The Chief of Medicine at UC San Francisco, Bob Wachter, has been a must-follow during the COVID-19 outbreak. Last month he posted a tweetstorm, upgraded to a Medium post, with some clear and sensible explanations of how masks work at preventing the spread of the virus, and where they’re most effective.
Of course the main point is that you wear a mask to protect others in case you’re infected and asymptomatic, not that you wear a mask as personal protection. Wearing a mask shows you are concerned about the welfare of others. It’s not a sign of fear, it’s a sign of compassion.
His anecdote about someone with COVID-19 riding on a packed airplane, coughing repeatedly, but wearing a mask—and not infecting anyone around them—really hits it home.
And there’s this sort of practical advice, too:
One of the most common questions is whether it is necessary to wear a mask when walking or exercising outside. Empiric and simulation studies have shown that there is practically zero risk of viral spread when one is outdoors and keeping a distance of greater than six feet from others. I personally don’t wear a mask when walking the dog (but I do keep one with me just in case I encounter someone at close range). But I always wear a mask inside, or if an encounter within six feet is likely.
This pretty much tracks with how I approach this, and it’s what my county’s health department has mandated: Wear a mask indoors or if you’re going to be close to people, but don’t sweat it if you’re out walking the dog and there’s nobody around. But carry a mask in case you run into anyone.