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How Particulate Respirator Masks Work

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Staff from the UC Davis School of Veterinary Medicine treat a dog at Butte County Fairgrounds. (Courtesy of UC Davis School of Veterinary Medicine)

Follow KQED's ongoing wildfire coverage.

Smoke from the deadly Camp Fire in Butte County continues to pour into the Bay Area, with a Spare the Air Alert extended until at least the end of the week. In response, the California Department of Public Health recommends the public wear particulate respirator masks labeled N95 or P100 to protect against polluted air.

These masks are used widely outside our current poor-air-quality recommendations, from medical to industrial to consumer settings. So, what are these masks and how are they protecting us?

Particulate respirator masks cover your nose and mouth, providing a physical barrier between you and the polluted air. A mask labeled N95 filters out at least 95 percent of the airborne particles. A P100 filters out at least 99.7 percent. The National Institute for Occupational Safety and Health tests and approves the filters.

The filters are made of fine strands of plastic blown onto a screen. This creates a complex netting. Manufacturers then take these filters of fine plastic fibers and form them into masks, said Jeffrey Birkner, vice president of technical services at Moldex-Metric, a company that makes respirator masks mostly for industrial use.

The netting creates a physical barrier, catching particles as they try to fly through. The N95 and P100 respirators can catch particles as small as 0.3 microns wide — particles far too small for the eye to see. For reference, red blood cells are 7 to 8 microns wide.

Their ability to filter out fine particles from poor-quality air is important because wildfire smoke contains particles of just the right size that can irritate the respiratory system and worsen chronic heart and lung diseases.


Particulate respirators do not protect wearers from dangerous gases and vapors, but the Bay Area is far enough from the wildfire that this is not a concern, said Brandon Hart, a program manager at the state's Division of Occupational Safety and Health (Cal/OSHA).

Additionally, many of these masks also use static electricity. An electric charge from the fibers attracts the particles like magnets, said Birkner. And unlike rubbing your socks against the carpet and touching a doorknob, the charge doesn’t cancel out when pollution sticks to the masks. The fibers retain their electric charge, allowing them to keep pulling particles out of the air.

However, manufacturers list expiration dates on their masks. That’s because there could be other issues with the masks over time, even if the filters are still electrically charged. Straps could lose their elasticity over time, or the plastic could be affected by ozone, said Birkner.

The California Department of Public Health also recommends that people stop using the masks when they become difficult to breathe through or if the inside gets dirty. They recommend using a new mask every day, if possible.

And the filters aren’t the only important part of these masks.

“You can have the best filter in the world, but if the mask doesn’t fit ... you’re probably getting no protection at all,” Birkner said. “If you have a large leak, you may be breathing through the leak.”

Hart, from Cal/OSHA, echoed this concern. “If you have facial hair, try to remove as much as you can to ensure that you have a nice tight-fitting seal.” He also advised against wearing a mask while wearing a baseball hat. That’s because the cap extends from the head and causes the straps to stretch away from the head by the temples. This also prevents a good seal.

As for the valves on some of the masks, they’re there to help wearers exhale. “They’re made in such a way that the amount of air drawn in on the inhale is minuscule,” Birkner said.

Regardless, wearers may feel exhausted wearing and breathing through masks.

“It can be fatiguing,” Hart said. If you find yourself breathing heavily or short of breath, take breaks, he said. “Those are typical signs that maybe you need to slow down, go indoors, remove the respirator, and just get some air into your lungs.”

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