因为干旱，该州没有淡水来排放有毒物质的缘故，内华达州是美国水污染最轻的。

Nevada has the lowest level of water pollution in the country because the generally arid state has very little fresh water to dump toxins into.

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生活在空气污染最严重地区的妇女，罹患乳腺癌的危险是空气污染最轻地区妇女的两倍。

Women living in the areas with the highest levels of pollution were almost twice as likely to develop breast cancer as those living in the least polluted areas.

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污染最轻的是美国和加拿大的城市，原因是它们的人口密度低，气候怡人，并且空气污染管理规定非常严格。

At the other end of the list are cities in Canada and the United States, which benefit from lower population density, favorable climates and stricter air pollution regulation.

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