During the winter, spring, and summer of a global pandemic, many people have become accustomed to the idea of wearing a fabric cover to reduce the spread of the coronavirus, the highly infectious virus that searches for as many human hosts as possible. Masks inhibit both the distribution and inhalation of breath droplets that can transmit the virus.
Recently, a number of homemade and mass-produced fabric face masks that are available for sale online have used a carbon or other filter that could improve the effectiveness of the mask and ward off more virus particles than the mask alone. Can this extra filtration really help?
According to health experts, the answer is yes ̵
Filters are typically placed between two layers of cotton. Alternatively, a mask provider can offer a mask with a pocket for a filter of your choice. Some filters claim to be HEPA certified, a standard for air filtration, or made from carbon, which can increase the risk of particles sticking to the material and preventing it from entering or exiting the mask. While these filters are used in air filter devices, their effectiveness when worn on the face has yet to be assessed.
“Carbon and charcoal are said to filter contaminants from water,” May Chu, Ph.D., clinical professor of epidemiology at the Anschutz Medical Center at the Colorado School of Public Health, told Mental Floss. “We don’t know if inhaling charcoal fragments or particles is good for your lungs.”
The same applies to the use of vacuum cleaner bags or coffee filters. Some, says Chu, are made of silica or glass. This is not something you want in your lungs. “These are not meant to be breathed,” says Chu.
According to Chu, the most important feature of a mask is that it reaches the “sweet spot” of particle binding while maintaining comfort and breathability. If a mask is very thick, you may not breathe in droplets, but you may have problems with the air flowing through and may breathe in through the open sides where there is no barrier.
Cotton is the preferred mask material. A tightly woven cotton allows air to flow and tends to capture particles better than synthetic materials that have a smoother structure. Cotton is also best suited for one or more layers, depending on whether light shines through it. If you can, it’s too thin.
Since cotton was not developed to protect against particles, it makes sense to add a filter. But like the mask itself, it must be made of a safe material that is comfortable to wear. Chu recommends polypropylene, a material that is available from Walmart and other retailers under the Oly-Fun brand and is also sold under the Spunbond name.
What is special about polypropylene? “It’s the same material used in N95 masks,” says Chu. The N95 mask is used by medical professionals and filters up to 95 percent of the particles in the air. Chu warns, however, that consumers need to be aware that there are differences between in-store polypropylene and the type used in a medical N95.
“The quality of the N95 polypropylene is calibrated and designed for a specific filtration level that has a standard,” she says. “It is more expensive and more precisely machined.” Two layers of spunbond polypropylene can approximate, but not duplicate, the protective properties of a single layer of medical grade mask filters.
Polypropylene is effective because it can hold a static charge that repels particles. While an N95 mask holds this charge for the entire life of the mask, you can create a charge with a homemade filter by ironing the polypropylene or rubbing with a plastic glove for 20 seconds. The charge takes about 12 hours or until it’s washed, Chu says.
If polypropylene is not available, there is an alternative that virtually anyone can use. Simply fill four Kleenex sheets (or two sheets folded in half) into the mask pocket. “The Kleenex offers good efficiency in blocking particles and breathability,” says Chu.
Whichever filter you use, it is important to remember that once a mask or filter gets wet, it will no longer be able to hold a load (for polypropylene) or block particles (facial tissue). The filter must be dried or turned off to maintain its effectiveness.
Obviously, medical professionals choose an N95 if they are available. But what does Chu do when it’s time to wear a fabric mask? “I wear a mask that is made on site,” says Chu. “There are two layers of polypropylene that are wrapped in cotton at the front and back.”
Combined with social distancing and hand washing, a breathable mask with a safe filter can slow the spread of the virus and reduce infectious droplets that come out of a person’s mouth. “That’s why masks are good,” says Chu. “We know that works.”