We know that the virus is spread primarily by respiratory droplets produced when an infected person coughs or sneezes. Droplets can also be generated by talking, laughing, singing or just exhaling. They can land in the mouths or noses of people who are nearby or be inhaled into the lungs, which is why people have been asked to stay at least 6 feet away from each other.
It's also possible for infected droplets to be transferred to surfaces and then spread by contact when another person touches the surface and then touches their eyes, nose or mouth. This is why public health officials have emphasized frequent hand-washing and cleaning of surfaces.
It has been suggested by institutions such as the Mayo Clinic that when someone infected with a respiratory viral illness wears a surgical mask it protects others from getting infected because the masks catch the droplets and keep them from reaching other people. But we have also learned that individuals can be infected with the coronavirus and be contagious even when they have no symptoms.
In fact, it is likely that in some outbreaks, a significant number of individuals were infected by those with no symptoms who were unknowingly spreading the virus. Could they have minimized spread of the virus if they had been using face masks before they felt sick?
Also, let's be realistic about how easy it is to be less than 6 feet away from others when we're at the grocery store or taking public transportation. Would it not be worth having an additional layer of protection? While brief encounters with others involving minimal interaction may not be high risk, why have we emphasized hand-washing and disinfecting surfaces after those interactions so much more than face masks when this virus is spread largely through respiratory droplets?
The scientific evidence is mixed regarding the effectiveness of masks at containing viral spread when worn by healthy people.
Some studies have not found a benefit. However, many were limited because the subjects didn't wear the masks as often as they were supposed to. In contrast, the CDC's own 2017 guidance on how to prevent pandemic flu in the community stated that "the use of face masks by well persons might be beneficial in certain situations (e.g., when persons at high risk for influenza complications cannot avoid crowded settings or parents are caring for ill children at home). Face mask use by well persons also might reduce self-inoculation (e.g., touching the nose with the hand after touching a contaminated surface)."
The same logic should apply to prevent the spread of the coronavirus.
What's more, we can look at the measures shown to be effective to slow community transmission of SARS in 2003 as an example of steps to be taken today.
A large study in Hong Kong found that wearing a face mask regularly in public places, frequent hand-washing and disinfecting one's home were all effective in limiting the spread of that virus in the community.
Another study conducted in Beijing found that always wearing a mask when going out was associated with a 70% reduction in risk of being diagnosed with SARS during the outbreak compared with never wearing a mask.