How to build a box fan air filter to escape the wildfire smoke blanketing NYC

Dec 18, 2023

The east coast, and New York in particular, is receiving an unwelcome taste (and scent) of the climate crisis to come, as smoke from massive Canadian wildfires billow out toward the Atlantic Ocean. Eerily reminiscent of what the west coast endured in 2020, the skies above New York City this week have turned a hazy orange, setting air quality index scores soaring across the five boroughs. New York on Wednesday ranked as having the second worst air quality on Earth behind Delhi.

That haze is a health hazard, especially to anyone dealing with respiratory disease, asthma, high blood pressure, diabetes, is elderly or an infant. It's not so great even if your lungs work just fine. Luckily, and I mean that in the most relative sense of the word, we’re coming off the peak of a global pandemic spread through aerosolized exhalations, so New York is already well versed in the practice of masking while in public. That's good, you’re going to need those skills – and any N95s you've still got tucked away – if you set foot outside for the next few days. Goggles too if you have them; fine particulate matter is murder on sensitive eyes.

Unless you reside in a hermetically sealed bro-sized terrarium, the hazy air from outside will eventually make it inside, where the particulate matter can concentrate further. And unless you feel like wearing your N95 non-stop until the firestorm has passed, you’re going to need a way to filter the air in your apartment.

Sure, you could blow a couple hundred bucks on some model — or you could get together some duct tape, a box fan and some good old American Ingenuity™ to build one of your own.

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You’ll need three things for this project:

One box fan: Doesn't matter how big, doesn't matter how old, doesn't matter how cheap, just make sure that the side lengths of the fan equal the lengths of the filters, so if you have a 20-inch box fan, get 20-inch filters as well. That way everything fits together evenly and you won't have weird gaps between the panels.

Four AC air filters rated either MERV 13-16 or MPR 1200-2800. These are standardized measures of filter efficiency and indicate that the products can effectively strain 2.5um smoke particles from the ambient air. They’ll even separate out bacteria and viruses if you spring for the higher grade materials.

Tape: The duct variety is always a winner, blue painters tape will also do well.

To construct it, place each filter on its end at a right angle to its neighbor so that all four form a square with the arrow indicators on each filter facing inward. Tape all of them together in this shape, making sure to not cover the actual filter bits with tape. Place the fan on top so that it blows air down into the square you just made and secure it with tape. Plug it in and you’re good to go. Fun fact: This also works wonders for covering the smell of intentionally-generated smoke in dorm rooms, not that I would have experience in such shenanigans.