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.
在《清洁空气法案》强制要求美国使用无铅气体的二十年里，铅在世界范围内的累积量显著下降。
In the twenty years that the Clean Air Act has mandated unleaded gas use in the United States, the lead accumulation worldwide has decreased significantly.
在《人类环境杂志》上发表的一项研究称，科学家们发现，自引进无铅汽油以来，美国东北部土壤中的铅含量已经显著下降。
In a study published in the journal Ambio, scientists found that lead levels in soil in the North-eastern United States had decreased markedly since the introduction of unleaded gasoline.
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