20世纪90年代，空气污染时最严重时（如图所示），臭氧这种最常见的污染物的平均浓度大约是安全浓度的一半。

The average concentration of ozone, one of the most common pollutants, is about half its level in the early 1990s, when the air was at its dirtiest (see chart).

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20世纪90年代，空气污染时最严重时(如图所示)，臭氧这种最常见的污染物的平均浓度大约是安全浓度的一半。

The average concentration of ozone, one of the most common pollutants, is about half its level in the early 1990s, when the air was at its dirtiest (see chart).in.

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本文针对该隧洞的特点依据保障隧道内乘驶机动车各人员健康、卫生和隧道的营运安全为原则对污染物浓度设计限值进行了研究。

This paper made some investigation about the permissible concentration limits about contaminations in roadway tunnel from the point of view of the effects on the body.

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