(每题2分，共10分) 1、 Forms of Environmental Disease
46 . How the diseases are expressed depends on how the particular environmental agent enters the body, how it is metabolized(引起代谢)，and by what route it is excreted. The skin, lungs, liver, kidneys, and 47 . Of particular concern is the capacity of many environmental agents to cause various cancers, birth defects or spontaneous affection (through fetal exposure), and mutations(突变)in germ cells, 48 .
Environmental illnesses can be mild or severe and range from transient to chronic, depending on the doses of toxin(毒素)received. Some diseases occur abruptly at a toxic exposure, whereas the time of onset of other diseases varies after exposure. 49 . Those illnesses that occur directly after a distinct toxic exposure are usually easily identified as being environmentally or occupationally caused. If the exposure is not clear-cut or illness is delayed, however, the cause is difficult to identify, as clinical features alone are usually nonspecific. 50 . In such instances, epidemiological studies of exposed populations can help relate exposures to the illnesses they cause.

1A Environmentally induced cancers, for example, commonly involve latency (潜伏期)periods of 15 to 30 years or more
B In addition, many different causes, environmental or otherwise, may produce identical illnesses
C However, the reverse is true
D nervous systems are commonly affected by different agents in different settings
E the last?named raising possibilities of environmentally caused genetic diseases in later generations
F Environmental diseases can affect any organ system of the body

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