What Is Mask Filtration Efficiency?

Every day, N95, KN95, KF94, and surgical face masks protect millions of people from smoke, dust, and other particulates, as well as germs that can cause serious illness. These masks act like a bouncer, keeping harmful substances from entering your mouth and nose via the air you breathe or by catching a ride inside tiny liquid droplets, like those found in a sneeze. And if you’re sick, face masks can also keep the people around you safe by restricting the germs you exhale from entering the open air. Mask filtration efficiency tells us exactly how protective each mask will be.

The “95” in N95 masks, which are the gold standard of COVID-19 face masks, stands for 95 percent filtration efficiency. And the “94” in KF94 masks stands for 94 percent filtration efficiency. But some tests have shown that these types of high-quality masks, when worn correctly, can block up to 98 or 99 percent of microscopic particles. Surgical masks, by comparison, are much less protective and can have filtration efficiencies as low as 50 percent. In many cases, regulatory agencies like the National Institute for Occupational Safety and Health (NIOSH) certify that the face masks you buy are actually working the way they are supposed to by preventing harmful microscopic particles from entering your body (or, in cases where you are sick, from entering the air around you).

The very best face masks, the ones that were used by doctors and nurses at the height of the pandemic, can trap aerosolized particles as small as 0.3 microns in size. That’s smaller than what we can see without a microscope, but to put it into context, 2 microns is about the width of a single strand of a spiderweb. These masks are made from multiple layers of nonwoven plastic material composed of randomly arranged microfibers, which are more efficient at catching microscopic particles — including viruses like COVID-19 — than the regular gridded weave of cloth masks. Additionally, these filter layers are often given an electrostatic charge that helps them attract particles in the air like a magnet. The percentage of particles a mask’s filter can capture and keep from passing through the mask determines the mask’s filtration efficiency.

There are lots of mask manufacturers that take it upon themselves to have their masks tested by third-party laboratories, like Nelson Labs. These tests usually involve passing air with microscopic particles suspended in it through a given mask and measuring the concentration of particles in the air before and after it passes through the mask. But most third-party labs only test the masks sent to them by brands interested in legitimizing their products.

During the height of the pandemic, when masks were often out of stock and counterfeits were a huge concern, Aaron Collins, a mechanical engineer with a background in aerosol science, decided to purchase hundreds of (mostly children’s) masks and test them in his home, sharing his results for free in a public Google spreadsheet. Collins used a “total inward leakage” test to measure the concentration of microscopic particles that could make their way inside a mask while it is being worn. This approach helped him get a good idea of a mask’s filtration efficiency as well as how good a seal it could form around the edges of the wearer’s face. He isn’t an accredited lab, but his data has shown agreement with accredited published data.

Masks should also be tested for breathability (or pressure drop) because, as Collins puts it, masks that are hard to breathe through won’t protect you from getting sick. Air has to be able to pass through a filter in order for the filter to work. So, the ideal mask has a good amount of filtration resistance to catch microscopic airborne particles, but not so much resistance that it forces the air to find another way in or out of the mask. (If the air can’t go through the mask, it’s going to go around the sides, skipping the filtering layers altogether.)

A mask’s filtration efficiency is one piece of the puzzle, but to get a complete picture of how well it will protect the wearer, you also have to consider its breathability. Unfortunately, masks don’t come with a specific breathability certification. But it is part of the criteria used by agencies like NIOSH when evaluating masks. Collins also notes his pressure-drop findings in all of his mask-testing data.

I have spent the last four years reporting on and writing about face masks. And in that time, the number of companies making masks for everyday use has grown significantly, with instances of counterfeit disposable masks skyrocketing. Depending on the country where a mask is manufactured, the regulatory standards and the enforcement of those standards are slightly different.

When buying N95 masks, which are the American standard for medical-grade masks, look for mentions of NIOSH approval and the code “TC-84A” followed by four more digits. For KF94 masks, which are the South Korean standard, check that they are actually made in South Korea. If a mask is also sold in South Korea, you can trust it’s real since the South Korean government requires masks to meet KF94 standards before they can be brought to market.

Counterfeiting is of special concern with KN95 masks, which are the Chinese standard. According to Michael Chang, M.D., assistant professor of pediatrics with McGovern Medical School at UTHealth Houston, this is because the process for meeting KN95 standards is based on self-regulation. When buying KN95 masks, look for third-party testing certificates and the most recent Chinese standard respirator code, GB2626-2019, stamped on the mask. You should also only buy them from reputable sources like Bona Fide Masks versus random sellers on Amazon.

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