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When smog descends on Beijing or other Asian cities, people rush to buy face masks. 当雾霾降临到北京等亚洲城市的上空时，人们争相购买口罩。 But how effective are the masks at filtering out tiny, harmful particles of pollution? 可是，在过滤微小的有害颗粒物方面，这些口罩究竟有多少效果呢？ The effectiveness varies tremendously, depending on factors like type, brand and fit. Simple, loosefitting masks do little to combat pollution, according to the United States Food and Drug Administration, whereas more advanced, government-approved respirators that bind tightly to the wearer's face can help but may be uncomfortable. More public education and research are urgently needed about face masks, health professionals say. 基于类型、品牌和贴合度等因素，它们的有效性千差万别。美国食品与药品管理局(United States Food and Drug Administration)称，松垮地戴在脸上的简单口罩在对抗污染上起不到什么作用，而如果是获政府审批的更为先进的呼吸防护罩紧贴在面部，则能起到作用，但或许不太舒适。卫生领域的专业人士称，在口罩方面亟需加强公众教育与研究。 "Consumers simply just don't know which of those masks are the best," Richard Saint Cyr, a physician in Beijing who writes a health column for the Chinese-language T Magazine once a month, said in an email. "And some indeed may be worse than helpful if people are falsely reassured and spend more time outside using a mask which doesn't work." | 
