What kind of filter should you put in a face mask?

Wearing a cloth face mask has become a standard part of life when leaving the house amid the coronavirus pandemic.

Face masks, which you can make from materials around the house, are meant to protect those around you from contracting the virus. Research shows up to 50% of people with COVID-19 don't show symptoms.

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But if you're looking to give yourself extra protection when out in public, adding a filter to your homemade mask may do the trick, Dr. Scott Segal, an anesthesiologist at Wake Forest Baptist Health in Winston-Salem, North Carolina, told TODAY. Segal's been researching the effectiveness of mask materials for the past several weeks.

Here's what you should know about face mask filters, should you decide to use one.

Ideally, face mask filters will "improve the capabilities of homemade, cloth masks," Segal explained.

Especially if your mask is made of thin fabric or has a looser weave, it might not catch that many "aerosol-sized particles that we're concerned about with the COVID virus," Joel Burken, Ph.D, chair of the department of civil, architectural and environment engineering at Missouri University of Science and Technology, told TODAY.

He added that an effective face mask filter will allow a lot of airflow while also capturing a high percentage of the virus's particles.

The effectiveness of cloth face masks depends on the material used, but in general, adding a filter is "more protective," Burken said.

Research from Missouri S&T, headed up by Yang Wang, Ph.D, assistant professor of environmental engineering, has found that "more open" fabrics block only 10-20% of particles, Burken said. But add several layers of a HEPA filter, and it will block 80-90%, "approaching (the rates) of N95 masks."

Ultimately, Burken said he recommends using a filter if you have an appropriate, safe material for it.

Both Burken and Segal spoke highly of HEPA filters used in common household appliances, like furnaces and air conditioners. Burken also advised checking the MPR (microparticle performance rating) and MERV (minimum efficiency reporting value) on these products.

"We tested a 3000 MPR with multiple layers, and that was giving us some of the best removal," he said.

If you can't access a HEPA filter, adding more layers of a tightly woven fabric can also boost protection. To check the effectiveness of the fabric, hold it up to the light. If you can see its individual fibers, then it's "probably not a great material," Segal said.

Missouri S&T's research, which has been submitted for publication, also tested vacuum bags.

"Some of those wouldn't let the air flow through," which is a drawback, Burken explained. "If the air can't get through, it'll just bypass around the sides."

You can tell if a filter won't allow adequate airflow by breathing through it while wearing glasses.

"If it fogs up your glasses, then the air ... is going up and around the filter. It's not filtering those aerosols," Burken said.

Another household item you might consider is coffee filters. The Missouri S&T research found three layers of coffee filters by themselves without fabric removed 24.1% of relevant particles.

"They’re not ideal, (but) in multiple layers, they add some measurable benefit," especially if your homemade mask has a looser weave, Burken said. "There's no harm in adding one to your mask."

He also stressed that HEPA filters are much more effective.

According to a video explainer from Missouri S&T, you can cut a piece of the filter material, and fold it into your existing, homemade mask. Many masks available for purchase online also come with sewn pockets where you can add a filter.

Burken cautioned that if you use a HEPA filter, you should always keep a a layer of fabric between the filter and your mouth. HEPA filters contain fiberglass, which you don't want to inhale. If you fold the filter fabric into the DIY mask (as per the above instructions), then you'll be protected.

But if you have a sewn mask without a pocket and want to use a HEPA filter, add another layer of fabric between the filter and your face and make sure you can breathe adequately. Even looser weave fabrics will greatly reduce the risk of inhaling particles from the filter itself.

Burken also recommended changing the filter every time you wash your mask.

Some companies have pivoted to making face masks and filters and selling them online. But because there's a lack of unbiased testing of the material or efficacy, right now it's "buyer beware," Segal said.

Burken echoed this idea, adding, "I'd be questioning ... I'd ask, 'What's the resistance to airflow? ... What's it made out of? Is it a breathable material?'"

Burken recommended buying HEPA filters online, which is how the Missouri S&T research team acquired their testing materials, and cutting them up yourself. (Just be careful when opening the filter packaging because they main contain sharp wires.)

"If you do it yourself, you know the material you're utilizing," he said.

Maura Hohman is the senior health editor for TODAY.com and has been covering health and wellness news and trends since 2015, when she graduated from journalism school. Her byline has appeared on TODAY, NBC News, US News & World Report, People, Everyday Health, WhatToExpect.com, History.com and more. Her interests include women's health, racial health disparities, mental health and COVID-19.