Masks with a leak
The effectiveness of medical masks was evaluated taking into account air seepage at the edges
Victoria Baranovskaya, N+1
A team of American researchers conducted an experiment in which 12 healthy volunteers talked and coughed into a device that analyzes the number of aerosol particles in the exhaled air. Moreover, the participants of the experiment followed the instructions of the researchers, referring to the device with different sides of the face (without a mask and in a mask). In a paper published in the journal Scientific Reports (Cappa et al., Respiratory aerosol particle escape from surgical masks due to imperfect sealing), researchers experimentally showed that air seepage along the edges of the medical mask (especially from above) reduced its overall effectiveness. But even despite the leak, medical masks were still good at containing the spread of aerosol particles.
Wearing respiratory protection is considered a reliable way to stop the spread of coronavirus. One of the commonly used means is the N95 respirator, which protects against contact with 95 percent of microparticles (up to 0.3 microns in size). Such masks fit snugly to the skin, and it is believed that they better protect against infections, unlike medical masks with gaps on the sides, from which aerosol particles can leak, especially when talking, coughing and sneezing. However, a 2019 study involving almost three thousand medical workers showed that surgical masks protect (from the flu) no worse than respirators. The number of cases after wearing respirators was 8.2 percent, and among those who wore medical masks – 7.2 percent.
There are also studies comparing the effectiveness of various protective equipment (fabric and medical masks, respirators) using a particle counter, in which participants in experiments in various masks cough (or just pronounce words). However, not all exhaled air passes directly through the mask, some of it seeps through the edges. And at the moment it is unclear exactly how this leakage of aerosol particles affects the overall effectiveness of medical masks.
Now researchers led by Christopher D. Cappa from the University of California, Davis, have evaluated the effect of aerosol particles seeping along the edges of medical masks on their overall effectiveness.
Scientists invited 12 healthy and non-smoking volunteers (7 men and 5 women) aged 18 to 45 years to participate in the study. The participants were placed in front of a device that analyzed the number of aerosol particles in the exhaled air, and asked to read aloud a text of 330 characters, as well as cough for 30 seconds.
The participants performed these tasks with and without a medical mask, as well as placing their face in front of the device in different ways. There were four different positions: straight, sideways, with lowered and raised head. The device counted particles (with a diameter from 0.3 to 20 microns) passing directly through the mask, as well as seeping along its edges – from below, above and from the side.
(a) The participant in the medical mask is facing the particle counter (illustration); (b) four different positions of the participant relative to the device (photo); (c) recording the voice of one of the participants when he read the text without a mask; (d) counting of aerosol particles by a counter.
It turned out that the average concentrations of particles coming out of the mask along with the air flows (top, bottom and side) were less than in the absence of a mask at all. However, the amount of aerosol particles in these streams was greater than in the air stream passing directly through the mask.
Experiments have shown that the overall effectiveness of the mask, taking into account the leakage of particles from the cracks, decreased (compared to the effectiveness of containment of particles passing through the mask) from 93 to 70 percent – when talking. And when coughing – from 94 to 90 percent. These results demonstrate that medical masks are still good at restraining the spread of particles, despite the gaps, but, of course, because of them, the overall effectiveness of these means of protection is reduced.
The researchers also found that wearing a mask reduces the number of particles directly passing through it by an average of 93 percent, in the exhaled air flow from below – by 91 percent, from the side – by 85 percent, and from above – only by 47 percent. That is, the airflow coming out of the upper part of the mask (and through the nose) carries the largest amount of aerosol particles. The authors of the article write that they were curious how wearing glasses would affect this figure.
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