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。

　　哪种口罩防雾霾,我们需要仔细甄别

　　"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."

　　为纽约时报国际生活网站撰写月度健康专栏的圣西睿智医生(Richard Saint Cyr)在北京执业，他通过电子邮件接受采访称，“消费者对哪些口罩最好完全是一头雾水。有些的确是会帮倒忙，比如人们获得了错误的保证，戴着不起作用的口罩在室外待更长的时间。”

　　Many masks worn around Asia are simple surgical-type masks. But these are designed to prevent problems like splattering blood, not to block tiny particles, Benjamin Cowling, an associate professor of public health at the University of Hong Kong, said in an email. "It is pretty common knowledge that surgical face masks have almost no filtration efficiency against pollutants," he said.

　　亚洲各地采用的许多口罩就是简单的医用口罩。香港大学公共卫生学副教授高本恩(Benjamin Cowling)在电子邮件中表示，这种口罩的设计用途是防止血液飞溅之类的问题，而不是阻挡细颗粒物。“医用口罩在过滤污染物方面几乎无效，这是很基本的常识，”他说。

　　Surgical masks are made of polypropylene, according to Wallace Leung, director of the Research Institute of Innovative Products and Technologies at the Hong Kong Polytechnic University. His tests found that at a standard airflow velocity, basic masks captured only 20 to 25 percent of tiny particles of 50 to 500 nanometers — a size common in diesel vehicles' exhaust. Such particles, less than 1 percent of the width of a human hair, are of particular concern because they can get buried deep in the lungs and end up in the bloodstream. The figures do not include any gap between the mask and the face that allows air to come in, further reducing efficiency

　　香港理工大学创新产品与科技研究所所长梁焕方(Wallace Leung)称，医用口罩的材料是聚乙烯。他做的实验发现，在标准空气流速下，基本款的口罩只能过滤20%到25%直径在50到500纳米的细颗粒物。这种尺寸的颗粒物常见于柴油车尾气中，直径不到人体毛发的1%，因为能深入肺部进入血液循环而尤为令人担忧。这些数据还没有考虑口罩与面部之间漏气的情况，而这种空隙会进一步降低过滤效率。

　　"What it means is, if you wear a face mask, you get 75 to 80 percent into the body," Dr. Leung said. "So that's not good."

　　“这就意味着，如果戴的是一般口罩，就会有75%到80%进入人体，”梁博士说。“所以情况不太妙。”

　　A better bet, experts say, are respirators that guard against at least 95 percent of small particles. Sometimes known as N95 respirators, they use thick layers of polypropylene, according to Dr. Leung, and are designed to fit tightly to the face. In the United States, such masks get tested by the National Institute for Occupational Safety and Health and must be shown to keep out at least 95 percent of all tiny airborne particles to gain approval. They are often used by industrial workers and are generally disposable.

　　专家们表示，更好的选择是能阻挡至少95%细颗粒物的呼吸防护罩。这种产品有时被命名为N95防护罩，梁博士说它由厚厚的多层聚乙烯制成，采用紧贴面部的设计。在美国，此类面罩接受国家职业安全与卫生研究所(National Institute for Occupational Safety and Health)的检测，要想过关必须证明能过滤空气中至少95%的细颗粒物。它们常被工业工人使用，基本是一次性用品。

　　One widely sought-after brand is 3M. The Minnesota-based company recently announced that it would invest $15 million in a Singapore plant to increase production of its N95 respirators by 70 percent.

　　一个广受欢迎的品牌叫3M，厂家的总部位于明尼苏达州。公司最近宣布，将投资1500万美元（约合9300万元人民币）在新加坡新建一座工厂，从而把N95防护罩的产量提升70%。

　　"We definitely are seeing an increase in sales for respiratory protection" for use in China and elsewhere in Asia during hazy conditions, said Nikki McCullough, the technical manager for 3M's personal safety division. The company said the price for its disposable respirators ranges from 30 cents to $6.70.

　　3M个人安全部门的技术经理尼基·麦卡洛(Nikki McCullough)称，在中国等亚洲国家，由于人们在雾霾天气里要保护呼吸系统，“我们明显发现相关防护产品的销量增加了”。该公司称自己生产的一次性呼吸防护罩的价钱从30美分到6.7美元不等。

　　The respirators block particles in a number of ways as the airflow encounters layered fibers. But respirators that guard against small particles can make it more challenging to breathe. That is especially true for people with heart or lung problems, said Tze-wai Wong, a research professor at the School of Public Health and Primary Care at the Chinese University of Hong Kong.

　　随着气流经过层状纤维，呼吸防护罩会以多种方式阻挡颗粒物。不过，能阻挡细颗粒物的呼吸防护罩可能会让呼吸变得更困难。香港中文大学公共卫生及基层医疗学院(School of Public Health and Primary Care at the Chinese University of Hong Kong)的黄子惠(Tze-wai Wong)教授表示，对有心肺问题的人群尤其如此。

　　To improve breathability and increase filtering efficiency at the same time, Dr. Leung of Hong Kong Polytechnic University wants to create masks and respirators that use multiple layers of nanofibers. He received a United States patent last year, and "a number of companies have approached us," he said.

　　为了改善透气性，同时增强过滤效率，香港理工大学的梁博士想发明出使用多层纳米纤维的口罩和呼吸防护罩。去年，他在美国获得了一项专利。“已经有多家公司跟我们接洽了，”他说。

　　Another issue is that while N95 respirators guard against small particles, they do not combat another form of traffic-related pollution: gases like nitrogen oxides or volatile organic compounds.

　　另一个问题是，尽管N95防护罩能阻挡细颗粒物，它们却无法抵御另一种与交通有关的污染：像氮氧化物这样的气体或挥发性有机化合物。

　　Some companies have created cartridges that can connect to certain respirators to block some gases. But they are expensive and cumbersome — and, Dr. Leung said, not a good choice for the public in Asia.

　　一些企业已经研制出了能和特定的呼吸防护罩相连以阻挡部分气体的滤芯。但它们很贵，而且很笨重。梁博士说，对亚洲的民众而言，它们不是很好的选择。

　　Dr. Leung hopes to create a system that uses sunlight and oxygen to turn nitrogen oxides and volatile organic compounds into "harmless substances, like carbon dioxide or water." Eventually, he hopes to be able to join it to a regular, particle-filtering respirator.

　　梁博士希望能制造一个用阳光和氧气，将氮氧化物和挥发性有机污染物变成“像二氧化碳或水那样的无害物质”的系统。他希望最终能把这个系统加进过滤颗粒物的常规呼吸防护罩中。

　　More testing of existing brands of masks and respirators should be a priority, Dr. Saint Cyr said.

　　圣西睿智表示，首要的应该是对现有的口罩和呼吸防护罩品牌进行更多的检测。

　　Aided by his blog, Dr. Saint Cyr is trying to organize independent testing for 40 brands that are commonly available in China. But he recently put the project on hold, citing the need to gain academic and scientific backing for the project.

　　通过自己的博客，圣西睿智正试图组织对中国常见的40个品牌进行独立检验。但前不久，他搁置了这个项目，理由是必须让这个项目有学术依据和科学依据。

　　"I feel it's very important to warn people against masks that don't work, as well as to tell them which work well," Dr. Saint Cyr said, adding that research is also needed on household air-purifying machines.

　　“我觉得提醒人们有些口罩不起作用，并告诉他们哪些口罩效果好很重要，”圣西睿智说。他还表示，也需要对家用空气净化器进行研究。

　　Dr. Wong said that one low-tech method of combating air pollution might also be the most effective — encouraging people not to go jogging or engage in other strenuous activities during times of heavy smog. The goal, he said, is "really to try to change the behavior of people on high-pollution days.”

　　黄子惠表示，对付空气污染的一种科技含量很低的方法可能也是最有效的，那就是建议人们不要在烟雾浓重的时候跑步或进行其他剧烈运动。他说，目的“实际上是试着改变人们在高污染天气中的行为”。