Worried about staying healthy in Detroit as Canadian wildfire smoke returns to the area? A cheap and simple, do-it-yourself air purifier can help. 

Inhaling smoke isn’t good for you. An air quality alert for Wayne County and much of Michigan advises anyone with sensitive lungs to limit exercising outdoors or spending a lot of time outside through Wednesday. The symptoms of inhaling wildfire smoke can include headaches, irritated sinuses and asthma attacks. Long-term exposure can increase the risk of lung and heart problems.

Here’s a quick guide to building an affordable filter that can help curb the effects of this smoky day and the next one. Wildfires are expected to become more frequent as a result of human-caused climate change.

Canadian firefighters, if you’re reading this: We’re rooting for you.

Make your own clean air

Detroiters who are sensitive to smoke, or are concerned about breathing the stuff, can reduce their exposure outdoors by wearing a KN95 or N95 mask, which are designed to filter out 95% of smoke particles.

Smoke particles can reach indoors, too. They are tiny enough to get through any cracks in the walls of a home or building.

“The air outdoors certainly impacts indoor air quality,” said Darren Riley, co-founder and CEO of JustAir, a tech company that aims to improve air monitoring in Detroit. “The more ready you are to equip yourself with tools to purify your air, the better.”

A cheap, DIY fix

Generally speaking, a fan pushing air through filters that can trap harmful particles such as pollen, dust and smoke will make your air cleaner.

Many central air systems have built-in filters and take care of cleaning the air for an entire house (as long as those filters are of decent quality and are regularly replaced). Standalone purifiers can also improve air quality in a room, but those purifiers can be expensive.

Luckily, there’s another option for the many Detroiters without central air, many of whom aren’t about to shell out $150, on the low end, for a store-bought air purifier.

For about $40 you can build an air purifier that will improve your air quality. You’ll need a box fan, duct tape and a furnace filter rated at least MERV 13, a grade that indicates how well a filter captures different-sized particles. You can find them at most hardware stores.

The DIY design was invented by air quality experts, popularized and tested during the pandemic as folks hunted for simple ways to remove dangerous airborne particles from homes and classrooms. The design became popular again after wildfires in California and now, Canada.

A white box fan with a knob control and a roll of gray duct tape and a white pleated filter sit next to each other on a red patterned rug.
A filter rated at least MERV 13 will clean tiny particles, including smoke, from the air. Photo credit: Koby Levin

Do these things work?

“It works, but it depends on a couple of things,” said Stuart Batterman, a professor of environmental health sciences at the University of Michigan who researches air quality.

A person with curly hair and a green shirt adds gray duct tape to a white fan with green house plants in the background.
Outlier reporter Koby Levin adds tape to his homemade air filter.

He cautioned DIYers to make sure they’re using a filter that will actually remove smoke from the air. Filters will efficiently clean the air if they’re rated 13 or above on the widely used MERV scale for air filtration, he said. He also noted using four filters in a cube, with the fan on top, works significantly better than a single filter, though it increases the cost.

Filters should be replaced regularly, and folks should double-check that their box fan is powerful enough to actually push air through the filter. You should be able to feel air moving through the fan, though the flow will be slower.

A well-built homemade air filter will noticeably improve the air quality in a room, he added, though not quite as much as a store-bought machine with a finer HEPA filter.

For long-term use, Batterman said he thinks a more expensive store-bought purifier is worth the money, because their filters generally need to be replaced less frequently.

How do I make my own air purifier?

If you’d prefer a video guide, the hosts of This Old House would be happy to explain how to make a multi-filter design that is more effective and can cost up to $120. This written guide has more details.

The New York Times has video instructions for building the cheaper, one-filter version, and this Twitter thread has useful photos.

If you’re ready to start building, here are the basics based on my experience building the simple, one-filter version.

  1. Start with a box fan, some duct tape and a 20 x 20 inch MERV 13 furnace filter.
  2. Align the filter and fan so that the airflow direction of the filter, indicated by an arrow on the casing, is the same as the airflow direction of the fan.
  3. Tape the filter and fan together, using enough tape to prevent air from leaking between them.
  4. Using cardboard or extra tape, cover the corners of the fan — like this — to improve the air flow.
  5. Turn the fan to its highest setting to begin filtering air. Close any windows in the room where your new filter is operating.

Koby (he/him) believes that love drives people to fight for their communities, and that curiosity is food for love. He enjoys the many moods of the Detroit River.