The CDC now recommends wearing a mask in some cases: A doctor explains why and when to wear one

By Dr. Thomas Perls, Boston University

The Centers for Disease Control and Prevention has changed its policy and is now advising everyone, whether or not they have symptoms of COVID-19, to cover their face with a mask or cloth covering whenever social distancing is difficult to maintain. To be clear, the CDC is not saying you should wear a mask wherever you go, but rather in places where people congregate, including grocery stores and public transportation and ride-shares.

The shift in recommendations reflects growing evidence that COVID-19 can be transmitted by a person’s exhalations and normal speech but also the fact that people are not effectively covering their sneezes and coughs.

The stealth virus

COVID-19’s middle name should be “stealth.” People can be shedding virus for one to three days before showing any symptoms – including no coughing, sneezing or fever – in what’s called “presymptomatic transmission.” A Singapore study suggests that 10% of infections are attributable to presymptomatic spread.

A study of the 3,711 passengers and crew on the Diamond Princess cruise ship indicates that close to 1 in 5 COVID-19 carriers never develop symptoms. Some of these people transmit the virus through “asymptomatic transmission.” The proportion of infected people that never develop symptoms could be more like one-third for the general population that is younger and healthier than typical cruise takers.

The virus’s ability to spread so easily from one person to the next is why people are being asked to physically distance themselves from one another. But people still have to go out to get essentials at places where people gather.

If a person is not coughing or sneezing, how are they spreading the virus? One way is by contact.

The virus lives on the mucous membranes in the throat and nose. With people touching their faces every two-and-a-half minutes, on average it’s easy to see how the virus gets on our hands, and then we can spread it to commonly used surfaces like doorknobs, a plastic handle in the subway or someone else’s hand. Steel and plastic surfaces can harbor live virus for three days.

The other manner of spread is by asymptomatic infected people simply breathing, talking, yelling or singing. These activities aerosolize virus, creating airborne virus particles – also called droplet nuclei – that are so tiny they can float around in the air for three hours. Coughing and sneezing produce larger water and virus-borne droplets, as well as producing aerosolized virus.

Common medical devices, like nebulizer machines for people with asthma and CPAP machines for those with sleep apnea, can aerosolize virus. But the concentration of aerosolized virus will be small in a large well-ventilated space and practically absent outdoors. The infectious aerosolized virus becomes more of a concern in a place like a small, poorly ventilated room. Places like a patient’s bedroom in their home, some nursing home rooms and a classroom would all be concerning to me as a physician. Hospital rooms are generally better ventilated.

Another key determinant of getting infected is the amount of time you are exposed – so your risk is much less with five minutes versus 30 or more minutes of exposure. People think about the danger of radiation exposure in very much the same way – how close you are to the source, the concentration of exposure and the amount of time you are exposed.