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 vegetations, or materials 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 vapor might be considered an air pollutant under certain conditions.
Many of the more important air pollutants, such as sulfur 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 cycle. These 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 form human activities. However, human production usually occurs in a localized area, such as a city.
In this localized regions, human output may be dominant and may temporarily overload the natural purification scheme of the cycle. 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, sulfur dioxide has detectable health effects at 0.08 parts per million (ppm), which is about 400 times its natural level. Carbon monoxide, however, ahs a natural level of 0.1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
41. What does the passage mainly discuss?
A. The economic impact of air pollution B. What constitutes an air pollutant C. How much harm air pollutants can cause D. The effects of compounds added to the atmosphere
50. Which of the following is best supported by the passage?
A. To effectively control pollution local government should regularly review their air pollution laws. B. One of the most important steps in preserving natural lands is to better enforce air pollution laws. C. Scientists should be consulted in order to establish uniform limits for all air pollutants. D. Human activities have been effective in reducing air pollution.
Another question.
These 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 form human activities.
A. The economic impact of air pollution B. What constitutes an air pollutant C. How much harm air pollutants can cause D. The effects of compounds added to the atmosphere
50. Which of the following is best supported by the passage?
A. To effectively control pollution local government should regularly review their air pollution laws. B. One of the most important steps in preserving natural lands is to better enforce air pollution laws. C. Scientists should be consulted in order to establish uniform limits for all air pollutants. D. Human activities have been effective in reducing air pollution.
These 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 form human activities.
"那些污染物其实起到空气自我净化的作用, 而自然产生的这些化合物数量其实比人类造成的多很多.但是人类造成的污染经常会在一个城市区域内产生造成不好的影响." - good understanding, but I still think that, according to the above understanding,the polution made by population, even small when compared with that of natural resource, has been effective in increasing, rather than reducing, air pollution.
stanhope, what's exactly I'm wrong about this.
By the way, your interpretation of 'that' is correct. thanks a lot!
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发表于 2005-7-28 00:39:00
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