美国空气最清新的城市
<p><span word="Forty">Forty</span> <span word="years">years</span> <span word="after">after</span> <span word="the">the</span> <span word="Clean">Clean</span> <span word="Air">Air</span> <span word="Act">Act</span>, <span word="more">more</span> <span word="people">people</span> <span word="are">are</span> <span word="breathing">breathing</span> <span word="easier">easier</span>. "<span word="In">In</span> <span word="general">general</span>, <span word="we">we</span>’<span word="ve">ve</span> <span word="seen">seen</span> <span word="tremendous">tremendous</span> <span word="improvement">improvement</span> <span word="in">in</span> <span word="air">air</span> <span word="quality">quality</span>," <span word="says">says</span> <span word="Jonathan">Jonathan</span> <span word="M">M</span>. <span word="Samet">Samet</span>, <span word="MD">MD</span>, <span word="director">director</span> <span word="of">of</span> <span word="the">the</span> <span word="Institute">Institute</span> <span word="for">for</span> <span word="Global">Global</span> <span word="Health">Health</span> <span word="at">at</span> <span word="the">the</span> <span word="University">University</span> <span word="of">of</span> <span word="Southern">Southern</span> <span word="California">California</span>, <span word="in">in</span> <span word="Los">Los</span> <span word="Angeles">Angeles</span>.</p><p>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<span word="Jonathan">Jonathan</span> <span word="M">M</span>. <span word="Samet">Samet</span>哲学博士说。</p><p><span word="But">But</span> <span word="some">some</span> <span word="air">air</span> <span word="is">is</span> <span word="cleaner">cleaner</span> <span word="than">than</span> <span word="others">others</span>. <span word="Here">Here</span> <span word="are">are</span> <span word="the">the</span> 5 <span word="cities">cities</span> <span word="with">with</span> <span word="the">the</span> <span word="cleanest">cleanest</span> <span word="air">air</span>, <span word="according">according</span> <span word="to">to</span> <span word="the">the</span> <span word="American">American</span> <span word="Lung">Lung</span> <span word="Association">Association</span>’<span word="s">s</span> <span word="annual">annual</span> <span word="report">report</span>, <span word="which">which</span> <span word="looks">looks</span> <span word="at">at</span> <span word="levels">levels</span> <span word="of">of</span> <span word="ozone">ozone</span> <span word="and">and</span> <span word="particulates">particulates</span>, <span word="which">which</span> <span word="are">are</span> <span word="tiny">tiny</span> <span word="particles">particles</span> <span word="that">that</span> <span word="can">can</span> <span word="get">get</span> <span word="lodged">lodged</span> <span word="in">in</span> <span word="your">your</span> <span word="lungs">lungs</span>.</p><p>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。</p><p><span word="Cheyenne">Cheyenne</span>, <span word="Wyo">Wyo</span>.</p><p><span word="Population">Population</span>: 88,854</p><p>怀俄明州夏延</p><p>人口：8 8854</p><p><span word="Cheyenne">Cheyenne</span> <span word="is">is</span> <span word="the">the</span> <span word="clear">clear</span> <span word="winner">winner</span>: <span word="It">It</span> <span word="has">has</span> <span word="the">the</span> <span word="lowest">lowest</span> <span word="level">level</span> <span word="of">of</span> <span word="year">year</span>-<span word="round">round</span> <span word="PM">PM</span> 2.5 <span word="particulates">particulates</span>, <span word="tiny">tiny</span> <span word="lung">lung</span> <span word="invaders">invaders</span> <span word="about">about</span> 1/30<span word="th">th</span> <span word="the">the</span> <span word="width">width</span> <span word="of">of</span> <span word="a">a</span> <span word="human">human</span> <span word="hair">hair</span>. <span word="Cheyenne">Cheyenne</span>'<span word="s">s</span> <span word="small">small</span> <span word="size">size</span> <span word="may">may</span> <span word="help">help</span>; <span word="fewer">fewer</span> <span word="cars">cars</span> <span word="travel">travel</span> <span word="the">the</span> <span word="roads">roads</span>, <span word="and">and</span> <span word="vehicles">vehicles</span> <span word="are">are</span> <span word="a">a</span> <span word="key">key</span> <span word="source">source</span> <span word="of">of</span> <span word="PM">PM</span> 2.5 (<span word="and">and</span> <span word="ozone">ozone</span>).</p><p>夏延是空气最清新的城市。它全年的微粒含量<span word="PM">PM</span>2.5最低，肺部侵略者——微粒<span word="PM">PM</span> 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<span word="PM">PM</span>2.5的主要来源。</p>
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