Is smoke in your home? Here's how to make an air purifier using a box fan
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A version of this story was originally published on Jan. 10, 2025. It has been updated to reflect the wildfires currently bringing smoke to the U.S.
Interest in air filters and purifiers is spiking as wildfire smoke spreads, including questions about how to build a homemade air purifier.
On Friday, the air in a large section of the U.S. — from Minnesota to Virginia — was listed as "very unhealthy" due to particulate matter from large wildfires, according to the federal AirNow air quality monitor.
The smoke is coming from wildfires in Ontario, Canada, and northern Minnesota — where Duluth and other cities are enduring hazardous air at record levels, as Minnesota Public Radio reports.
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On AirNow's map, a purple zone signals "very unhealthy" air quality index values from 201 to 300. That level triggers public health alerts, because "everyone may experience more serious health effects," the Environmental Protection Agency says. Of particular concern are children, older adults, and people with heart of lung disease.
Within the warning area, many communities are currently in an even more dire "hazardous" category, reflecting an AQI of at least 301 — a level the EPA considers "extremely rare."
A big worry, experts say, is fine inhalable particulate matter known as PM 2.5 (for 2.5 microns) that can lodge deep into our lungs.
And as researchers at the University of Washington note, "Smaller ultrafine particles (PM 0.1) can pass into the bloodstream and organs, including the brain."
Here's a quick guide to getting cleaner air:
Search filters by their MERV rating
Filters are key, whether you're using them to upgrade your home's HVAC system or put them in an air purifier or a DIY air cleaner. A filter's ability to pull particulates and other matter out of the air are rated by Minimum Efficiency Reporting Values, or MERV. Filters with higher values can catch smaller contaminants like smoke, bacteria and viruses.
A MERV rating of 13 means the filter is efficient at catching items from 0.3 to 1 micron in size: "Bacteria, droplet nuclei (sneeze), most tobacco smoke, insecticide dust," according to the California Air Resources Board, or CARB.
"Upgrading to a filter rated MERV 13 or higher can be especially important during smoky periods to effectively remove fine particle pollution from smoke in the indoor air," the EPA says. It adds that most HVAC systems work fine using MERV 13 filters, as long as the filter is replaced frequently — but if you're in doubt, ask an HVAC technician to be sure.
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One thing to remember: If you're using your HVAC system to clean your home's air, it only does so when the unit is running. You can learn more about filter rating systems at member station LAist.
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Build an air cleaner at home
All you need is a box fan, good air filters and some duct tape, and you're in business.
NPR has previously posted a design and guide to building an air purifying cube known as a Corsi-Rosenthal Box, using four 20-inch MERV 13 filters, a 20-inch box fan and two 20-inch panels of cardboard.
A smaller and more affordable approach is to attach one MERV filter to the back of a box fan, an idea laid out in a website from the Montana group Climate Smart Missoula. The group notes that people using that option should use newer box fans, and be sure the motor is clean to reduce the risk of overheating.
The Centers for Disease Control and Prevention says these DIY purifiers can be effective for a temporary solution, especially if you don't have any other alternatives.
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Use HEPA air purifiers and other methods
HEPA (for high-efficiency particulate air) filters are used in many air purifier machines. HEPA is roughly equivalent to a MERV 17 filter, the California Air Resources Board says.
"A well-sealed HEPA filter is more than 99.9% efficient for all sizes of particles," according to the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, or ASHRAE.
If you plan to buy a portable air cleaner or purifier, the EPA says you should make sure it has a HEPA filter. To help your search, the EPA has a guide on matching your room size with the appropriate unit. And California has a table of state-certified air cleaners that lists more than 1,000 products by brand, model number, and type.
Whichever strategy you choose, experts say you should also make sure doors, windows and air ducts are well sealed, to keep out smoke and pollution. You should also drink plenty of water to help ease irritation — and wear a mask if you go outside, as NPR has reported.
