In 1997, EPA established annual and 24-hour NAAQS for PM2.5 for the first time.

在1997年，美国环保署就首次制定了年度，和24小时国家环境空气质量标准来监测PM 2.5。

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NAAQS (National Ambient Air Quality Standards) define attainment and protect human health by setting a limit on SO2 concentration nationwide.

国家环境空气质量标准规定了全国的二氧化硫浓度限值来保护人民健康。

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This NAAQS review process has evolved since it was instituted in 1970, with many of the current features having been in place for over 20 years.

自1970年制定之后，国家环境空气质量标准评审流程已经逐步形成，并且呈现许多当前特征业已存在了20多年。

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Yet the Clean Air Act requires States to adopt implementation plans adequate to attain primary (health-based) NAAQS within five or at most 10 years.

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