A standard medical face mask is more effective at preventing the wearer from inhaling aerosols without causing substantial breathing resistance than various cloth, medical, or respirator masks, new research shows.
“Medical face masks with good filtration efficacies can provide even better protective effects than KN95 respirators,” write Christian Sterr, MD, from Philipps University of Marburg in Germany, and his colleagues. “FFP2 respirators, on the other hand, could be useful in high-risk situations but require greater breathing effort and therefore physical stress for users.”
Extensive evidence has shown that face masks are an excellent form of source control, preventing infectious people from spreading the SARS-CoV-2 virus into the environment. But evidence has been less clear about how well masks protect the wearer from inhaling particles containing the virus.
The researchers conducted three experiments to test 32 different face masks. The findings were presented at the 31st European Congress of Clinical Microbiology & Infectious Diseases and published online in PLOS One.
First they tested pressure drop, which “relates to how easily air can pass through the material,” said Chris Cappa, PhD, professor of civil and environmental engineering at the University of California, Davis, who was not involved with the study.
“Higher pressure drops mean that there is greater resistance to the air passing through. A higher pressure drop will typically mean breathing through the material will be slightly more challenging compared to a low pressure drop. There is no relationship between pressure drop and the mask effectiveness,” he told Medscape Medical News.
Pressure drop was lowest with type II medical face masks, the typical three-ply surgical masks designed to stop large particles expelled by the wearer from entering the environment, was highest with respirators, including KN95 and FFP2 masks, and varied with the different cloth masks tested.
Next the researchers compared filtration efficacy, which “refers to how well the material removes particles from the air that passes through the mask material,” Cappa explained. They did this by placing each mask over the opening to an air collector that measured how many particles got through. “A mask that has 100% filtration efficacy will remove all particles from the air that passes through it and 0% means that no particles are removed.”
Cloth masks had the lowest filtration efficacy, at 28%. Certified face masks that met European Standards (EN) had a relatively high efficacy, at 70%; for uncertified face masks, filtration efficacy was 63%. As expected, KN95 and FFP2 masks had the highest filtration efficacy, at 94% and 98%, respectively.
Most of the particles that we exhale will travel right around a face shield.
Finally, the researchers tested as-worn filtration efficacies. They placed each mask on a dummy head with an artificial airway that collected airborne particles. They then pumped a mixture of aerosol particles — ranging in size from 0.3 to 2.0 µm — and particle-free pressurized air into the air-proof acrylic chamber in which the head was placed.
In this experiment, cloth masks and noncertified face masks were least effective, filtering less than 20% of aerosols. Interestingly, the cloth face mask with the highest filtration on its own (84%) had the lowest filtration efficacy (9%), apparently because of its very high pressure drop (breathing resistance). When more effort is required to breathe through a mask, more air can bypass the filtration system.
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,” 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, Sterr and his colleagues write. “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 write. “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 note.
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 writes. “High pressure drop results in higher breathing resistance and therefore supports leakage, especially if combined to a loosely fitting mask.”
If the mask does not fit well, then it will only provide moderate protection for the wearer.
These findings are “in line with what we already knew,” 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, 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 Cappa have disclosed no relevant financial relationships.
PLoS One. 2021;16:e0248099. Full text
31st European Congress of Clinical Microbiology & Infectious Diseases (ECCMID): Abstract 1760.
Tara Haelle is an independent science/health journalist based in Dallas.