There's something called the Air Quality Index, or the AQI, and it keeps track of how clean or populated the air is on a day to day basis.

空气质量指数，简称AQI，是对每天空气清洁或污染的情况进行追踪记录。

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Air quality Index (AQI) measures the overall quality of the air on a scale of 0 to 500. A low number means good air quality while higher Numbers means worse air quality.

空气质量指数衡量大体的空气质量，数值从0到500，数值越小说明空气质量越高，反之越低。

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Local media reported that the air in Beijing on Friday pushed past 300 AQI on the air Quality Index, a level deemed 'hazardous'.

当地媒体报道说，北京星期五的空气质量指数超过了300 AQI，达到了“危险”的级别。

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While air pollution has remained a nagging problem since then, Beijingers have learned to live with the situation, and there have been enough blue sky days in the capital city to keep the Air Quality Index (AQI) out of the news until last week that is.

FORBES: Air Quality In China