在二十世纪六七十年代，铅沉积就在土壤和雪中不断累积，这主要是美国的含铅汽油排放的后果。
Lead deposits, which accumulated in soil and snow during the 1960s and '70s, were primarily the result of leaded gasoline emissions originating in the United States.
通过观察北极不同地区的铅沉积量。
Byobserving the lead accumulations in different parts of the arctic area.
在过去的20年里《空气洁净法案》要求在美国只能使用无铅汽油，因此世界范围内的铅沉积有了显著下降。
In the twenty years that the Clean Air Act has mandated unleaded gas use in the United States, the lead accumulation world-wide has decreased significantly.
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