With coughing and sneezing both common COVID-19 symptoms, there has been an unprecedented global surge in the sales of medical face masks, leading to shortages for health workers in some parts of the world. But with some countries making mask-wearing mandatory for anyone in public, and others actively discouraging people from wearing them, what should we do?
Analysis of over 75,000 confirmed cases in China found no evidence of airborne transmission of COVID-19. Instead the overwhelming evidence suggests that it is transmitted primarily through respiratory droplets and through contact.
Transmission through respiratory droplets differs from airborne transmission in a number of critical ways. While both are capable of transmitting respiratory infections via droplets, it's the size of the droplets that matters.
With an airborne disease, such as measles, a pathogen can be transmitted through aerosols, “droplet nuclei” that are less than 5 micrometres in size (5 millionths of a metre). These are small enough to stay suspended in the air for several hours and transmit infection through inhalation. This is one reason why measles is so infectious and capable of infecting someone entering a room even hours after someone with measles has left it.
With COVID-19 the virus is spread via much larger respiratory droplets, of between 5-10 micrometres in size. These are too large to remain in the air for long and so instead fall onto nearby surfaces. Anyone close enough can be infected if the droplets land on or around their mouth, nose and eyes, or by touching a surface on which droplets have landed on and then touching their face.
This is why governments are advising people to stay out of reach, at least two metres apart, to avoid droplets landing on them when someone sneezes or coughs, and to wash their hands regularly, to avoid becoming infected through contact with contaminated surfaces.
In light of this, the World Health Organization (WHO) recommends that individuals caring for others sick with COVID-19 wear masks to protect themselves, as they will need to come closer than two metres with patients and so be within the range of droplets. WHO also recommends that someone infected with COVID-19 wear a mask because it can act as a barrier when they sneeze or cough, reducing the spread of droplets.
Although these are the situations where wearing a mask is most beneficial for reducing the chances of someone infected spreading COVID-19, it may also be useful for other healthy individuals. For example, people may find it difficult to stay consistently at least two metres apart, such as workers who need to take public transport to work or people living in crowded slums or refugee camps. Such individuals may be at lower risk from COVID-19 than people who need to come into close contact with people who are known to be sick with COVID-19 as part of their job, but a mask could still provide valuable protection, especially since there is evidence that asymptomatic individuals can spread the COVID-19 virus. Similarly, it may prevent people from infecting themselves, even if they keep their distance, by unwittingly touching a contaminated surface and then unconsciously touching their face.
At the same time, mask-wearing does not replace other measures to reduce the spread of COVID-19. Handwashing, physical distancing when possible and individuals staying home when they are ill all remain very important. Wearing a mask should not provide a false sense of security, particularly as it works best in combination with other measures.
Governmental advice on the use of masks during this pandemic varies dramatically, given different circumstances and different assessments of governments of the risks and benefits of healthy people wearing masks if they are not in close contact with known COVID-19 cases. Some governments have gone beyond WHO’s recommendations to make it mandatory to wear a mask in public, giving police the power to issue on-the-spot fines to those who don’t. Nevertheless, individuals must be careful to observe all precautions against the spread of COVID-19 in force in their areas, whether or not that includes wearing a mask when healthy.