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  3. A useful definition of an air pollutant is a compound added directly or indirectly by humans to the atmosphere in such quantitie

A useful definition of an air pollutant is a compound added directly or indirectly by humans to the atmosphere in such quantitie

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问题     A useful definition of an air pollutant is a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or material adversely. Air pollution requires a very flexible definition that permits continuous change. When the first air pollution laws were established in England in the fourteenth century, air pollutants were limited to compounds that could be seen or smelled—a far cry from the extensive list of harmful substances known today. As technology has developed and knowledge of the health aspects of various chemicals has increased, the list of air pollutants has lengthened. In the future, even water vapour might be considered an air pollutant under certain conditions.
    Many of the more important air pollutants, such as sulphur oxides, carbon monoxide, and nitrogen oxides, are found in nature. As the earth developed, the concentrations of these pollutants were altered by various chemical reactions; they became components in biogeochemical cycles. These reactions serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. On a global basis, nature’ s output of these compounds dwarfs that resulting from human activities. However, human production usually occurs in a localized area, such as a city.
    In this localized region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an increased concentration of noxious chemicals in the air. The concentrations at which the adverse effects appear will be greater than the concentrations that the pollutants would have in the absence of human activities. The actual concentration need not be large for a substance to be a pollutant; in fact the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally in the area. For example, sulphur dioxide has detectable health effects at 0.08 parts per million(ppm), which is about 400 times its natural level. Carbon monoxide, however, has a natural level of 0.1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
The author puts forward the examples of sulphur dioxide and carbon monoxide at last to show that______.
选项 A、sulphur dioxide is more dangerous than carbon monoxide
B、sulphur dioxide is less dangerous than carbon monoxide
C、the numerical value of noxious chemicals in the air is not the key in defining pollution
D、it needs a great increase in concentration for noxious chemicals in the air to be pollutants
答案C
解析 细节题。根据原文第三段最后三句“实际上，浓度值并不能告诉我们什么，除非我们能知道这个浓度值相对于这种物质在该地区自然状态下的浓度的增长幅度。比如．当每百万个气体分子中含有0．08个二氧化硫气体分子时，人体就会明显感到不适。这个浓度大约是正常浓度的400倍。而一氧化碳的正常浓度是百万分之零点一．在不超过百万分之十五时就不算污染物。”可知化学物质的浓度值不是界定污染物的关键．和选项C的表述一致。所以答案选C。
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