Wearing two face covers can nearly double the effectiveness of filtering SARS-CoV-2-sized particles, preventing them from reaching the wearer’s nose and mouth and causing COVID-19.
The reason for the improved filtration is not so much the addition of layers of fabric, but the removal of any gaps or areas that do not fit the mask well, according to a new study published in JAMA Internal Medicine .
Adjusted filtration
To test the tight filtration efficiency of a wide range of masks, the researchers filled a stainless steel exposure chamber with small aerosols of salt particles, and had the researchers don mask combinations to test how effective they were at maintaining particles out of your breathing space.
Each individual mask or combination of layered masks was fitted with a metal sample port, which was connected to a tube in the exposure chamber that measured the concentration of particles entering the respiratory space under the investigator’s mask .
A second tube measured the ambient concentration of particles in the chamber. By measuring the concentration of particles in the breathing space under the mask compared to that in the chamber, the researchers determined the efficiency of the filtration.
They also had the researchers in the chamber undergo a series of range-of-motion activities to simulate the typical movements a person can do throughout the day: bending at the waist, talking, and looking from left to right, from above. down, etc.
Based on their findings, the base-set filtration efficiency of a mask differs from person to person, due to the unique fit of each person’s face and mask. But in general, a mask without altering the fit is about 40-60% effective at keeping COVID-19-sized particles out. A cloth mask is approximately 40% effective . Face mask duplication shows that when a cloth mask is placed over a surgical mask, filtration improved by approximately 20% and further improved with a tight-fitting mask. When layered over masks, fabric masks improve fit by eliminating gaps and keeping the mask closer to the face, constantly covering the nose and mouth, improving 16%.