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As thick smoke from Canadian wildfires blankets much of the Northeast, many people are once again donning masks -- but which is best to protect against the tiny particulates in wildfire smoke?

Two experts -- a pulmonologist and an air quality researcher -- told MedPage Today that well-fitting N95s or KN95s are preferable, because they offer the best chance of blocking PM2.5, or particulate matter that is approximately 2.5 microns in size, which is the main concern with wildfire smoke.

Sameer Khanijo, MD, a pulmonologist with Northwell Health in Long Island, New York, said in general, cloth masks and surgical masks, "with this kind of particulate matter are probably not going to do much. KN95 and N95 masks have been graded to remove this type of particulate matter."

Peter DeCarlo, PhD, an associate professor of environmental health and engineering at Johns Hopkins Whiting School of Engineering in Baltimore, said wildfire smoke is comprised of "thousands of chemicals" but is largely a mix of organic material including carbon.

"It's essentially the same stuff that you get from the campfire or in your fireplace, just on a much larger scale," DeCarlo said. "Basically, it's these tiny microscopic particles about 100 times smaller than the width of our hair, made up of chemicals that get into our lungs and then get deposited into the rest of our bodies."

That's why keeping them out of the body is important, though it may seem counterintuitive that surgical masks aren't good at that since physicians have recommended them to prevent viral spread during the COVID-19 pandemic -- and viruses are smaller than particulate matter.

But there's more to mask science than just particle size, experts say.

N95 masks generally filter down to 0.3 microns, but SARS-CoV-2 is about 0.1 micron (a micron, also known as a micrometer, is a thousandth of a millimeter).

Yet the virus doesn't usually travel alone, from the U.S. Occupational Safety and Health Administration (OSHA). It's connected to other particles, such as water and mucus, in droplets and aerosols, which tend to be larger than 1 micron.

"These larger particles are easily trapped and filtered out by N95 respirators because they are too big to pass through the filter," according to OSHA, which notes that this kind of "mechanical" filtration is just one component of N95 filtration. "An electrostatic charge also attracts particles to fibers in the filter, where the particles become stuck."

OSHA also said that the National Institute for Occupational Safety and Health (NIOSH) tests respirators using particles that simulate a 0.3-micron diameter "because this size particle is most likely to pass through the filter. If worn correctly, the N95 respirator will filter out at least 95% of particles this size. An N95 respirator is more effective at filtering particles that are smaller or larger than 0.3 microns in size."

Thus, OSHA concludes that N95s are indeed "very effective at protecting people" against SARS-CoV-2 infection -- and by extension, the larger particulates in wildfire smoke.

This doesn't mean there's no place for surgical masks in protecting against wildfire smoke. Surgical masks that use polypropylene offer a great physical filter, but also hold an electrostatic charge, like N95s, May Chu, PhD, an epidemiologist at the Colorado School of Public Health, .

That electrostatic charge helps to trap droplets and particulates; it is even unaffected by humidity, although it will lose its charge when washed, according to the NPR article. Rubbing it with a plastic glove for about 20 seconds reportedly will bring back that charge.

The challenge with surgical masks, however, is that they don't seal the same way N95s do, so particulates can seep in through those openings.

Respirator masks, , are also effective at filtering PM2.5, though many may find them impractical to wear for longer periods of time.

James McDonald, MD, MPH, New York's acting commissioner of health, that "any mask you do have is better than not wearing one."

"The point is to use the best available mask you have access to," McDonald said. "If you have a surgical mask, that will give you some protection, but N95 and KN95 are better masks to use in times like this."

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