Type II medical face masks, however, filtered 47% of aerosols, KN95 masks filtered 41%, and FFP2 masks filtered 65%. Face shields did not prevent the inhalation of any aerosols.
“We know that face shields will only be effective in stopping very large droplets, essentially visible spittle,” Dr. Cappa explained. “Most of the particles that we exhale will travel right around a face shield.”
The “optimal mask effect is a combination of high filter performance and low filter resistance,” which applies to most of the FFP2 and medical type II face masks tested, Dr. Sterr and colleagues wrote. “The type II medical masks in our random sample showed very good as-worn filtration performances with a low additional work of breathing at the same time.”
Although this study showed how well different masks filtered out particles, it could not assess how well different masks prevent actual infection.
“Like any virus, SARS-CoV-2 can only infect people as long as it is viable,” the researchers wrote. “Moreover, a certain number of viable virus particles need to be inhaled to trigger an infection. Thus, the assessed filtration efficacy may differ from the provided protection rate against SARS-CoV-2.”
In addition, particles containing the virus could dry out while going through the mask and become less infectious. “Even a small reduction in inhaled particles might prevent infection or at least lead to a less severe infection,” they noted.
In fact, filtration efficacy does not necessarily indicate how well the mask filters out particles while being worn. “This might be due to the combined effects of mask fit and pressure drop of the mask material and therefore tendency for mask leakage,” the team wrote. “High pressure drop results in higher breathing resistance and therefore supports leakage, especially if combined to a loosely fitting mask.”
These findings are “in line with what we already knew,” Dr. Cappa explained. “Even if the mask material filters out nearly all particles that pass through it, as is the case for high-efficiency masks such as N95 and FFP2, if the mask does not fit well, then it will only provide moderate protection for the wearer.”
Although the findings reaffirm the different levels of filtration provided by various cloth masks, they do not “provide any guidance on which types of cloth masks are better or worse,” he said. But they do show that “medical face masks will generally provide more protection to the wearer.”
It’s not surprising that face shields offer little protection from aerosols, Dr. Cappa said, but they can provide added protection when worn with a mask.
“A face shield could prevent large droplets that might shoot out when a person coughs or sneezes from depositing on a person’s eye,” he pointed out. And it can help “redirect the plume of particles that an infected person exhales, which could be useful in close quarters. However, even then those particles will keep moving around and could be inhaled. A mask can really help to decrease the amount inhaled.”
The study did not use external funding. The authors and Dr. Cappa disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.