“Given that much transmission of [Covid-19] is suspected to occur before someone who has the virus actually shows any symptoms, encouraging everyone to wear masks in public regardless of whether they are symptomatic could be a useful way to break the chain of transmission.
There is no doubt that masks form a barrier to transmission, by stopping droplets passing from infected to uninfected people close by. The WHO recommends standard surgical masks as part of the personal protective equipment to be worn by doctors and nurses who are caring for covid-19 patients in clinics and hospitals. The same goes for anyone caring for a patient at home. In most instances, this is good enough, according to a meta-analysis of four randomised controlled trials that was published this month in Influenza by Mark Loeb of McMaster University, in Canada. Indeed, Dr Loeb found no significant differences between surgical masks and their more sophisticated cousins, n95 respirators, when it came to protecting health-care workers from viral infections transmitted by droplets—including those caused by coronaviruses.
A study published in the current edition of Nature Medicine backs this idea up. Ben Cowling, an epidemiologist at Hong Kong University, measured the amount of virus shed, in half an hour of breaths and coughs, by participants infected with a variety of respiratory viruses, including influenza, rhinovirus and coronaviruses (though not sars-cov-2). In the case of those with coronaviruses, 30% of droplets and 40% of aerosol particles exhaled by participants without a surgical face-mask on contained virus particles. When they wore masks, that dropped to zero.
An experiment carried out in 2013 by Public Health England, found that a commercially made surgical mask filtered 90% of virus particles from the air coughed out by participants, a vacuum cleaner bag filtered out 86%, a tea towel blocked 72% and a cotton t-shirt 51%—though fitting any DIY mask properly and ensuring a good seal around the mouth and nose is crucial.”