Do masks protect?

Airborne transmission is the main pathway for spreading infection in respiratory viruses such as SARS-CoV-2. Masks reduce contamination, both for infected people and those who risk infection. While type N95 or FFP2 masks only allow 5% of particles through, with surgical or similar masks the figure is between 30% and 70%. It is therefore understandable that overall mask effectiveness is still a subject of debate. In addition, clinical and observational studies have been contradictory.

In order to evaluate the effectiveness of masks, Chinese and German researchers (at the Academy of Environmental Sciences in Shanghai and the Max Planck Institute for Chemistry in Mainz) developed a quantitative model of airborne virus exposure. What were the results? The probability of infection seems to be largely dependent on the quantity of the virus to which one is exposed, and therefore on the environment. While an individual can exhale up to 3 million diverse particles by breathing for 30 minutes (especially if he is also coughing), the number of viruses emitted when infection occurs is inferior to a hundred.

This is why, in an environment that is enclosed and with high virus concentration (medical centres treating COVID-19), surgical masks only reduce the probability of infection slightly. N95 or FFP2 type masks do a better job, but do not give full protection. The likelihood of infection is between 0,1 and 10% and this variability reflects the enormous differences in viral loads observed in the respiratory tracts of patients.

But in an environment where quantities of the virus are limited, the mask substantially reduces risks if all other necessary conditions are met: if masks are worn correctly, and if there is social distancing and adequate ventilation.

In both these cases, the airborne virus is carried by aerosol particles (small solid or liquid particles) or larger droplets. Analyses show that it is the smaller aerosol particles that carry the most risk since they remain suspended in the air for longer, penetrate more effectively and reach the lungs of infected individuals who inhale them more easily. The larger droplets carry more viruses but fall to the ground quicker, due to gravity, and are more efficiently blocked by masks, limiting the danger to close contacts. In a situation where not everyone is wearing a mask, it is preferable that infected individuals do wear one, since particles that have just been exhaled are in general bigger, and therefore more easily blocked.