What To Know About Wildfire Smoke

Lately, when summer approaches, there’s an underlying feeling of dread in the Western U.S. Although the skies start off bright and blue above fields of cheery mountain wildflowers, in the back of our minds we’re all wondering when wildfire smoke will change all of that and drive us indoors.

Everyone I know out west lives under the shadow of impending smoke, and a lot of us have had to evacuate at some point due to an encroaching fire. The size, duration and frequency of those wildfires have ramped up dramatically in the last 20 years, fueled in large part by human-caused climate change. In 2023, the midwestern and eastern U.S. got a taste as well when smoke from Canadian wildfires drifted into town.

All of that smoke is not just gloomy, it’s dangerous to our health. Here’s what to know about what’s in it and how to keep yourself safe when it’s in the air.

What Is Wildfire Smoke?

Wildfire smoke comes from uncontrolled fires burning in remote areas such as forests and grasslands. Wildfires can also occur in wildland-urban interfaces, which is where human development meets wild areas. Those fires can sometimes travel into urban areas, too, as they did during the 2021 to 2022 Marshall Fire in Colorado.

Wildfires can be caused by lightning strikes or by elements associated with people, like arson, faulty electrical infrastructure, campfires, barbecue grills and errant cigarette butts. Once a fire starts, it injects aerosols (tiny suspended liquid and solid particles) into the atmosphere that block and scatter sunlight, creating visible smoke plumes.

What Is in Wildfire Smoke?

Particles from burning vegetation and building materials, or particulate matter (PM), are part of what makes up wildfire smoke.

“The PM in wildfire smoke can range from larger visible particles, like ash and soot, to smaller particles that can be inhaled deeply into the lungs by bypassing our natural defense mechanisms [like coughing, sneezing and swallowing] that prevent us from inhaling larger particles,” says Dr. Vijay Limaye, senior scientist at the Natural Resources Defense Council.

Those small particles are called PM2.5s because they are 2.5 microns or less, which is about 30 times smaller than the width of a human hair.

Wildfire smoke also contains gases, including carbon monoxide (CO), volatile organic compounds (VOCs), nitrogen oxides (NOx) and ozone.

“The chemical composition varies not only from fire to fire, but also within a fire, [like whether it’s] flaming vs smoldering,” says Dr. Kang Sun, assistant professor at the University at Buffalo. Other factors include the types of vegetation and structures in the burn area, weather conditions and the presence of toxic metals like lead.

How Does Wildfire Smoke Travel?

Smoke gets heated by a wildfire and then rises into the atmosphere. From there it gets transported long distances, sometimes hundreds or thousands of miles. Factors that influence how it travels include wind patterns, atmospheric stability and topography. Rain is one of the most efficient ways of clearing particles from the air, says Sun, but the smallest particles can remain aloft for days afterward.

Wildfire Smoke Health Risks

Wildfire smoke poses direct dangers to humans, pets and wildlife. It also contributes to ozone smog air pollution. Most pressingly, PM2.5 is a dangerous air pollutant regulated through the Clean Air Act, and recent evidence suggests it is 10 times more dangerous than the same types of particulates generated from burning coal and driving gas-powered cars. Once inhaled, PM2.5 can enter the bloodstream, causing a range of harmful effects from coughing and shortness of breath to adverse effects on cardiovascular health.

“More frequent wildfire smoke days are already threatening this country’s progress to clean up its air,” says Limaye. It’s also proving to be expensive. One study identified billions of dollars in health-related costs stemming from wildfires and smoke exposures in just two states.

Beyond human health, smoke affects nature by blocking out sunlight, changing the chemistry of lakes and streams and stressing plants. Direct pollution from firefighting chemicals is also a danger to ecosystems and humans.

How To Find the Most Reliable, Up-To-Date Information About Wildfires

The U.S. Environmental Protection Agency (EPA) provides real-time air-quality data, information on wildfires and local smoke conditions through its AirNow website and through its AirNow app. The site also includes short-term forecasts for air quality.

How To Get Wildfire Smoke Out of Your Home

It’s best to try to reduce your exposure to wildfire smoke in the first place, but if it does get in your home, the EPA recommends the following steps:

• Keep doors and windows closed to prevent smoke from entering your home.
• Use high-efficiency air purifiers with HEPA filters to reduce indoor smoke levels.
• Avoid activities that can introduce air pollution, like smoking and burning candles.
• Create a clean air room for vulnerable people by using a portable air cleaner or running the air conditioner.

“Close ventilation when the air quality index is orange or above, and open when it turns green,” says Sun. “Filters and air purifiers for PM2.5 work for the smoke.”

A DIY air filter known as the Corsi–Rosenthal Box is also a helpful tool, says UC Davis Professor Anthony Wexler.