N-95 face masks are the gold standard, but what protections do surgical and procedure masks offer against particles that could carry virus? University of North Carolina School of Medicine researchers, in collaboration with the EPA, researched the protectiveness of various kinds of consumer-grade and modified masks, with the assumption that the mask wearer was exposed to the virus, like when interacting with an unmasked infected person.
Which face masks are the most effective
Medical face masks and the modifications that are used to make them fit better were tested. Below are their filtration efficiency percentages and photos of each.
- 3M 9210 NIOSH-approved N95 Respirator: 98%
- Surgical mask with ties: 71.4%
- Procedure mask with ear loops: 38.5%
- Procedure mask with ear loops + “loops tied and corners tucked in”: 60.3%
- Procedure mask with ear loops + “Ear Guard”: 61.7%
- Procedure mask with ear loops + “23mm claw hair clip”: 64.8%
- Procedure mask with ear loops + “Fix-the Mask (3 rubber bands)”: 78.2%
- Procedure mask with ear loops + “nylon hosiery sleeve”: 80.2%
As for face masks that the community wears, here are the filtration efficiencies for many popular styles listed from most effective to least:
- 2-later woven nylon mask, ear loops, w/ nose bridge, washed, no insert: 79%
- 2-layer woven nylon mask, ear loops, w/ nose bridge, 1 non-woven insert: 74.4%
- 2-layer woven nylon mask, ear loops, w/ aluminum nose bridge: 56.7%
- Cotton bandana – folded Surgeon General style: 50%
- Cotton bandana – folded “Bandit” style: 49%
- 2-layer woven nylon mask, ear loops, w/o aluminum nose bridge: 44.7%
- Single-layer woven polyester/nylon mask with ties: 39.7%
- Single-layer woven polyester gaiter/neck cover (balaclava bandana): 37.8%
- Non-woven polypropylene mask with fixed ear loops: 28.6%
- Three-layer woven cotton mask with ear loops: 26.5%
How face mask efficiency was determined
Multiple studies have been done looking at the efficiency of face masks to filter out virus aerosols. One blew aerosol particles through different cloths, another had people scream while wearing various face masks and measure droplets that come through.
For this study published in JAMA Internal Medicine, researchers used an approach based on the OSHA Fit Test to determine the fitted filtration efficiency (FFE) of each of the face masks and modifications tested.
The tests were conducted in a custom-built exposure chamber, where the atmosphere reflected typical indoor conditions, with exposure to small particles that are slightly smaller than individual SARS-CoV-2 virions. In an addition, a sampling port was installed in each mask to allow sampling behind the mask. A pair of condensation particle counters were run in single-particle analysis mode to continuously monitor ambient particles in the chamber just outside the face mask and particles in the breathing space behind the face mask at a sampling rate of one second.
FFE measurements were collected during a series of repeated movements of the torso, head, and facial muscles as outlined for approximately three minutes. The FFE corresponds to the concentration of particles behind the mask expressed as a percentage of the particle concentration in the chamber air.
Why FFE is important
It turns out that at some point mask wearing doesn’t provide a benefit and that’s when the FFE is less than 30%.
Research published in Physics of Fluids examines how a three-layer surgical mask alters airflow and affects the inhalation and depositing of virus particles in the upper airways.
Using a computational model that simulated a person wearing a surgical mask with pleats, researchers were able to track how particles travel through the mask, onto the face, into the airway, and eventually, where the aerosols land in the nose, pharynx, or deep lung. They found that the mask does change airflow around the face.
Instead of flowing through the nose and mouth via specific paths, air enters the nose and mouth through the entire mask surface. Once in the space between the mask and the face, the air moves more slowly than if there was no mask. It is this slower speed that aids the inhalation of aerosols into the nose. So even though masks filter out a certain number of particles, there is a chance that the particles that make it through can enter the respiratory tract.
And here is where the type of mask matters. Filtration efficiency can make much of this moot; researchers found that once a mask only filters less than 30% of particles, it might be better to not wear a mask. So, as one would expect, the higher the filtration efficiency the better whenever possible.
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