Face masks are emerging as one of the most powerful weapons to fight the new coronavirus, with growing evidence that facial coverings help prevent transmission—even if an infected wearer is in close contact with others.

Robert Redfield, director of the Centers for Disease Control and Prevention, said he believes the pandemic could be brought under control over the next four to eight weeks if “we could get everybody to wear a mask right now.” His comments, made in mid-July with the Journal of the American Medical Association, followed an editorial he and others wrote there emphasizing “ample evidence” of asymptomatic spread and highlighting new studies showing how masks help reduce transmission.

The research Dr. Redfield cited included a recently published study suggesting that universal use of surgical masks helped reduce rates of confirmed coronavirus infections among health-care workers at the Mass General Brigham health-care system in Massachusetts.

The CDC currently recommends the use of cloth face coverings in public, and several states have made the coverings a requirement for most people in wide-ranging public situations. An analysis published in July in the BMJ, a medical journal, found that face coverings are now recommended or mandated in 160 countries to reduce coronavirus transmission.

Researchers from around the world have found many different kinds of masks can significantly reduce the spread of coronavirus, though new studies have shown that some masks are better than others. Many researchers are also now examining the possibility that masks might offer some personal protection from the virus, despite initial thinking that they mostly protect others.

Experts caution that widespread masking doesn’t eliminate the need to follow other recommendations, like frequent handwashing and social distancing.

In the absence of widespread availability of N95 masks—considered among the most effective but typically reserved for health-care workers—transmission can still be reduced with simple and affordable face coverings, the research shows. A case study by Australian researchers published in July in the journal Thorax found that a three-ply surgical mask made of nonwoven material noticeably reduced droplets dispersed while speaking, coughing and sneezing. The surgical mask proved more effective than two-layer and one-layer cotton facial coverings, the researchers found, noting that efficacy diminished as masks grew thinner.

The study, which analyzed the droplet spread of a healthy volunteer after capturing it on video, hasn’t yet been peer-reviewed. Researchers concluded from their observations that homemade cloth masks likely need several layers—ideally at least three—to prevent the transmission of the virus.

A Shield Against Covid-19

Face shields can significantly reduce the short-term exposure to larger aerosol particles but are less effective against smaller particles, which researchers believe can remain airborne longer and can possibly be inhaled due to gaps between the shield and face. The CDC doesn't recommend face shields for everyday activities or as a substitute for masks, citing a lack of evidence of their effectiveness for reducing Covid-19 spread.

During a simulation, researchers found that wearing a face shield helped reduce exposure to an influenza-laden cough.

The study found that aerosol-size droplets expelled from the mannequin with the double-layered cotton mask traveled forward about 2.5 inches on average, and that most of the leakage escaped from gaps between the nose and face. Loosely fitting facial coverings, including a folded cotton handkerchief with ear loops, as well as a bandanna were less helpful, the study found. With those masks, droplets traveled on average about 1.25 and 3.5 feet, respectively. In contrast, the study found droplets traveled about 8 inches on average with an off-the-shelf cone-shaped mask.

The study found that aerosol-size droplets expelled from the mannequin with the double-layered cotton mask traveled forward about 2.5 inches on average, and that most of the leakage escaped from gaps between the nose and face. Loosely fitting facial coverings, including a folded cotton handkerchief with ear loops, as well as a bandanna were less helpful, the study found. With those masks, droplets traveled on average about 1.25 and 3.5 feet, respectively. In contrast, the study found droplets traveled about 8 inches on average with an off-the-shelf cone-shaped mask.

Researchers are hopeful that more evidence about the personal protection masks could lead to more use in coming weeks. The CDC said the use of cloth face coverings while in public in the U.S. increased to 76.4% in mid-May, compared with 61.9% in April, according to internet surveys sent to roughly 500 adults each month.

Researchers say the benefits of widespread mask use were recently seen in a Missouri hair salon, where two stylists directly served 139 clients in May before testing positive for coronavirus. According to a recent report published by the CDC, both wore either a double-layered cotton or surgical mask, and nearly all clients who were interviewed reported wearing masks the entire time.

After contact tracing and two weeks of follow-up, no Covid-19 symptoms were identified among the 139 clients or their secondary contacts, the report found. Of the 67 who were willing to be tested, all were negative for coronavirus.