A higher level of humidity improves the protective effect of masks because fewer infectious aerosols get into the environment when you exhale.
San Diego (U.S.A.). The interaction between aerosols and droplets with the mask material of the different mask types was hardly researched at the beginning of the Covid-19 pandemic. However, many scientists expressed the thesis that the filter effect of mouth and nose protection decreases when it is damp. A team led by Abhishek Saha from the University of California San Diego has now refuted this assumption.
According to their publication in the specialist magazine Physical Reviews Fluids a wet mask is even more effective because it prevents large droplets from breaking up into many small droplets that could then penetrate the mask. In a previous study, the researchers were able to show that the small secondary droplets in particular reduce the effectiveness of masks because they create floating aerosols.
Moisture increases the protective effect of masks
The physicists’ experiments show that the more water there is in the tissue of a mask, the more water droplets have to move, so that the aerosols produced during the bounce can penetrate the mask. Unfortunately, the moisture also increases the breathing resistance and makes the mask unusable after a while.
The working group examined the impact of droplets one to two millimeters in diameter on three types of masks in order to find out the critical speed for aerosol formation. Although these relatively large particles are rare in the air we breathe, they still make up a large part of the volume of liquid. The formation of split-off secondary droplets from these droplets during exhalation is therefore of great importance.
Both a medical mouth and nose protection that repels water and two cloth masks that attract water were examined. According to the study, all three mask types hold secondary droplets better at higher humidity.
Different material properties of the masks
The better protective effect due to the higher humidity arises with the different materials of the masks for different reasons. With the water-attracting masks, the water soaks up and the pores become smaller. A higher speed is therefore necessary for droplets to penetrate the mask when you exhale.
In the case of the water-repellent mask, on the other hand, fine water pearls accumulate on the fibers, which slow down the impact of the larger droplets. A higher energy is therefore necessary so that these can penetrate the mask.
Physical Reviews Fluids, doi: 10.1103/PhysRevFluids.6.110510