We’ve done extensive research into the tools to help you prepare for the next wildfire, but if the air is full of smoke right now, and you’re missing a few items on that list (such as an air purifier), this temporary fix may help mitigate the conditions in the meantime.
In 2018, I tested a popular claim: that sticking a furnace/HVAC filter on a standard box fan produces a useful DIY air purifier. I taped a 20-by-20-inch Honeywell FPR 9 filter to a Lasko 20-inch box fan and ran that combo through our standard 35-minute, five-match test in our 200-square-foot New York test space, with the fan on high. And you know what? It did okay, cutting the initial particulate load by 87% over 35 minutes on medium. That’s nothing like the 99% reductions our purifier picks achieved on their high settings, but the results were better than one might expect.
Some caveats apply.
I was careful to seal the filter around its entire perimeter with clear pro-strength packing tape—any gap would have let unfiltered air pass through, same as on dedicated air purifiers. You should do the same if you try this hack.
Most box fans aren’t engineered to withstand the extra workload of driving air through a filter, so we wouldn’t consider this hack a long-term solution for air-quality issues.
And long-term, it’s more expensive to use a fan-plus-filter than a purifier. Fans are far less energy efficient than the purifiers we recommend, and HVAC filters have short lifespans. Because most of them rely on electrostatic attraction to efficiently capture particles, their efficiency plummets as they clog up (unlike the HEPA filters used in purifiers, which actually become more efficient). That means you’ll need to replace your fan filter at least every couple of months, and after any major air quality crisis.
But if you have an air-quality emergency on your hands—regional wildfires, or you charred your dinner under the broiler—and you have a box fan, tape, the right sort of filter, and no time to get an air purifier, it’s worth a shot.
For more context, info on our test methods, and details about the air purifiers we recommend if you’d prefer something more sophisticated than the fan-and-filter rig, see our guide to the best air purifiers.
