这结果至少超过了美国国家空气质量标准的4倍。

That's at least four times greater than exposure standards set by U.S. national air quality standards.

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许多发达国家(如美国、欧洲和加拿大等)已把大气中BTEX作为常规监测内容，并规定了其空气质量标准。

Therefore, many developed countries (for example the US, Europe and Canada and so on) take BTEX in the atmosphere as the routine monitoring content, and has stipulated its strict air quality standard.

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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。

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

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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。

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

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