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Forty years after the Clean Air Act, more people are breathing easier. "In general, we’ve seen tremendous improvement in air quality," says Jonathan M. Samet, MD, director of the Institute for Global Health at the University of Southern California, in Los Angeles. 空气清洁法实行后40年来，越来越多人呼吸到更加清新的空气。“总体上，我们见证了空气质量的巨大改善。”落砂机南加利福尼亚大学全球健康研究所主任Jonathan M. Samet哲学博士说。 But some air is cleaner than others. Here are the 5 cities with the cleanest air, according to the American Lung Association’s annual report, which looks at levels of ozone and particulates, which are tiny particles that can get lodged in your lungs. 不过，一些地方的空气比其他地方的更加清新。下面是根据美国肺协会的年度报告列出的5个空气最清新的城市，肺协会观察了臭氧和微粒物质的含量，这些细微颗粒可能进入肺部。 Cheyenne, Wyo. Population: 88,854 怀俄明州夏延 人口：8 8854 Cheyenne is the clear winner: It has the lowest level of year-round PM 2.5 particulates, tiny lung invaders about 1/30th the width of a human hair. Cheyenne's small size may help; fewer cars travel the roads, and vehicles are a key source of PM 2.5 (and ozone). 夏延是空气最清新的城市。它全年的微粒含量PM2.5最低，肺部侵略者——微粒PM 2.5是人类头发直径的约30分之一。夏延城市面积小也有帮助；路上来往的车辆更少，机动车是PM2.5的主要来源。 | 
