Three years after the pandemic began, there's still no scientific consensus on how much protection masks offer.
Laboratory studies are clear that masks – especially N95s and KN95s – decrease the number of COVID-19 particles that enter the body, and the less virus that is inhaled, the lower the risk of infection. But research has led to mixed findings on how much masks reduce the spread of COVID-19 in communities, in part because levels of mask-wearing have varied over time and not everyone wears the masks correctly.
Some studies, including some conducted earlier in the pandemic when mask-wearing was more common, concluded masks significantly reduce rates of COVID-19. Others found they only have a small effect, and still others were unable to reach firm conclusions.
A review of studies published in the June Annals of Internal Medicine concluded that “masks may be associated with a small reduction in risk for SARS-CoV-2 infection in community settings.”
Another review of previous studies, some completed before the pandemic, looked at community spread of different types of respiratory viruses, including COVID-19, and found “there is uncertainty about the effects of face masks.”
Some interpreted that review, published in January in the Cochrane Database of Systemic Reviews, as concluding masks don't work. But the editor-in-chief of the Cochrane Library said that is "an inaccurate and misleading interpretation."
A significant caveat in the Cochrane review illustrates the difficulty in measuring the impact of wearing masks in the real world: “Relatively low numbers of people followed the guidance about wearing masks.”
In addition, the CDC warns, masks must fit properly to prevent leaks.
The CDC no longer has a blanket recommendation for mask-wearing, although, the agency says, "Masking is a critical public health tool for preventing spread of COVID-19."
Current CDC guidelines recommend that everyone wear masks in all indoor public spaces only when hospital admission levels are deemed high, and high-risk people do so when hospitalizations are at a medium level. Despite the recent rise in hospitalizations, current levels are considered low on Long Island and in more than 98% of U.S. counties, CDC data as of Aug. 5 shows.
Ali Khosronejad, an associate professor of civil engineering at Stony Brook University, said masks do much more to prevent someone with COVID-19 from spreading the virus than protect someone from contracting it.
“If someone has COVID, wearing a mask is very effective” at stopping that person from passing it on to others, he said.
However, even the best N95 masks don’t prevent breathing in some saliva of a nearby person with COVID-19, although they reduce the number of particles that get through, he said. (The amount of virus that a person is exposed to determines whether the individual is infected, studies show.)
Khosronejad said a study he recently led that likely will be published in the coming weeks confirmed his previous research that shows masks significantly reduce how much virus a person with COVID-19 emits. Other researchers have reached similar conclusions.
Tightfitting N95 masks are most effective, followed by KN95s, and then surgical masks and cloth masks, he said.
Even cloth masks can curb the spread of saliva to less than 2 feet, by diminishing the momentum of the "jets" of saliva as someone exhales, Khosronejad said.
“These jets have kinetic energy that can carry the particles far away, and if you have a mask, it reduces the energy of those jets significantly,” he said.
With masks more effective at blocking the spread than the inhalation of the virus, if you're the only person in a crowded place wearing a mask — or not wearing one — you have a much greater chance of getting infected than if most people were donning masks, Khosronejad said.