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How to Stay Safe from Wildfire Smoke Indoors

Is it safe to turn it on an AC? If you have an air purifier, where should you put it? An air quality expert answers our pressing questions.

A blueprint and smoke.
Heatmap Illustration/Getty Images

You’re in your apartment, windows closed, hiding out from the wildfire smoke blanketing your city. But it’s starting to smell a little bit like barbecue, and your eyes are getting watery. What should you do?

Wildfire smoke contains tiny particles, invisible to the human eye, that can enter your lungs and bloodstream. Those particles can exacerbate the risk of having an asthma attack, heart attack, and stroke. They also have lasting effects on your heart and respiratory system, and can lead to premature death. You really want to take the likelihood that smoke is getting into your home seriously.

But that can lead to a lot of questions.

Maybe you have an air purifier. Where should you put it?

Maybe you don’t have an air purifier, but you have a window air conditioner. Is it safe to turn it on?

I called up John Volckens, a professor of mechanical engineering at Colorado State University, and grilled him on every home configuration I could think of to understand how you can protect yourself against the dangers of smoke. Volckens studies air quality, exposure science, and air pollution-related disease. He has even pioneered the development of new pollution sensor technologies. Hehad a lot of helpful tips to offer.

Is smoke entering my home?

It’s important to understand there’s really no way to fully prevent smoke from getting inside your home. Even if you don’t smell it or feel its effects, you should do what you can to protect yourself.

That’s because most homes and buildings “breathe.” As the sun heats the upper reaches of the building, it warms the air, which expands and wants to escape. As the air flows out, the lower part of the house, where it’s cooler, draws new air in to replace it.

“The analogy would be like if a 6-foot flood of water came to your house. It doesn’t matter how many sandbags you have, the water’s coming in, right?” Volckens said. “If the air quality index is like 400 outside for a few hours, it’s going to get to like 200 inside your home no matter what.”

The number one thing you can do is get an air purifier. You might not be able to find one in stores right now or have one delivered in time, but there are other options, too, as I’ll discuss below.

I have an air purifier, where should I put it?

Place it wherever you are.

We spend a third of our lives in bed, so Volckens said he likes to have one in the bedroom. “If you have seasonal allergies, creating that kind of safe space for your immune system can be really helpful,” he told me.

But if you only have one device, when you’re done sleeping, just pick it up and move it into the kitchen, office, or living room with you. It should only take about an hour to work its magic and get whatever room you’re in to the best air quality that it’s capable of.

I don’t have an air purifier. Should I turn my air conditioner on?

For Window AC users:

Window AC units work by recirculating the air in your apartment, so they won’t exacerbate the issue and are generally safe to use. The filters in your window units won’t do much to improve your air, though — they are designed to catch larger particles like dust and animal fur, and smoke particles will slip right through.

For Central AC users:

If you have a central air conditioning system that delivers cool air through ducts and vents, that’s another story. Those are typically designed to draw in air from the outside. In that case, the best thing to do is install what’s called a MERV filter, which you can purchase at most hardware stores or big box stores. Volckens recommends picking up a filter rated MERV-13, which can capture the smallest particles at a relatively high rate.

“It will probably be like 75% efficient. So if 100 wildfire smoke particles pass through that filter, only 25 will get through,” he said. “You're going to knock down the concentration significantly, especially as that filter keeps cycling air through your home.”

The one thing to keep in mind is that these filters are so good, they will get clogged quickly. A clogged filter will cause your HVAC system to work too hard, which could lead to mechanical issues, so make sure you remember to replace it every couple of months.

What's the alternative to an air purifier?

DIY air filters are surprisingly easy to make and incredibly effective. No, really. All you have to do is buy a box fan, duct tape, and a MERV filter. Tape the filter to the back of the box fan. That’s it.

“They work just as well as commercial air cleaners,” said Volckens. They’re a little bit louder, but otherwise, it’s the exact same idea. “A commercial air purifier might have a fancier fan and a fancier filter, but it’s still just a fan and a filter.”

And if you want to get a little fancier, you can build what’s called a Corsi-Rosenthal box. It’s the same idea, but envelops the fan intake in four filters instead of one. TheWashington Post has a very easy-to-follow video showing how to build one. “They work as best as the highest-end air cleaners you could buy for one-fifth or one-tenth of the cost,” said Volckens.

I’ll buy or build an air filter eventually, but what can I do in the meantime?

N-95 masks, like the kind recommended to protect against COVID-19, also effectively filter out pollution, and are your best bet. Even a blue surgical mask will be somewhat helpful, said Volckens.

If you’re tired of being cooped up at home, you can also find what Volckens likes to call “clean air zones.” He recommended the public library, a Starbucks, or any other public building. Go support your local movie theater.

“Most public buildings actually have more efficient air cleaning than homes because the buildings were built more recently and they’re built to code standards that require cleaner air.”

Once the smoke clears, then what? What if my house still smells?

Wildfire smoke is truly disgusting. The particles can contain thousands of chemicals, and they will stick to any surface they touch — the ceiling, the carpet, your clothing. You can certainly wash your clothes and linens, but it might not be possible to scrub every surface of your home.

“The best thing you can do when the air does clean up is to just open all your windows and get some good air exchange going,” said Volckens. “I guarantee, yes, you’ll have that wildfire smell for a couple of days, but it will eventually go away.”

Before we hung up, I asked Volckens if there were any other tips we didn’t cover.

“The only thing I’ll say is that this problem isn’t going away,” he said. “And it’s our doing, right? This is the result of a warming planet.”


This article was last updated on June 28, 2023.

Read more about wildfire smoke :

The 5 Big Questions About the 2023 Wildfire Smoke Crisis

Wednesday Was the Worst Day for Wildfire Pollution in U.S. History

When There’s Smoke, Getting Indoors Isn’t Enough

How Many People Will This Smoke Kill?

Nowhere Is ‘Climate Proof’

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Emily Pontecorvo

Emily is a founding staff writer at Heatmap. Previously she was a staff writer at the nonprofit climate journalism outlet Grist, where she covered all aspects of decarbonization, from clean energy to electrified buildings to carbon dioxide removal. Read More

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