Read the following passage & mark the letter A, B, C or D on your answer sheet to lớn indicate the correct answer to each of the question from 1 to 7

An air pollutant is defined as a compound added directly or indirectly by humans to the atmosphere in such quantities as khổng lồ affect humans, animals, vegetation, 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 khổng lồ compounds that could be seen or smelled - a far cry from the extensive danh sách of harmful substances known today. As giải pháp công nghệ has developed và knowledge of the health aspects of various chemicals has increased, the các mục of air pollutants has lengthened. In the future, even water vapor might be considered an air pollutant under certain conditions.

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Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, & nitrogen oxides, are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds khổng lồ move from the air to the water or soil. On a global basis, nature"s output đầu ra of these compounds dwarfs that resulting from human activities.
However, human production usually occurs in a localized area, such as a city. In such a region, human output may be dominant & 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 khổng lồ 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, has a natural level of 0.1 ppm & is not usually a pollutant until its màn chơi reaches about 15 ppm.

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