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Face Masks: Do They Work? Not very well. They do make you more cautious. (A brief explainer)


photo by Juraj Varga courtesy of

A new study published on May 4 by the Royal Society DELVE Initiative states that face masks will be beneficial to prevent transmission of SARS-COV-2 (the virus responsible for COVID-19) from asymptomatic or presymptomatic people to others in the public.  Masks may not be so effective to prevent the wearer from becoming infected.

 Evidence supporting their potential effectiveness comes from analysis of: (1) the incidence of asymptomatic and pre-symptomatic transmission; (2) the role of respiratory droplets in transmission, which can travel as far as 1-2 meters; and (3) studies of the use of homemade and surgical masks to reduce droplet spread.

Note that the study did not support the conventional view that masks would prevent the wearer from becoming infected.  Preventing others from catching the virus is viewed as the major benefit.  Respiratory droplets are generated during normal breathing and talking as well as coughing and sneezing (in which asymptomatic people presumably don’t engage).

Droplets that you breathe out range in size from 10 microns on down, while those you inhale are generally 10-100 microns (1/1000’s of a millimeter) in size.  For comparison, a SARS-COV-2 virus is roughly 120 nanometers in diameter (1 micron equals 1000 nanometers).  Droplets generated when you cough are bigger and “snottier”– they contain more mucus proteins.

A “droplet” is considered to be more than 5 microns in diameter; particles smaller than this are considered “aerosols”.  The important difference is that aerosols tend to hang in the air longer than droplets– they are suspended and don’t settle down to surfaces for hours.

Surgical masks were originally intended to prevent surgeons from contaminating their patients’ open wounds during surgery.  As such, they are highly effective.  They prevent droplets containing bacteria (which are much larger than viruses) from falling into wounds and have reduced the bacterial infection rate in most surgery to negligible levels.  Surgical masks do not prevent aerosols emitted during normal breathing from entering the air– a point generally ignored by hospitals and their leaders, who tend to equate the more effective N95 respirators with unnecessary expense and bother.

Surgical masks do reduce the emission of viruses in exhaled breath by 3-4 fold, according to the first-mentioned study.  This is a significant improvement over the open air.  N95 respirators are more effective: first, they seal around the sides (masks have no seal, respirators do– while ventilators breathe for you).  Second, they filter 95% of all particles over 2.5 microns out of the air.  There are also N99 and N100 respirators, which filter larger percentages of particles.

One caveat: the “N” (as opposed to “P”– “partial”– or “R”– “resistant”) means that the filters are not resistant to oily particles, which are slippier.  Remember that the virus itself has a lipid envelope and “lipid” means oily.

Cloth masks are even less effective than surgical masks at filtering the air we breathe.  However, the first-mentioned study indicates that there is some protection, especially against the larger droplets associated with coughing.  Other studies support mask protections.  Some studies (to which I can’t find references right now) say that coughing and sneezing causes large particles to go right through a mask.

The largest effect of wearing masks may be simply psychological: they remind us to be careful of the air we exhale and where it lands.  This is not a trivial matter.  People not wearing masks may be less cautious and they certainly don’t feel that they are in any danger– otherwise, if you are at risk, why aren’t you at least wearing a mask?

The bottom line: Do wear a mask, to show that you care and you are trying to help.  But don’t expect the mask to protect you.

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