非典十年后，港人摘下口罩

    “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!"