UW professor clears the air about filters and face masks in this stubborn haze of smoke

SEATTLE — Since this toxic noxious cloud of wildfire smoke is going to be hanging around for a while, everyone wants to know the most effective ways to keep it out of our homes and our lungs.

When UW Bothell Professor of Environmental Chemistry Daniel Jaffe showed KIRO-7 how effective his homemade indoor smoke filter was--by attaching a box fan and a furnace filter together, most area stores quickly sold out of the furnace filters.

If you were fortunate enough to find a filter and run the fan, you’ll notice the filter can build up layers of dark buildup in smoky conditions within hours. Turns out, those smoke particles building up in the filter become part of the overall filtration system, and the professor says even dirty filters can still be effective keeping dangerous smoke particles out of the air you breathe.

“As long as the fan is still blowing air, it will work,” Professor Jaffe said. “If it gets so dirty that it cannot pull air through the filter, then it’s going to overheat the fan and that will not do you any good, but so as long as you get a pretty good flow from the filter, I would keep using it," said Jaffe.

We asked Professor Jaffe about recent demand for PM 2.5 filters in cloth face masks, since some doctors recommended them as an effective way to filter out smoke particles outdoors.

“The term PM 2.5 filter is a vague term,” Jaffe said, adding the PM 2.5 filters may all have slightly different specifications, and that’s important because he says the dangerous, toxic parts of the smoke are incredibly tiny particles the human eye can’t see.

He says the closest comparison is a strand of human hair. A smoke particle can be .03 microns--which is about one-one hundredth the width of a single human hair.

He says many masks, unless they’re rated N-95, don’t have the ability to filter those tiny smoke particles out. He says what’s really important for any mask in smoke is fit.

“It turns out, most of the loss of effectiveness with a mask is the fit around your face,” Jaffe said. “The actual material may be capable of collecting 90% of the particles, but if you not have a good seal around your face the mask, then the air is flowing around it.  If you have to be outside in this kind of bad air for more than thirty minutes, I think that’s when you want to think carefully about how you’re going to protect your lungs.”

Cox Media Group