????“非典”疫情已過去整整十年。十年前，SARS曾造成全球8,000多人感染，774人死亡。亞洲各大城市隨處可見戴口罩出行的人。

????當時香港共299人死亡，是中國境內疫情最嚴重的城市。然而，近日香港大學民意研究計劃的調查結果表明，港人防疫意識已逐漸松懈。只有一半的受訪者表示，他們會在傷風感冒時在公共場所戴口罩。《英文虎報》（The Standard）援引一位傳染病專家的話表示，這一調查結果“令人擔憂”。

????不過說到戴口罩，專家們的意見并不一致。調查結果發布后第二天，我拜訪了香港衛生署衛生防護中心感染控制科學委員會前主席司徒永康博士，對他進行了為期一小時的采訪。采訪的氣氛很熱烈，之所以這么說，是因為司徒博士講起話來太激昂了，以至于他的助手來敲了兩次門，懇請他小聲點，因為吵到了隔壁的人。后來我們到外面去了。

????司徒博士的觀點，一言以蔽之就是：“這簡直太蠢了！”他說，要想保護自己和他人，完全可以采取更好、更簡單的方式。他承認，戴口罩有時候確實有些用處，“但是使用方法不當反而有害！”

????司徒博士表示，要想了解“非典”、新冠狀病毒等癥狀類似流感的呼吸系統疾病，首先要知道的是，它們從技術上講并不是通過空氣傳播的疾病 ：“很多年以前，我剛做醫生那陣子，大家都認為所有呼吸道疾病都是通過空氣傳播的，因為，”說到這里，他咳嗽了一聲，作為示范，“你咳出來的東西都散播到空氣中去了。”

????那么，咳嗽時會產生哪些噴出物呢？司徒博士表示，噴出物包括大粒子，還有飛沫，大飛沫很快就掉落到地上，不會飛出一米遠。只有足夠小的粒子才可以在空氣中傳播。這種傳染性的小飛沫有個專門的術語，叫做氣溶膠。美國疾病控制中心和世界衛生組織都認為，只有三種常見傳染病會在患者咳嗽或者出皮疹時產生氣溶膠。“肺結核，知道吧？”司徒博士說，“還有水痘。還有麻疹。沒了，就這么三種！”

????香港科技大學健康、安全及環境處處長關繼祖博士并不同意這個觀點。他不是醫生，而是工程師。他說：“任何工程師，只要研究了患者咳嗽時噴出氣溶膠的機制，就會毫不猶豫地告訴你，不管是咳嗽還是打噴嚏，都會產生多分散氣溶膠。”也就是說粒子的大小各不相同。他承認大粒子會掉落在地，但小粒子會滿天飛散。關博士表示，把安全距離設定為一米，而不考慮實際情況，“肯定不是對公共衛生最有利的選擇。”

????司徒博士對此嗤之以鼻。“市政府說要定成兩米，”他說，“我們不同意。你定成兩米了，醫院就要花費很多成本，而且這個標準還沒有被證實。一米就夠了！了解了這一點，事情就很簡單了！”

????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!"