
4fourteenth century, air pollutants werelimited to compounds that could be seen or smelled—a farcry (悬殊的差别. from the
5extensive list of harmful substances known today. Astechnology has developed andknowledge of the health aspects of
6variouschemicals has increased, the list of air pollutants has lengthened. Inthefuture, even water vapor (水蒸气. might be
7consideredan air pollutant under certain conditions.
8Many of the more important airpollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides,arefound in
9nature. As the Earth developed, the concentrations ( 浓度. of these pollutants were altered byvariouschemical reactions;
10they became components in biogeochemical (生物地球化学的. cycles. These serve as an airpurification scheme by allowing
11thecompounds to move from the air to the water or soil. On a global basis,nature'soutput of these compounds dwarfs that
12resulting from human activities. However,human productionusually occurs in a localized area, such as a city.
13In this localized region, human outputmay be dominant and may temporarily overload the natural purificationscheme ofthe
14cycles. The result is an increased concentration of noxious ( 有害的. chemicals in the air. Theconcentrations atwhich the
15adverse effects appear will be greater than the concentrations thatthe pollutants wouldhave in the absence of human
16activities. The actualconcentration need not be large for a substance to be apollutant; in fact thenumerical value tells
17us little until we know how much of an increase thisrepresents over the concentration that would occur naturally in the
18area. Forexample, sulfur dioxide has detectable health effects at0.08 parts per million(ppm., which is about 400 times
19its natural level.Carbon monoxide, however, has a naturallevel of 0.1 ppm and is not usually apollutant until its level
20reaches about 15 ppm.
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