The Centers for Disease Control and Prevention (CDC) said in an updated guide in early 2022 that loose cloth masks played the least role in preventing the spread of covid-19, while N95 and kn95 masks played the most protective role. Nevertheless, more than two years after the beginning of the pandemic, people still do not fully understand the characteristics of masks in order to obtain the best protection. [fluid physics] published by AIP press( https://aip.scitation.org/doi/10.1063/5.0086320 ) 》Using principal component analysis (PCA) and hydrodynamic simulation models, researchers at Florida State University and Johns Hopkins University showed the critical importance of the correct fit of all types of masks and how face shape affects the optimal fit**
The study shows that wearing two layers of inappropriate masks may not significantly improve the efficiency of masks, but also produce a "false sense of security".
More layers mean that the mask is less porous, resulting in more air flow forced out of the peripheral gaps (side, top and bottom) of the mask, while the mask is less safe. The double-layer mask can improve the filtration efficiency only under the good fit of the mask, but it may also lead to dyspnea.
The researchers simulated moderate coughing airflow from the mouth of an adult male wearing a cloth mask and elastic bands around his ears. They calculated the maximum volumetric velocity through the gap at the front and periphery of the mask at different material porosity levels.
To obtain a more realistic three-dimensional face shape and size, the researchers used PCA, which integrates the head data of 100 adult men and 100 adult women retrieved from the head scan data of the University of Basel, Switzerland. PCA condenses a large number of variable sets and retains most of the information.
Their model shows how the typical slight asymmetry in all facial structures affects the correct mask fit. For example, the mask may fit better on the left side of the face than on the right side.
"Facial asymmetry is almost imperceptible to the eyes, but the cough airflow through the mask is obvious," said co-author Tomas Solano of Florida State University. "For this particular case, the only unfiltered leakage observed is through the top. However, for different face types, leakage through the bottom and side of the mask is also possible."
Creating a "designer mask" customized for everyone's face is impractical in scale. Nevertheless, PCA based simulations can still be used to design better masks for different populations, revealing general differences in facial structures between men and women or children and the elderly and the associated airflow through masks.