5 Questions: Stanford scientists on COVID-19 mask guidelines

Scientists say we should wear masks to control the spread of COVID-19. Stanford experts share the evidence that informed the World Health Organization’s recommendations.

A video produced for the World Health Organization explains source control in the context of the COVID-19 pandemic.
Larry Chu and Amy Price

On June 5, the World Health Organization revised its guidelines about when people should wear cloth masks. Previously, the organization had recommended that only those with symptoms of COVID-19, the respiratory disease caused by the novel coronavirus, or those caring for them, wear cloth masks over the nose and mouth. 

On June 18, California Gov. Gavin Newsom issued a statewide order requiring people to wear masks in most public spaces.

The new guidelines recommend that everyone who comes in close contact with others in crowded or close quarters, such as on a bus or in a store, wear a cloth mask composed of at least three different layers of material. People 60 or older, or those with underlying health conditions, should wear medical masks, such as surgical masks, in public, and physicians and health care providers should wear medical masks in all areas of a hospital, even if the area in which they are working has no COVID-19 patients, according to the organization.

The new guidelines were devised after WHO officials reviewed information from researchers at Stanford and elsewhere about the ability of cloth masks to slow the spread of the disease, which has now infected more than 8 million people worldwide and caused more than 400,000 deaths.

The revised recommendations more closely echo those of the U.S. Centers for Disease Control and Prevention, which since early April has recommended cloth masks in public settings where social distancing is difficult to maintain.

Science writer Krista Conger spoke with two Stanford researchers involved in the change to the WHO guidelines: Amy Price, PhD, a senior research scientist at Stanford’s Anesthesia Informatics and Media Laboratory, and Larry Chu, MD, a professor of anesthesia and director of the AIM Laboratory. They recently co-authored an article in Nanotechnology Letters assessing the filtering and breathability of various household fabrics often used to make masks.

 1. How do cloth face coverings prevent the spread of COVID-19? 

Chu: In order to answer this, it’s first important to understand the concept of source control. We’ve learned that as many as 40% of people infected with the virus that causes COVID-19 may have no symptoms. But when they talk, cough or sneeze, they can still spread the virus to others in the form of respiratory droplets expelled into the air. Those droplets evaporate into fine particles that may linger. The mask traps these larger droplets before they can evaporate. So, wearing a mask regularly can prevent spreading at the source even when we don’t know we are sick. But masks are just one important way to prevent this disease from spreading. Washing your hands regularly and thoroughly and keeping at least 6 feet apart from one another are still vitally important.  

Price: Many people argue that cloth masks can’t be effective because they can’t filter out viral particles, which are extremely tiny. But, as Larry explained, most of these particles leave the mouth and nose in much larger droplets that become smaller through evaporation as they move away from the body. Trapping droplets with the mask means not nearly as many viral particles escape. So, when all parties in a gathering are wearing well-constructed, well-fitting masks, it provides an extra layer of safety for everyone. If two people are wearing masks, the viral particles can travel about 5 feet away from each individual. When an infected person is not wearing a mask, those particles can floatthrough the air 30 feet or more and stay alive for up to 30 hours.

2. How do you respond to people who feel that wearing a mask can be harmful?

Price: I’ve heard so many misconceptions about cloth masks. Some people think that if you wear a mask for long periods of time you will trap and breathe in excess amounts of carbon dioxide, which could lead to brain damage. That’s just not true. A properly constructed mask provides more than enough ventilation. In fact, one way to test if your mask is well made is to try to blow out a candle through the mask from about 1 foot away. If you can’t do so, your mask might be too tightly woven. Other people feel that wearing a mask encourages people to touch their face and to loosen their adherence to other safety precautions like social distancing and hand washing. We’ve found the opposite. Wearing a mask reminds people to continue to be cautious. With a mask on, you actually touch your face less. People who experience skin irritation should ensure their mask has a layer of wicking fabric, like cotton, against the face, and everyone should change the mask if it becomes wet or dirty. Finally, it’s been suggested that mask-wearing may increase the concentration of viral particles around an infected person’s mouth and could increase the severity of the illness. While it’s true that some studies of health care workers have suggested that the viral dose is an important determinant of infection, it’s different for someone who is already infected. If you are sick, you already have the virus in your lungs; it’s not going to get any worse. 

3. What’s the best way to make and use a mask? 

Chu: Our studies show that, if constructed properly with high-quality materials, a homemade cloth mask can function as well as or better than a surgical mask. Based on our studies, the WHO now recommends a cloth mask of at least three layers of different materials. The outermost layer should be made of a fabric that is at least somewhat water resistant. That can be a fabric that is a combination of cotton and polyester, nylon or rayon. The middle layer should either be a polypropylene — a spunbond material used in some reusable grocery bags, mattress covers and craft projects — or three-ply disposable facial tissues like Kleenex. Finally, the innermost layer should be a wicking material to draw moisture away from the face. One hundred percent soft cotton works well here. 

Larry Chu narrates a video on the best nonmedical mask materials.
Larry Chu and Amy Price

Fit is also important. It shouldn’t fit too tightly, but it should sit against the skin all the way around from the middle of your nose to under your chin and almost to your ears, and it shouldn’t gape or bulge away when you move your head or speak. 

 Finally, treat your mask like your toothbrush. Don’t share it with anyone, and keep it in a plastic Ziploc bag when not in use. When you remove it, do so in a way that doesn’t spread germs from the front of the mask to your face, and wash your hands after touching the mask.  

 4. What does it mean to charge a mask? Does it work for all materials? How is it accomplished? 

Price: When we began researching cloth masks, we realized there was not a lot of evidence about their effectiveness when used as source control in a community setting. We collaborated with some phenomenal materials scientists here at Stanford, including Nobel laureate Steven Chu, PhD, and Yi Cui, PhD, to conduct a study of the effectiveness of various readily available household fabrics, including cotton, polyester, polypropylene, nylon and silk. We found we could make some materials as effective as a surgical mask by charging them with static electricity by rubbing them with latex gloves for 30 seconds prior to use. A similar approach is used in the construction of medical protective equipment, including surgical masks and N95 respirators. Overall, charging masks can increase their efficiency by as much as 30%, and the effect can last for many hours.

 5. If everyone wore a face mask in public, what would it accomplish?

Chu: The timing of these new recommendations is critically important. Across the country, communities are beginning to end shelter-in-place and to return to work and community settings. Nonmedical face masks will become an increasingly important way, in conjunction with frequent hand washing and social distancing, to prevent the resurgence of disease.  

Price: We know that during the first wave of the pandemic, those countries that implemented masking early were more successful than others at reducing the spread of the virus. Wearing a mask doesn’t mean that you are weak or afraid or a coward. It’s a way to protect the vulnerable around you. It’s our duty to keep each other healthy.



Stanford Medicine integrates research, medical education and health care at its three institutions - Stanford University School of Medicine, Stanford Health Care (formerly Stanford Hospital & Clinics), and Lucile Packard Children's Hospital Stanford. For more information, please visit the Office of Communication & Public Affairs site at http://mednews.stanford.edu.

COVID-19 Updates

Stanford Medicine is closely monitoring the outbreak of novel coronavirus (COVID-19). A dedicated page provides the latest information and developments related to the pandemic.

Leading In Precision Health

Stanford Medicine is leading the biomedical revolution in precision health, defining and developing the next generation of care that is proactive, predictive and precise.