DIY Wildfire Smoke Filters: How They Work, Safety, Cost

Do-it-yourself air filters are safe, effective, and can be used to protect your California lungs forest fire smoke.

Smoke from wildfires is noxious and can travel for hundreds of kilometres. Smoke from the 2021 Dixie Fire in California was felt as far away as Denver, The New York Times reported.

Here are two safe options, according to the University of California, Davis, the U.S. Environmental Protection Agency, and the California Air Resources Board:

When should I use an air filter?

Good air filters can remove dangerous smoke particles from your home. According to the California Air Resources Board, IIndoor air purifiers help filter small particles which can cause health problems.

Wildfire smoke produces harmful air pollutants that can aggravate existing health conditions and increase the risk of heart attack or stroke.

The Resource Council recommends using a certified air purifier whenever the Air Quality Index is at an unhealthy level, which you can check on The agency also says that if a board-certified commercial system isn’t an option for your home, a DIY is an acceptable alternative.

“These temporary air purifiers should be used with extreme caution, and only if other air purification options are not available,” the council writes on its website.

It says never leave the unit unattended and only use box fans manufactured within the last 10 years (after 2012) as these fans “will have a fused socket, which will prevent electrical fires if the device is overturned”.

Types of DIY Air Filters

Option 1

The EPA tested a version of the DIY air cleaner with a flat MERV-13 filter against a box fan.

The agency outsourced in 2021 to Underwriters Laboratories Inc. to conduct a safety study. The company found that covering the front and back of a 20-inch box fan for seven hours”posed no observable fire hazard.” Covering only the suction side of a 20 inch box fan would cause the surface to get hot, but not hot enough to cause a superficial burn.

The California Air Resources Board writes that you should “close all windows and doors when the box fan filter is in use” and “change the air filter when dirty.”

Option 2

Another design, known as the Corsi-Rosenthal Box, does not have the air filter placed directly on the fan, but incorporates multiple filters and a cardboard bottom to create a cube. University of California, Davis Scholars Richard Corsi, Theresa Pistochini and others found that it is effective enough to combat airborne particles during the COVID-19 pandemic.

The EPA also lists this method as a inexpensive way to protect yourself from wildfire smoke.

If you try this option, Pistochini told The Bee that you should use the safety instructions attached to the box fan model you’re using.

How to build yours

The CR Box was “designed by experts but can be built by an amateur in less than an hour”, NPR reported.

What you need for option 1, the filter fan, according to the EPA:

At Walmart, for example, a box fan, filter, and roll of tape might cost around $50.

A single filter DIY air purifier instruction chart. United States Environmental Protection Agency

Check for an arrow or indicator on the filter to ensure proper airflow. Attach the filter to the suction side of the fan, per EPA instructions.

What you need for a CR box, according to the UC Davis study:

According to the Kentucky State Air Quality Division, its total cost is between $70 and $100.

The APE suggest buying new box fans if you intend to do this project.

Visual instructions on how to build a Corsi-Rosenthal air purifier box. Kentucky Air Quality Division

Build the fan shroud out of cardboard, only the fan blades should be visible. Attach a shroud to the box fan. Glue all sides of the box together.

You can find the detailed assembly Kentucky Air Quality Division instructions.

How much does it cost to buy an air filter?

The filters recommended by the EPA and the creators of the Corsi-Rosenthal box specify using air filters (20x20x1) MERV13 or better, which cost about $20 each at a hardware store. One or more of these materials are needed to build the DIY.

Air Resource Board certified filters, based on a UC Davis comparison of HEPA filters tested by ULmodels for large rooms can range from $99 for the Whirlpool Whispure WPT60P at $899 for the ORANSI EJ120.

Are DIY air filters safe?

The EPA hazard conclusions for the use of a 20-inch box fan are comparable to those of the CR Box model. The fan in this model is not directly obstructed. Pistochini, with UC Davis, told The Bee that the fan in a CR Box is more stable than the single filter option because the shape reduces resistance and prevents it from falling off.

She uses the model around her own children and doesn’t think it’s any more dangerous than other electrical devices found around the house, she said. The box is cost-effective and efficient, but it and the EPA recommend people buy UL-tested HEPA filters as a long-term solution. Insurers’ laboratories, ULis an OSHA-certified electrical test facility that mitigates risk, injury, or hazard, according to its website.

You should turn off the fan if it gets too hot or if you leave the house.

While HEPA filters are the most qualitative option for an air filter, using a MERV-13 oven version will still collect particulate concentrations, Corsi, Pistochini and colleagues wrote in their report on the performance of the CR Box.

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