“非典”疫情已过去整整十年。十年前，SARS曾造成全球8,000多人感染，774人死亡。亚洲各大城市随处可见戴口罩出行的人。

    当时香港共299人死亡，是中国境内疫情最严重的城市。然而，近日香港大学民意研究计划的调查结果表明，港人防疫意识已逐渐松懈。只有一半的受访者表示，他们会在伤风感冒时在公共场所戴口罩。《英文虎报》（The Standard）援引一位传染病专家的话表示，这一调查结果“令人担忧”。

    不过说到戴口罩，专家们的意见并不一致。调查结果发布后第二天，我拜访了香港卫生署卫生防护中心感染控制科学委员会前主席司徒永康博士，对他进行了为期一小时的采访。采访的气氛很热烈，之所以这么说，是因为司徒博士讲起话来太激昂了，以至于他的助手来敲了两次门，恳请他小声点，因为吵到了隔壁的人。后来我们到外面去了。

    司徒博士的观点，一言以蔽之就是：“这简直太蠢了！”他说，要想保护自己和他人，完全可以采取更好、更简单的方式。他承认，戴口罩有时候确实有些用处，“但是使用方法不当反而有害！”

    司徒博士表示，要想了解“非典”、新冠状病毒等症状类似流感的呼吸系统疾病，首先要知道的是，它们从技术上讲并不是通过空气传播的疾病 ：“很多年以前，我刚做医生那阵子，大家都认为所有呼吸道疾病都是通过空气传播的，因为，”说到这里，他咳嗽了一声，作为示范，“你咳出来的东西都散播到空气中去了。”

    那么，咳嗽时会产生哪些喷出物呢？司徒博士表示，喷出物包括大粒子，还有飞沫，大飞沫很快就掉落到地上，不会飞出一米远。只有足够小的粒子才可以在空气中传播。这种传染性的小飞沫有个专门的术语，叫做气溶胶。美国疾病控制中心和世界卫生组织都认为，只有三种常见传染病会在患者咳嗽或者出皮疹时产生气溶胶。“肺结核，知道吧？”司徒博士说，“还有水痘。还有麻疹。没了，就这么三种！”

    香港科技大学健康、安全及环境处处长关继祖博士并不同意这个观点。他不是医生，而是工程师。他说：“任何工程师，只要研究了患者咳嗽时喷出气溶胶的机制，就会毫不犹豫地告诉你，不管是咳嗽还是打喷嚏，都会产生多分散气溶胶。”也就是说粒子的大小各不相同。他承认大粒子会掉落在地，但小粒子会满天飞散。关博士表示，把安全距离设定为一米，而不考虑实际情况，“肯定不是对公共卫生最有利的选择。”

    司徒博士对此嗤之以鼻。“市政府说要定成两米，”他说，“我们不同意。你定成两米了，医院就要花费很多成本，而且这个标准还没有被证实。一米就够了！了解了这一点，事情就很简单了！”

    It has been exactly 10 years since the deadly SARS outbreak that infected more than 8,000 people worldwide, killed 774, and made surgical masks a common sight in cities across Asia.

    Hong Kong, where 299 died, was the second-hardest hit in the world after China, and the University of Hong Kong Public Opinion Programme recently released the results of a survey suggesting that Hongkongers had grown dangerously complacent. Barely half of the respondents claimed they bother to wear a mask in public anymore if they have a cold or the flu. One infectious diseases expert quoted in the Hong Kong daily, The Standard, described the results as "alarming."

    But when it comes to masks, the experts don't all agree. The day after the survey was published, I spent a boisterous hour with Dr. Wing-hong Seto, former chairman of the Scientific Committee on Infection Control at the Centre for Health Protection, Department of Health, Hong Kong. I say boisterous because he got so animated during our interview that twice his handler knocked on the door and asked him to keep it down, please, because he was bothering the people in the next room. Eventually we moved outdoors.

    Dr. Seto's bottom line: "This is so stupid!" He says there are better, simpler ways to protect yourself and others. A mask might help, he allows, sometimes, "but if not used properly it will hurt you more than help!"

    The first fact to understand about SARS, as well as the new coronavirus and other respiratory conditions that produce flu-like symptoms, Dr. Seto says, is that they are not, technically speaking, airborne diseases: "Years ago, when I first became a doctor, we used to think that all chest infections were transmitted airborne, because" -- he coughs now, demonstrating -- "you cough into the air."

    But what comes out when you cough? Large particles, he says; wet, heavy droplets that fall to the floor within one meter of your mouth. To go airborne the particles must be very small. The term for small-particle infectious spray is aerosol, and both the U.S. Centers for Disease Control and the World Health Organization agree that there are only three diseases active among humans that produce aerosol, by cough or by rash. "TB, okay?" says Dr. Seto. "Chickenpox. And measles. Full stop. That's it!"

    Dr. Joseph Kwan, director of Health, Safety, and Environment across town at Hong Kong University of Science and Technology, disagrees. He's not a medical doctor, he's an engineer, and he says, "engineers who have actually studied the mechanism of aerosol emission from coughing patients would readily tell you that any cough or sneeze, they are essentially poly-dispersed." This means the particles are different sizes. Big particles drop, he admits, but smaller ones can fly. To apply the one-meter rule indiscriminately, Dr. Kwan argues, "certainly is not in the best interests of public health."

    Dr. Seto just scoffs. "City says two meters," he acknowledges. "We disagree. You make it two meters, it costs a lot of money for hospitals, and it's not proven. One meter is all you need! If you understand this, things become very simple!"

    假设你在香港挤公交车，现在是流感季节，而你不想生病。“看看周围，”司徒博士建议，“如果有人咳嗽，离他远点就行了！哈哈！至少保持一米的距离！如果你看到有人表现出流感的症状，那就闪人！”

    如果有人正好对着你的脸打了个喷嚏，那该怎么办呢？“遇到这种情况，你得记住一点，”司徒博士说，“没有什么细菌是能在接触到你的一刹那就让你患病的。只有电视上的东西才会那么夸张，比如恐怖科幻片《幽浮魔点》（The Blob），上面描述的那种细菌能让人瞬间融化。医学当中有个词叫‘潜伏期’；病毒需要潜入你的身体，它需要以正确的方式附着在细胞上，然后自我复制。要是有人对着我的脸咳嗽，我怎么办？没事！洗脸，洗脸，再洗脸！别的什么都不用做！”

    至于戴口罩防病，“我不推荐这种方法。”司徒博士说。首先得选对口罩，要双层的，里层能透气，外层能防水。（“告诉你吧，你去公交车里看就知道了，好多人都戴错口罩了。”）戴口罩时要把嘴巴、鼻子都遮住。（“要戴眼镜。”）不能整天都戴同一个口罩。（“你得记住，戴上口罩在外面转了一圈之后，口罩就变脏了。要是不会变脏，那我们还戴它做什么？”) 脱下来的时候要非常、非常的小心。（“要是你戴着口罩出去，路上有患病的人，那么这个口罩就会沾上病菌。你一碰到它，病菌就跑到手上去了。”）

    司徒博士承认，关于最后要提的这个事实，虽然也不是没有争议，但他的立场是坚定的：“我们现在基本上认为，如果一个人没有症状，就可以视为尚未感染。”也就是说只要你没打喷嚏，就算感觉身体不适，也不会威胁到其他人。

    好吧，假如一个人身体真的很不舒服，又咳嗽、又流鼻涕、又打喷嚏的，那么是不是应该至少戴个口罩呢？别的不说，至少在拥挤的公交车里也该为其他乘客想想吧？“如果你病了，”司徒博士说着，咧开嘴笑了，“那就老老实实在家里呆着。”非常简单。

    译者：Nasca

    Say you're on a crowded Hong Kong bus, it's flu season, and you don't want to get sick. "Look around," Dr. Seto advises. "If someone is coughing, just stay away from that guy! Hah-hah! At least one meter! If you see someone with symptoms, move away!"

    And if by chance someone sneezes in your face? "This you must believe me," Dr. Seto says. "There is no bacteria on earth that clobbers you the moment it touches you. That's TV. The Blob, the guy melted right away. There is something in medicine called the incubation period. The virus needs to get into you; it needs to attach the right way, multiply. Someone coughs in my face, what do I do? No problem! Wash, wash, wash, wash! That's all you need to do!"

    As for wearing a mask to protect yourself, "I would not recommend it," Dr. Seto says. You need the right kind of mask, first of all, one with two layers. Absorbent side in, waterproof side out. ("I tell you, if you go to the bus you see all these idiots wearing it the wrong way.") You need to cover your mouth and your nose. ("And wear spectacles.") You don't want to wear the same mask all day. ("It's good practice to say once you put it on, consider it dirty. If it's not dirty, why are you wearing it?") And be very, very careful when you take it off. ("If you're infectious and I'm wearing a mask, now the mask is infectious. I touch it and it's on my hands.")

    Last fact. Well, not an undisputed fact. There is still some debate about this, Dr. Seto allows, but his own view is firm: "Generally we now believe that if you have no symptoms, you can be considered not infectious." Meaning as long as you're not sneezing, even if you don't feel quite right, you're not a threat to others.

    Okay but let's say you really are sick. Runny, slobbery, sneezy sick. Can we at least agree that then you should wear a mask? If for no other reason than to protect your fellow passengers on that crowded bus? "If you are sick," says Dr. Seto, grinning, "stay home." Very simple.