For example, daily air quality warnings — like those sometimes issued in Beijing — do little to help the average person, since the real threat is exposure to dirty air over extended periods.
例如，北京每日的空气质量报告对普通人的帮助不大，因为长期暴露在肮脏的空气中才是真正的威胁。
Annual average pollution levels are one of the most commonly used scientific measurements of air quality.
年平均污染水平是衡量空气质量的最常用指标之一。
The study of cities in China found the average annual expose to PM2.5 was 56 micrograms per cubic metre, compared to the WHO guideline limit for safe air quality of 10 micrograms.
对中国城市的这项研究发现，PM2.5的年均接触量为每立方米56微克，而世界卫生组织的空气质量安全标准为每立方米10微克。
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