People across the United States are donning homemade masks in an effort to curb transmission of the coronavirus. But there isn’t enough data to know for sure whether such cloth masks will prevent an infected person from spreading the virus to someone else, experts say.

In the face of evidence that the coronavirus may spread by talking and breathing, on top of coughing or sneezing, the U.S. Centers for Disease Control and Prevention recommended April 3 that people cover their faces with cloth or fabric when going out in public (SN: 4/2/20).

Cloth may cut down on some large respiratory droplets from a cough or sneeze, but it’s unclear whether it will also catch smaller droplets called aerosols that are released by just breathing or talking.

Cloth masks, as well as surgical masks, are designed to protect others from virus spread by the mask wearer, not the other way around. Those infected with the virus that causes COVID-19 can transmit it to others before they begin showing symptoms (SN: 3/13/20). When the masks are worn as a general habit, they aim to prevent people who are unaware that they are sick from unwittingly transmitting the virus to others. Wearing a mask is not meant to be a replacement for social distancing, handwashing and other efforts.

But there are few studies evaluating the effectiveness of fabric masks at preventing respiratory diseases from spreading, researchers from the U.S. National Academies of Sciences, Engineering and Medicine wrote in an April 8 letter to the White House Office of Science and Technology Policy.

Those that do exist suggest that fabric masks may capture large respiratory droplets, like those from a cough or a sneeze. Those made of different types of cloth have a wide-ranging ability to filter virus-sized particles, with a trade-off between filtration and ability to breathe.

In one study, a mask that used 16 layers of handkerchief fabric was able to filter out 63 percent of 300-nanometer-sized particles. (The coronavirus is between 50 to 200 nanometers in diameter.) But that mask was harder to breathe with compared with thick, tight-fitting N95 respirators, often used in hospitals, that can block minuscule particles. Wearing a cloth mask with that many layers would be uncomfortable and may “cause some to pass out,” the researchers wrote in the letter.

Surgical masks are somewhat less mysterious. Those masks may help reduce transmission of influenza and seasonal coronaviruses that cause common colds from people with symptoms, researchers report April 3 in Nature Medicine. Researchers quantified the amount of virus exhaled by participants both with and without a surgical mask over 30 minutes.

Those masks significantly reduced the amount of detectable influenza virus in respiratory droplet particles, as well as the amount of seasonal coronaviruses in aerosols.

Regardless of how well they work, the success of cloth or surgical masks at protecting others depends on whether people wear the gear properly — including keeping it in place — and making sure it doesn’t get too wet. Moisture, such as from breathing, can trap virus in a mask and make it a strong source of contamination when the wearer takes it off.

Though the evidence for fabric masks is sparse, health officials should still encourage people to wear face masks, other researchers write in an April 9 analysis in the BMJ. Limited protection could still save lives. “As with parachutes for jumping out of aeroplanes, it is time to act” without waiting for evidence, the authors say.