Let me start with the punch line and then work backwards.
You do not wear a face mask to protect yourself, though it may offer some modest protection. You wear one to protect your fellow human beings. Refusing to wear a mask does not indicate bravery and toughness but rather a lack of concern for others.
How does the coronavirus spread? It spreads almost entirely when viral particles are expelled from the respiratory tract of an infected person and inhaled by another person. Huge numbers of viral particles are spread when an infected person coughs or sneezes, large numbers when they sing or shout, and fewer but enough to spread with normal talking or even breathing.
Who can be a virus spreader? Alas, it is not only people who are obviously ill. There are still gaps in our knowledge, and it is hard to give exact numbers, but all experts agree, backed up by increasing data, that many people who harbor coronavirus and can spread it have minor or no symptoms. The most recent study I found, published in a British medical journal, found that 81% of passengers on a cruise ship who tested positive had no symptoms when they were tested. Moreover, even those who develop symptoms are most infectious just before symptoms develop. Lack of symptoms seems to be more common in women than men, and in younger adults. Roughly half the Covid-19 patients in China appeared to have been infected before the patient from whom they caught it knew they were sick.
How much do masks protect the wearer? Probably not a lot. Only N95 masks, which remain in short supply and are needed by healthcare workers, filter out most viral particles, which are very small and can pass through the fabric of cloth or disposable surgical masks. They do offer modest protection against moist droplets which carry viral particles from a sneeze or cough.
The masks do cut down on spread of the droplets generated by speaking, coughing or sneezing. The key point is not that masks do not block every virus particle, but that they filter out many. Every virus particle that does not escape into the air is a virus that will not be inhaled by others or fall on surfaces and perhaps be picked up on the hands of others.
Just as wearing a seat belt does not confer immortality if you get in a car accident, but does cut down on deaths, wearing a face mask will not prevent all spread of coronavirus but will reduce it. Wearing a mask is an easy, inexpensive and harmless way to show that you care about others.
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