地表臭氧和颗粒污染物是人类健康的最大威胁。
Ground level ozone and particle pollution are the greatest threat to human health.
举例来说，1997年，美国环保署意图降低地表臭氧量标准——一种部分由石油精炼排放的污染物造成的污染。
In 1997, for instance, the E.P.A. moved to reduce surface ozone, a form of pollution caused, in part, by emissions from oil refineries.
在火星和我们太阳系的其他行星上没有氧气，这意味着这些行星也缺乏保护地表生物免受紫外线辐射的臭氧层。
The absence of an oxygen atmosphere on Mars and other planets in our solar system means that these planets also lack an ozone shield that would protect surface-dwelling life from UV radiation.
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