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It didn’t happen overnight. The problem of polluted air has been festering for centuries. Suddenly the problem of air pollution is becoming critical and is erupting right before our eyes. Not only do our eyes bum as they focus through murky air, but when the air clears, we see trees and vegetation dying. We must realize that this destruction can no longer be pinned to some mysterious cause. The one major culprit is air pollution. Today’s air pollution is an unfortunate by-product of the growth of civilization. Civilized man desires goods that require heavy industrialization and mass production. Machines and factories sometimes pollute and taint the air with substances that are dangerous to man and the environment. These substances include radioactive dust, salt spray, herbicide and pesticide aerosols, liquid droplets of acidic matter, gases, and sometimes soil particles. These materials can act alone to irritate objects and forms of life. More dangerously, they join together to act upon the environment. Only lately have we begun recognizing some of their dangerous consequences. Scientists have not yet been able to obtain a complete report on the effects of air pollution on trees. They do know, however, that sulfur dioxide, fluorides, and ozone destroy trees and that individual trees respond differently to the numerous particulate and gaseous pollutants. Sometimes trees growing in a single area under attack by pollutants will show symptoms of injury or will die while their neighbors remain healthy. Scientists believe this difference in response depends on the kind of tree and its genetic makeup. Other factors, such as tree’s stage of growth and nearness to the pollution source, the amount of pollutant, and the length of the pollution attack also play a part. In short, whether or not a tree dies as a result of air pollution depends on a combination of host and environment factors.

For the most part, air pollutants injury trees. To conifers, which have year-round needles, air pollution causes early balding. In this event, trees cannot maintain normal food production levels. Undernourished and weakened, they are open to attack by a host of insects, disease, and other environmental stresses. Death often follows. Air pollution may also cause hardwoods to lose their leaves. Because their leaves are borne only for a portion of the year and are replaced the following year, air pollution injury to hardwoods may not be so severe.

66. The author attributes today’s air pollution to .

A. the growth of civilization B. man’s carelessness

C. environmental imbalance D. some mysterious cause

67. The resistance of some trees to disease can be traced to .

A. protective foliage B. thick bark C. genetic makeup D. tainted air

68. Air pollution causes the most damage to ?

A. hardwoods B. conifers C. fruit trees D. fluorides

69. The author implies that the greatest source of pollution is .

A. heavy industry B. chemical processing plants

C. urban expansion D. salt spray

70. We can conclude that .

A. air pollution is easier to control than water pollution

B. the problem of polluted air is a problem overnight

C. the impact of air pollution has been known for centuries

D. research on the effects of air pollution is incomplete

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