Wearing a mask is critical to prevent COVID-19 — Some findings by aerosol scientists

Author: Jorge Saturno

Thanks to COVID-19 we have learned that many respiratory diseases actually spread through the air, via aerosol transmission. The tiny particles we expel when talking, sneezing and coughing, remain suspended in air for minutes/hours and infect other people. This is especially dangerous in indoor spaces. That's why wearing face protection is one of the most effective ways to avoid getting infected. However, not all masks will protect you in the same way. There are significant differences in the efficiency of the masks available in the market. Choosing the right one and wearing it properly will keep you and others protected.

Aerosol or droplet?

Historically, it has been believed that respiratory diseases spread via droplets and fomites (large droplets landing on surfaces and contaminating them). However, it has been found that the main transmission path for COVID-19 is airborne, i.e., aerosol particles which remain suspended in air and can be inhaled. In the early stages of the pandemic, the belief that droplets landing on surfaces were the main transmission pathway made the WHO and national health authorities to establish cleaning and disinfection guidelines that did not consider airborne transmission. After some research based on super-spreading events was made available, it was understood that aerosol transmission was playing the main role. The most important lesson for scientists was that the threshold between aerosol and droplet, that was previously defined at 5 µm diameter, should actually be located at about 100 µm in diameter. A 100 µm particle is the largest particle that can remain suspended for 5 seconds 2 meters away from the emitter, and can be inhaled by other people. Redefining the difference between aerosols and droplets was crucial to understand how COVID-19 spreads.

Figure 1. A lab generated virus-laden respiratory particle. Adapted from Vejerano and Marr, 2008.

Different masks will offer different protection

There are different masks in the market and also some masks people can manufacture themselves at home using fabric or clothing. The most common types of masks available are the following:

- Non-woven and cotton fabric masks: Typically self-made. These kind of masks are done with one or more fabric layers. Since they are so different from each other, there is high variability in their filtration efficiency.

- Surgical masks

- N95, KN95, FFP2 respirators

Recently, Brazilian aerosol scientists studied more than 200 sample masks and measured the filtration efficiency for particle with diameters between 60 and 300 nm, which are the ones that have longer atmospheric lifetime and could keep the virus in the air for hours. The scientists designed a laboratory setup where the mask was placed in a chamber and exposed to sodium chloride aerosol particles. This salt was used as a proxy because it can easily be aerosolized and particle size can be measured in an accurate way. In principle, any particle floating in air should behave as the particles used in the experiment, regardless if they host a virus or not. A differential pressure measurement was used as a proxy of breathing resistance. Scanning electron microscopy was used to investigate the structure of the different materials.

Figure 2. Filtration efficiency for different kinds of face masks. Data from Morais et al. 2021.

The authors found that sewed masks are less efficient to filter the air flow compared to non-woven fabric masks. The best filtration was found for surgical (80-89 % filtration efficiency) and N95 and FFP2 masks (90-98 % filtration efficiency). The lead author, Fernando Morais, mentioned that masks made with some fabric containing metal fibers, were more efficient because the electric charge of the material could capture some more particles. Another important finding was that fabric masks were more efficient when two layers of fabric were used to produce the masks, but a third layer did not add a significant extra protection.

The fit is important

Only wearing an efficient mask is not enough, it has to properly fit your face. Even though there are official WHO guidelines for wearing a mask, these are based on visual checks that do not offer any guarantee that the mask is properly fit. Given the critical role of the fit regarding filtration efficiency, it is important to always check if the mask is properly protecting you. One common issue is that brand new FFP2 or KN95 masks are V-shaped in the nose fit. When wearing a mask for the first time it is important to manually turn the V into an U shape so that it properly fits on the nose.

Figure 3. Correct way to fit KN95 mask to nose. Image by Frank Constantini (@frankcos)

Alternative methods to check mask fit have been recommended by researchers. For example, an aroma diffuser could be used at home to detect leaks by nebulizing a sodium saccharin solution that should taste sweet when smelled if there are leaks in the mask fit. Although, this method is not validated, it could be more efficient than a visual inspection only.

At the end, the user should make sure the mask is properly filtering inhaled and exhaled air. Face masks will be around for quite a long time and we need to learn how get the best out of them. As recommended by Prof. Linsey Marr’s 2 out of 3 rule, the only scenario where not wearing a mask during the pandemic is acceptable is outdoor with at least 2 m distance from other people.