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Protecting your indoor space from wildfire smoke

A zoomed in shot of a HEPA filter
Daniel Foster
Flickr Creative Commons
HEPA filters can capture more than 99% of the tiny particles in wildfire smoke, according to the EPA.

When wildfire smoke fills the air, public health officials advise going indoors.

However, the tiny, toxic particulate matter in wildfire smoke, known as PM2.5, can enter our homes through cracks, doorways, and heating and cooling systems.

Given that the majority of people in the U.S. spend about 90% of their time indoors, it's within these indoor spaces that they likely experience the bulk of their exposure to wildfire smoke and its fine particles, which can penetrate deep into the lungs.

As climate change continues to create conditions conducive to longer and more intense fire seasons, the risk of smoke exposure in the Western U.S. is expected to increase. Consequently, public health officials increasingly emphasize that merely staying indoors when it’s smoky out is not enough; proactive measures are necessary to ensure clean indoor air. Here's where to start:

1. Keep the smoke out

Source control

The best way to control any indoor air quality problem is to get rid of whatever's causing it, said Bill Bahnfleth, a professor of architectural engineering at Penn State University.

When it comes to a wildfire, you can't avoid the smoke entirely, but you can minimize its intrusion into your house by keeping windows and doors tightly shut. For older homes, weatherization strips and tape can seal cracks and crevices.

Additionally, scientist recommend limiting activities that can generate even more indoor pollutants, such as burning candles, vacuuming or frying food.

Recirculate the air

In general, the next step for ensuring healthy indoor air is ventilation, or bringing fresh, outside air in, said Steven Emmerich, a mechanical engineer in the indoor air quality and ventilation group at the National Institute of Standards and Technology.

"Of course, for smoke from outside, you don't want to do that," he said. "You want to reduce that to a minimum."

So, shut off vents and fresh air intakes on your HVAC or bathroom fan systems, and utilize the air recirculation setting, instead.

If you have a central heating and cooling system, consider replacing the filter with one that is rated MERV 13 or higher. These filters can efficiently trap fine particles found in wildfire smoke, but you may need to consult a technician to ensure your system can work with the higher-efficiency filter.

Smoke from California wildfires up to 200 miles away blankets a residential neighborhood in Sparks, Nev. A study released by the Desert Research Institute, July 13, 2021, suggests a correlation between exposure to the particulate matter in wildfire smoke and risk of contracting the coronavirus. Scientists found that test positivity rates increased last year in Washoe County when the region was blanketed with wildfire smoke from fires in neighboring California. (AP Photo/Scott Sonner, File)
Scott Sonner
Smoke from California wildfires blankets a residential neighborhood in Sparks, Nev. in 2020.

Check the smoke map

You’ll want to keep an eye on the air quality forecast (Idaho DEQ posts localized ones, too) to know when to implement these preventive measures effectively.

If you want to check how much smoke is coming into your home, as well as whether your mitigation efforts are working, you may want to invest in an indoor air monitor. PurpleAir makes the most well-known products in this category, often referred to as “low-cost” sensors because they’re much cheaper compared to the Environmental Protection Agency’s official outdoor sensors. The EPA also has a guide to using these indoor air pollution sensors.

2. Clean the air

Air filtration, which basically means cleaning the air, is key when it's smoky out because you likely can't stop the smoke from getting inside entirely and you also aren't able to ventilate your space as well. There are two primary filtration options recommended by scientists for residential use. Note that during wildfire smoke events, these supplies can sell out, so preparing early is important.

Consider buying an air filter

First, you can purchase a portable air purifier with a High Efficiency Particulate Air (HEPA) filter. When selecting one, consider the following:

  • To effectively capture fine PM2.5 particles, the EPA recommends a filter rated MERV 13 or higher. Filters may need more frequent replacement and cleaning during smoky conditions.
  • Evaluate how much square footage the purifier is designed to clean.
  • Pay attention to the noise rating. The purifier only works if it’s running, and if it’s too disruptive, it may not be used as intended.
  • Avoid purifiers that emit substances like ozone and ions, as these can introduce pollutants.
  • Place the purifier where you spend the most time indoors.
How to make a DIY air cleaner

Do it yourself

If a portable air purifier doesn't align with your budget, you can build your own. You need a box fan, a 20” by 20” MERV 13 filter and some duct tape. Just secure the filter onto the back of the fan with the duct tape and turn the fan on.

The good news? The DIY box fan filters are more cost-effective and the supplies should be available at a local hardware store. Also, scientists at the EPA have studied them, and have found that they’re effective at reducing smoke concentrations. The agency also has some variations to the box fan design that can capture even more particles.

However, the box fan model isn’t designed for continuous use. You’ll need to shut it off when leaving the house, and it tends to be noisier.

Create a “clean room”

Air purifiers can be useful if you are unable to keep smoke particles out of your house effectively, whether due to leaky design or the need to open windows in the absence of air conditioning.

Either way, you may want to designate a specific “clean room” where your household can spend time comfortably. Bedrooms or living rooms often make good candidates. Ensure that windows and doors in this designated room can be sealed to minimize smoke infiltration, and make sure to run a fan and air filter in there. Still, the EPA advises that if you can’t stay cool and smoke infiltration persists, you may want to seek shelter somewhere else.

Find reporter Rachel Cohen on Twitter @racheld_cohen

Copyright 2023 Boise State Public Radio

As the south-central Idaho reporter, I cover the Magic and Wood River valleys. I also enjoy writing about issues related to health and the environment.

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