From the Journals

Standard medical mask can protect wearer from aerosols


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,” Christian Sterr, MD, from Philipps University of Marburg (Germany), and colleagues wrote. “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 said in an interview.

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,” Dr. 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 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.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 mcm – 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.


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