An air pollutant is defined as a compound added directly or indirectly by humans to the atmosphere in such quantities as to 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 to compounds that could be seen or smelled – a far cry from the extensive list of harmful subtances 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 notrigen 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 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 as city. In such a 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 subtance to be a pollutant; in fact, the numerial value tells us little until we know how much of an increase this represents over ther 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 and is not usually a pollutant until its level reaches about 15 ppm.
According to the passage, the numerical value of the concentration level of a substance is only useful if ________.
A. the other substances in the area are known
B. it is in a localized area
C. the natural level is also known
D. it can be calculated quickly
Đáp án C.
Key words: numerical value, concentration level.
Câu hỏi: Theo bài khóa, giá trị bằng con số của mức độ dồn lượng một chất sẽ chỉ hữu dụng khi nào?
Clue: “…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”
Phân tích: Ngay sau khi nhắc đến giá trị bằng con số, tác giả đưa ra ví dụ trong đó có sự xuất hiện của hai biến là “concentration level” và “natural level”. Do đó, để “concentration level” có ý nghĩa thì cũng phải biết “natural level”. Chọn đáp án C. the natural level is also known.
Các đáp án khác không phù hợp:
A. the other substances in the area are known: các chất khác trong tự nhiên được biết.
B. it is in a localized area: các chất có ở khu vực chuyên môn hóa.
C. it can be calculated quickly: nó phải được tính nhanh.