全国职称外语等级考试综合类概括大意九_职称英语阅读理解

全国职称外语等级考试综合类概括大意九

　　The ultimate source of metals within the human body is rocks， which weather into soil， gaining or losing some of their chemical constituents. The crops we eat selectively rem

　　ove from the soil the elements that they require for growth. The water we drink contains trace elements leached from rock and soil. Thus the geology and geochemistry of the environments have effects on the chemistry and health of plants， animals and people.

　　So far there is no data to suggest that people living on metal-rich soils experience a potential health hazard. The levels of metals within naturally contaminated soils are generally not high enough to cause serious health problems. Living on metal-rich soils does not represent a health risk unless large quantities of soil are digested or metal-rich dust is inhaled. However， small children are particularly exposed to metal-rich topsoil in playgrounds and gardens. They are also the most likely ones to eat potentially dangerous metal-rich soil.

　　Heavy metals are persistent; they do not break down to other chemicals in the environment. Industrially polluted sites usually undergo intensive clean-up and rehabilitation because heavy metals are a health concern once they enter the food chain. Some trace metals are alleged to cause cancer and are also known to cause poisoning.

　　In contrast， naturally contaminated soils have not been subject to risk assessment studies and rehabilitation measures， despite the fact that they frequently possess metal concentrations well above those of such polluted by humans and above environmental quality criteria.