The evidence concerning how effective homemade masks are effective in preventing the spread of the novel coronavirus is "inconclusive," and more research is needed, doctors on an infectious diseases panel within the National Academies of Sciences, Engineering and Medicine told the White House in a letter Wednesday.

The research memo, requested by the White House, comes as the Centers for Disease Control recommends that Americans wear non-surgical face 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. And the limited laboratory testing that has been done shows homemade masks are not nearly as effective as medical-grade masks.

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The committee, which has been providing rapid-response research on coronavirus at the White House's request, was asked to look at whether homemade face coverings can prevent infected people who are asymptomatic or pre-symptomatic from spreading the disease. Dr. Rich Besser, one of the experts who authored the memo, explained there is very little research on how effective homemade face coverings are in preventing the spreading of aerosols — the very small particles transmitted by breathing or talking, which can linger in the air much longer than the larger, heavier droplets produced by coughing or sneezing. It's these small aerosol particles that asymptomatic or pre-symptomatic infected people can spread, he said.

"The question was whether cloth masks would protect in that early period, particularly would they protect against these aerosols, these small particles," Besser, a former acting director for the CDC, told CBS News. "So we were asked to look at the scientific evidence on this. And our conclusion is, really, it's inconclusive. There are no studies looking at homemade fabric masks worn by people going about the course of their typical activities. And so the only thing we could really look at was laboratory data, and there there was some evidence of variable levels of protection against these respiratory droplets, these bigger particles. It depended on what the material was made of. But there was very little if anything to show value for aerosols. Our conclusion is that the extent of protection is going to depend on how masks are made, what they're made of, how people use them."

The handful of laboratory experiments cited by Besser and his co-author, Dr. Baruch Fischhoff found significant penetration of particles through homemade masks. A 2010 study by Rengasamy cited in their research memo 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," Besser and Fischhoff's memo said.

Another experiment by doctors in the UK 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 study concluded.

Overall, the doctors wrote in their memo that "the available evidence is inconclusive about the degree to which homemade fabric masks may suppress spread of infection from the wearer to others." Moreover, the research memo says it's unclear whether face coverings will remind people to implement other safeguards, including social distancing, or make people more complacent and comfortable interacting, which would be a negative effect.

Just last week, the CDC issued guidance recommending Americans wear masks, including homemade masks, in public. Surgeon General Jerome Adams filmed a video of himself showing Americans how they can make their own homemade face coverings.

"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 memo to the White House says.

"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."

Besser warned that any use of face coverings is meant to supplement, not replace, any of the social distancing and other mitigation measures.

"It's very important that any recommendation about masks doesn't undercut the primary recommendation around social distancing, hand washing, trying to limit touching your face, and the importance of those measures," Besser said.