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With fires raging on the West Coast, air purifiers are quickly going out of stock.

Polluted air and smoke from fires can have serious adverse health effects and irritate the sinuses. These devices help keep air quality at a safe level.

If you find yourself unexpectedly breathing smoke or simply want to reduce the pollution you’re inhaling, you can make your own air purifier with a few essential parts.

Are these DIY air purifiers effective?

Sarah B. Henderson, PhD, senior scientist in environmental health services at the British Columbia (BC) Centre for Disease Control, was involved in a study on homemade air purifiers earlier this year.

According to Henderson, “What we found in our trials is that this does work. So, they can be quite effective for removing smoke particles from the area around the box fan.”

Purifiers contain filters that remove particulate matter (PM) from the air. This matter is composed of chemicals such as sulfates, nitrates, carbon, or mineral dusts. It can be found in burning organic matter, like fire smoke.

A subsection of PM found in the composition of wildfire smoke is PM2.5 — fine particulate matter 30 times thinner than the average human hair.

This ultrafine particulate can be inhaled into the respiratory tract and travel deep into lung tissue. This can contribute to health problems, including respiratory and cardiovascular disease.

Smoke from wildfires, in particular, are shown to increase the risk of:

According to the New York State Department of Health, exposure may also cause short-term effects, such as:

  • shortness of breath
  • coughing
  • sneezing
  • runny nose
  • throat, eye, or nose irritation

Smoke exposure can worsen existing conditions, such as asthma and heart disease.

One study from 2008 showed that remaining indoors while using an air cleaner can effectively reduce PM2.5 exposure.


Research from 2015 shows that for air filters to work effectively, they require airflow to assure adequate ventilation. They must also effectively filter out various small particle sizes, including PM2.5.

Many air purifiers on the market contain High Efficiency Particulate Air (HEPA) filters, which remove most airborne particles through mechanical filtration.

Other high-quality filters, such as those with Minimum Efficiency Reporting Values (MERV) ratings, work too. The higher the rating, the more effective. But you’d want to aim for a rating of 11 or higher.

Research shows that HEPA filters are standardized at a minimum 99.97% efficiency rating for filtering particles greater than or equal to 0.3 micrometers (μm) in diameter. That’s a diameter of 1/83,000 of an inch.

This means that for every 10,000 particles that are 0.3 μm in diameter, 3 will pass through the filter. The rest will be trapped by the filter.

This makes HEPA filters the best choice for DIY air purifiers, as they filter out the ultrafine particulates contained in fire smoke.

“We tested a filter that’s equivalent to about a MERV 13, but there are different filter rating systems,” says Henderson.

According to Henderson, it’s best to go for the highest number possible.

“In general, the numbers get higher as the filter gets better. If you’re using a filter on the MERV system, a 13 is a good place to start. But if you can get a MERV 14 or 15, that’ll be even better,” she says.

Room size

It’s also important to be aware of room size. Henderson says that homemade air purifiers are appropriate for small rooms of about 10 by 10 feet.

If the room is much bigger, they won’t do the job.

“They’re not going to be very effective in your wide-open living room or kitchen plan design. It’s just not what they’re designed to do,” explains Henderson. “But if you put one in a small room, you should be able to keep that small room quite clean.”

Place your air purifier where you spend the most time, whether that be the bedroom or an enclosed living room. You can also place multiple purifiers in each room of the house.

If you’ve only got the means to make one air purifier, Henderson recommends choosing one room in your home that you can keep comfortable and clean. Maintaining the air quality of a smaller space is much easier than an entire house or home.

There are two types of DIY air purifiers: the filter plus fan and the box fan filter.

Both of these require one or more HEPA or high-quality filters.

Ideally, you want a filter that’s 20 by 20 inches, but any size that will adequately cover the size of your box fan will work.

Regardless of the type of purifier you make, you’ll need a box fan. Again, you’ll want to aim for 20 by 20 inches, but any size that your filter covers sufficiently will work.

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Filter plus fan air purifier

This DIY purifier is essentially a filter taped to a fan. It’s easy to make and proven to be effective.

What you’ll need:

  • 20-by-20-inch box fan
  • 20-by-20-inch HEPA or other high-quality filter
  • duct tape


  1. Make sure the fan isn’t plugged in, and turn the knob to the highest setting. The filter will cover the switch. This is OK.
  2. Place the filter on the front of the fan.
  3. Tightly tape around the filter to hold it in place.
  4. Place in your desired room, making sure there are no obstructions to airflow.
  5. Turn the fan on and off at the plug to avoid disrupting the filter.

Periodically check the motor to make sure it doesn’t overheat.

Box fan air purifier

You may have seen the box fan air purifier on Marshall Hansen Design or Tom Builds Stuff, where you can also buy supplies for your filter.

This method involves using two filters instead of one to increase surface area and reduce stress on the motor.

It’s a little bit more finicky, but it’ll alleviate the pressure on the box fan while adding an extra filter to work more efficiently.

What you’ll need:

  • 20-by-20-inch box fan
  • two 20-by-20-inch HEPA or other high-quality filter
  • cardboard (you can use the box the filters come in)
  • duct tape
  • utility knife or scarf scissors


  1. Unbox the filters, keeping the cardboard as intact as possible. Be careful not to puncture the filters inside with scissors or knife.
  2. Take the two filters and lay them on top of each other with the black carbon sides facing each other. This means the airflow arrows or labels should be pointing towards each other.
  3. Firmly tape the filters together on one side to create a hinge when opened.
  4. Lay the box fan face down on a flat surface and place the filters on top so that the two non-taped sides meet the edge of the fan. Tape them in place.
  5. Fill the triangular empty space made by attaching the cupboard. To do this, first place the cupboard on the top and bottom of the filters. Using a pen or pencil, mark where the cupboard meets the filters and box fan.
  6. Cut the cupboard and attach it to the box fan and filters using tape.
  7. Seal any gaps in your device with extra duct tape.
  8. Place in your desired room, making sure there are no obstructions to airflow.

It’s a good idea to monitor the outdoor air quality index, which you can do by visiting the Environmental Protection Agency website or using this handy tool.

It’s also important to know your indoor air quality, and you can get a device to keep track. These devices use real-time data to determine current air quality.

You can also try spending time in buildings with better air filtration systems, such as shopping centers or offices.

Henderson says it’s important to remember that box fans aren’t designed to do this.

“The motor isn’t designed to have that extra load of having to pull air through the filter,” she says. “So, we did run some checks on the temperature of the motor and found that it definitely gets hotter than it would if you didn’t have the filter on it.”

This means that a DIY filter has an increased potential for fire risk.

For this reason, it’s crucial that you only use an improvised device when someone is present to keep an eye on it. DIY air filters should never be left on unattended.

You’ll also want to make sure there’s nothing obstructing the airflow to the purifier, such as hangings, drapes, or curtains.

Research indicates it’s also important to reopen windows once smoke from the outside air starts to clear to release any trapped particles from inside the home. Keeping them inside will increase indoor exposure.

With air purifiers selling out at many retailers, a DIY option can be a good alternative. You only need a few basic materials. It’s important to get a HEPA filter or filter with a high rating, or your air purifier may not be effective.

If used in small rooms with doors and windows closed, homemade air purifiers can help improve your in-home air quality.

Marnie Vinall is a freelance writer living in Melbourne, Australia. She’s written extensively for a range of publications covering everything from politics and mental health to nostalgic sandwiches and the state of her own vagina. You can reach Marnie via Twitter, Instagram, or her website.