01/28/21 Update: This article describes the benefits of filtration in homemade masks, which are similar to the protection afforded by the surging practice of double-masking. You can read more here.
Through the winter, spring, and summer of a global pandemic, many people have gotten used to the idea of wearing a cloth face covering to reduce the spread of the coronavirus, the highly infectious virus looking to find as many human hosts as possible. Masks both inhibit the distribution and inhalation of respiratory droplets, which can carry the virus.
Recently, a number of homemade and mass-produced cloth face masks for sale online have boasted of having a carbon or other kind of filter in place that might improve the efficacy of the mask, warding off even more viral particles than the mask alone. Can this additional filtration really help?
According to health experts, the answer is yes—but you should exercise caution in what type of filter you choose.
Filters are typically sandwiched between two layers of cotton. Alternately, a mask provider might offer a mask with a pocket for a filter of your choosing. Some filters claim to be HEPA-certified, a standard for air filtration, or made with carbon, which may increase the potential for particulates to stick to the material, blocking it from entering or exiting the mask. But while these filters are used in air filtration devices, their effectiveness has yet to be evaluated when worn on the face.
“Carbon and charcoal are meant to filter contaminants from water,” May Chu, Ph.D., a clinical professor of epidemiology at the Colorado School of Public Health Anschutz Medical Center, tells Mental Floss. “We don’t know if breathing in charcoal fragments or particles is good for your lungs.”
The same holds true for using vacuum cleaner bags or coffee filters. Some, Chu says, are made with silica fibers, or glass. That’s not anything you want in your lungs. “These are not designed for breathing,” Chu says.
According to Chu, the most important feature of a mask is that it hits the “sweet spot” of locking in particles while maintaining comfort and breathability. If a mask is very thick, you’re not likely to inhale droplets through it, but you might have trouble with air passing through it and would likely wind up inhaling through the open sides, where there’s no barrier.
Cotton is the preferred mask material. A tight weave cotton allows for air flow and tends to trap particles better than synthetic materials, which have a smoother structure. Cotton also tends to work best with one or more layers depending on whether you can see light shine through it. If you can, it’s too thin.
Because cotton isn’t intrinsically designed to protect against particles, there is value in adding a filter. But like the mask itself, it needs to be made of a safe material that’s comfortable to wear. Chu recommends polypropylene, a material that can be purchased under the brand name Oly-Fun from Walmart and other retailers and is also sold under the name Spunbond.
What’s so special about polypropylene? “It’s the same material used in N95 masks,” Chu says. The N95 mask is used by medical professionals and filters up to 95 percent of airborne particles. But Chu cautions that consumers need to be aware that there are differences between store-bought polypropylene and the kind used in a medical-grade N95.
“The quality of the N95 polypropylene is calibrated and designed for a particular level of filtration that has a standard,” she says. “It’s more expensive and tooled more precisely.” Two layers of Spunbond polypropylene can approach—though not duplicate—the protective qualities of a single layer of the medical quality mask filters.
Polypropylene is effective because it can hold a static charge that repels particles. While an N95 mask is made to hold that charge for the life of the mask, you can generate a charge with a homemade filter by ironing the polypropylene or rubbing it with a plastic glove for 20 seconds. The charge will last for roughly 12 hours or until it’s washed, Chu says.
You can also use a surgical-style mask as your filtration layer, as medical-grade masks are made with polypropylene.
If polypropylene isn’t available, there’s an alternative that virtually anyone can use. Simply stuff four sheets of Kleenex (or two sheets folded in half) into the pocket of the mask. “The Kleenex gives you good efficiency in blocking particles and breathability,” Chu says.
No matter which filter you use, it’s important to remember that once a mask or filter gets wet, it loses its ability to hold a charge (for polypropylene) or block particles (facial tissue). The filter needs to dry or be switched out in order to maintain its effectiveness.
Obviously, medical professionals opt for an N95 when they’re available. But what does Chu do when it’s time to wear a cloth mask? “I wear a mask that’s locally made,” Chu says. “It’s two layers of polypropylene pocketed by cotton in the front and back.”
Coupled with social distancing and hand washing, a breathable mask with a safe filter can slow the spread of the virus, reducing infectious droplets that come out of a person’s mouth. “That’s why masks are good,” Chu says. “That’s what we know works.”