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.”

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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!”

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!”

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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.