People Are Making DIY Air Purifiers to Cope With Wildfire Smoke Indoors

With air purifiers selling out, taping some filters to a box fan isn't a bad alternative.
​Image via Derek Mead's Twitter
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As the west coast burns in apocalyptic wildfires, people in areas socked in with smoke and ash filling the sky are scrambling to pick up air purifiers to make the air in their homes more breathable. Dozens of people are missing and at least 35 are dead, and even outside of fire evacuation zones, people with asthma and heart or lung conditions are in serious danger while inhaling the smoke—even inside.


According to Google Trends, searches for "DIY air purifier" shot up in the last month, with the majority of searches coming from Oregon, Washington, and California. That's probably because there's a run on "real" purifiers right now: Amazon's stock is spotty—a lot of the air purifiers offered there are on backorder until November or available via third-party sellers with slower shipping. Online hardware stores like True Value and Home Depot, especially in the Bay Area, are already showing air purifiers sold out and filter replacement sets selling out, too. 

Air purifiers were already in high demand because of COVID-19, with people looking for ways to circulate and clear the air in their homes or offices. When Wired looked at the potential effectiveness of these DIY filter-box fans on removing virus particles from the air, they found that one 2018 study from Singapore specifically examined soot and smoke from wildfires. The researchers concluded that a fan with a filter slapped on the front and mounted in a window to pull air inside through the filter reduced small particulate matter by around 75 percent. 

In short, they don't look that sophisticated, but DIY air purifiers can improve air quality, and are definitely better than nothing if you're struggling with smoke and don't have an alternative. Studies show that indoor air purification, when the air outside is badly polluted, reduces inflammation and has cardiopulmonary benefits, even in young, healthy people.


Before this month, as an east coaster my whole life, I didn't fully grasp how terrible annual wildfires make the air quality out West—but this year's fire season is unprecedented. One LA-based friend said she's sick just sitting in her room. Another sent photos of the hazy red sun disappearing into smoke a little more every day.

But nothing hit me quite like this tweet from former Motherboard editor-in-chief and now Bay Area resident Derek Mead of his DIY air purifier: 

Mead put his alternative together for less than half the cost of consumer-quality purifiers. People have been turning a box fan and a couple filters into a janky air purifier for years, either from wildfire smoke or wanting to make their own, cheaper version of expensive air purifiers. Several people in one Reddit thread last week said they'd been doing this one weird box fan trick for years, and how-to's about it have circulated since at least 2011, when Jeffrey Evans Terrell, director of the Michigan Sinus Center, demonstrated his $25 build.

Terrell is using one of several methods; for his, he affixed one HEPA filter to the front of a box fan. Others use two filters taped to the back of a box fan, to form a triangle shape.

Thomas Talhelm, Associate Professor of Behavioral Science at University of Chicago Booth School of Business, has been fascinated with air quality and particulates since living in Beijing. He founded Smart Air, a corporation that educates people on indoor air quality and sells air purifiers. 

"The data on DIY purifiers is very clear: they work," Talhelm said. "They can even rival the $1,000 purifiers." 

Talhelm spent years making DIY air purifiers and testing air quality and particles on his own. Here are some tips he shared with us:

  • Make a good seal around the fan and the filter, with tape if you have to, and if you're doing the one-filter method, use a fan with a flat surface so you can get that seal. "The most common mistake that I see is that people don't have a flat fan," he said. Strapping a flat filter to a convex fan face will just pull air in around the filter, bypassing it for the most part.  
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  • Close all the doors and windows. "Just by closing doors and windows, indoor air has 40 percent less particulate than outdoor air." He tested this himself.
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  • The design shape—two-filter triangle versus one filter on the front—doesn't matter too much for air quality improvement, and neither does room placement. The triangle does allow more surface area and makes the fan work less hard to pull the air through, but slapping a filter to the front works fine, too, especially if they're in short supply. 
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  • With the front approach, make sure the filter covers the whole face of the fan. If it overlaps, on a circular fan with a square filter for example, that's fine.
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  • A weak filter is better than nothing. Since filters are selling out fast, any type would do in a pinch—it doesn't have to be the highest-rated HEPA to work. "If people can only find a furnace filter in Walmart or whatever it's definitely better than nothing," Talhelm said.
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Out-of-control wildfires have been a problem on the West coast for years, and as climate change makes droughts worse and weather more extreme, the fires will likely only become more devastating each year. California's annual burned area is five times what it was in 1972, due to an eightfold increase in summer forest‐fire extent, studies show. Just as we saw with COVID-19 supplies like PPE and paper goods, panic-buying mixed with neglectful government leadership means we're mostly on our own—and we'll have to start making our own solutions to survive, even if it's some filters duct-taped around a fan.