The extent to which homemade masks are effective in preventing the spread of the novel coronavirus is unclear, doctors on an infectious diseases panel within the National Academies of Sciences, Engineering and Medicine told the White House in a letter dated Wednesday. 

The letter comes as the Centers for Disease Control recommends that Americans wear masks, including homemade masks, out in public, a measure meant more to prevent unknowingly infected individuals from spreading the virus than to keep healthy people who wear masks from becoming infected. There is little research on the effectiveness of homemade masks, doctors on the Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats told the White House Office of Science and Technology Policy in a paper requested by the White House. But what research there is shows homemade masks can still allow for a significant amount of transmission of virus particles. 

“Overall, the available evidence is inconclusive about the degree to which homemade fabric masks may suppress spread of infection from the wearer to others,” the doctors wrote in their paper, urging that more research on the subject is needed since the CDC has recommended that Americans wear masks in public. The National Academies of Sciences, Engineering and Medicine has been doing rapid-response research on various topics related to the deadly COVID-19 for the White House. 

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The doctors who wrote the eight-page letter, Dr. Richard Besser and Dr. Baruch Fischhoff, noted that how well the fabric fits on an individual’s face plays a role in the effectiveness of such homemade masks, as do the size of the particles transmitted. The paper also notes that this strain of coronavirus can be spreadable not just by invisible droplets, such as those in sneezes, but also by smaller bioaerosol particles, from an infected person’s normal breathing process.

A 2010 study by Rengasamy cited in the letter found the filtration penetration rate of five common household materials used as masks — sweatshirts, T-shirts, towels, scarves and cloth masks — ranged from 40 to 90%. That means 40 to 90% of the particles still passed through the makeshift mask. 

“The authors concluded that common fabric materials may provide a low level of protection against neoparticles, including the size ranges of virus-containing particles in exhaled breath,” the doctors on the panel told the White House office. 

Another 2013 experiment by doctors in the United Kingdom cited in the paper tested the effectiveness of three types of masks in reducing emissions from a simulation dummy that produced “exhalations.” That experiment found that cloth masks reduced emitted particles, or “leakage,” by just one-fifth, compared to surgical masks, which reduced emitted particles by half, and N95-equivalent masks, which reduced emitted particles by roughly two-thirds. 

“Our findings suggest that a homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection,” that 2013 study concluded

Additionally, the White House-requested letter says it’s unclear whether face coverings will remind people to implement other safeguards, such as social distancing, or make people more comfortable interacting, which would be a negative thing. 

“The evidence from these laboratory filtration studies suggests that such fabric masks may reduce the transmission of larger respiratory droplets,” the paper concludes. “There is little evidence regarding transmission of small aerosolized particulates of the size potentially exhaled by asymptomatic or presymptomatic individuals with COVID-19. The extent of any protection will depend on how the masks are made and used.”

Last week, the CDC issued guidance recommending Americans wear masks, including homemade masks out in public, still urging people to leave medical-grade masks for health care workers on the front lines. And Surgeon General Jerome Adams filmed a video of him showing Americans how they can make their own homemade scarves. 

“It is critical to emphasize that maintaining 6-feet social distancing remains important to slowing the spread of the virus,” the CDC says on its website. “CDC is additionally advising the use of simple cloth face coverings to slow the spread of the virus and help people who may have the virus and do not know it from transmitting it to others. Cloth face coverings fashioned from household items or made at home from common materials at low cost can be used as an additional, voluntary public health measure.”

Additional research is needed, the doctors who wrote the paper say. 

“Given the current Centers for Disease Control and Prevention (CDC) recommendation to wear cloth face coverings in public settings in areas of significant community-based transmission, additional research should examine the ability of the general public to produce properly fitted fabric masks when following communications and instructions.”

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