The case for universal face-covering
Universal face covering is the most effective mitigation to control the coronavirus infection. Unfortunately, its significance has been either exaggerated or dismissed by many. Here is a brief explanation of why it works. There are mountains of evidence from either lab studies and empirical data analysis to support universal face covering. One can easily find references to support the assumptions in the following explanation.
Key point: face covering is primarily for protecting others. The protection for the wearer is limited unless it is properly worn N95 masks.
How face-covering works
Most coronavirus transmission is through droplets, especially microdroplets that can float in the air for hours. Surface transmission is minimal.
Symptomatic patients can be easily isolated to prevent the spread of their infectious droplets. However, asymptomatic and pre-symptomatic patients are difficult to detect and isolate. Universal face-covering significantly reduce the spread of these transmitters' virus-containing droplets.
Good face-covering, depending on its type, can reduce droplets by over 90%. It requires the virus load to exceed a threshold which varies from person to person to infect someone. Face-covering reduces the number of virus-containing micro-droplets in the air, hence decreases the virus load inhaled by people.
Therefore face-covering is primarily for protecting others. The protection for the wearer is limited unless it is properly worn N95 masks.
Why face-covering is effective
The goal of coronavirus mitigations is not to eliminate the infection instantly but to bring the effective reproduction number Re to below 1 so that the infection will gradually diminish. Face-covering is not bullet-proof. Even if only 80% of people use face-covers that reduce droplets by only 80%, it may well be sufficient o bring Re to below 1 to make the infection decrease steadily.
Where face-covering is most needed
Almost all coronavirus transmissions are through indoor activities. Face-covering is most effective in confined spaces such as subways, churches, hospitals, stores... Since a small portion of virus-containing micro-droplets from an asymptomatic person still enters the circulating air, it is important for buildings to have good HVAC air filters that can trap droplets, or even better, virucidal UVC filters that deactivate coronavirus.
Possible adverse side-effects of face covering
When considering the side effects. please always keep in mind that people of some countries (e.g. Japan) wear masks to prevent seasonal flu every year, some professions (e.g. surgeons, some construction workers, paint shop workers) wear masks for an extended period of time every day, and they do not have any health problems as a result of masking. For fighting coronavirus, the vast majority need to cover their faces no more than 5% of the time (when they are outside their homes and are unable to distance from others). However, there is a potential risk of having germs growing on a face cover if it is used repeatedly without disinfection (e.g. washing, UVC processing).