Even after the flames have died down, invisible particles in the air can contribute to a number of health problems.

By Amanda MacMillan
Updated September 10, 2020
Credit: Luis Sinco/Getty Images

Whether due to a stray ember from a smoldering campfire, a cigarette butt, or a failed gender-reveal pyrotechnics stunt, humans are responsible for nearly nine in 10 wildfires in the US. Of course, lightning strikes can also ignite wooded and dry, grassy areas. Add heat and high winds to the mix, as is common in California, and you've got the ingredients for an uncontrolled and dangerous burn.

In addition to the immediate threat of wildfires sweeping through residential areas, these out-of-control blazes also produce smoke that can linger in the air for days or even weeks afterward. The gases and fine particles that wildfires produce can travel hundreds or even thousands of miles and affect the health of people well out of the fire’s path.

Instagram users captured eerie images in the wake of recent conflagrations. Ash and smoke cast an orange pall over the San Francisco Bay Area, 150 miles southwest of one outbreak. "Did I wake up on Mars or is the apocalypse coming?" posts bigmoonmandybot. Fires in Oregon turned the Pacific Northwest sky an ominous blood-red hue.

Wildfire smoke is filled with really nasty stuff

Breathing in any kind of smoke is unpleasant, but scientists are particularly worried about wildfire smoke because of the combination of chemicals it tends to contain. In addition to burning trees, these fires are also consuming homes, automobiles, businesses, and industrial facilities—which all contribute their share of toxic gasses and particulate matter.

“Just think about all the chemicals people keep in their garages,” says Jennifer Horney, PhD, professor of epidemiology and member of the Disaster Research Center at the University of Delaware. “Pesticides, paint, you name it—and when those things burn, along with everything else, you’re breathing in the product of all that combustion.”

Forests and crops are increasingly being treated with flame-retardant chemicals and pesticides as well, according to a 2017 review in the journal Current Topics in Toxicology. “The research on air pollution used to focus on the health effects of breathing in one specific chemical or another,” says Horney, “but recently there’s been a lot more interest in the health impact of these types of mixtures of all different compounds.”

Air pollution has been linked to real health risks

A review in Science of the Total Environment concluded that exposure to wildfire smoke or particulate matter was associated with an increased risk of asthma, chronic obstructive pulmonary disease (COPD), bronchitis, and pneumonia. It was also linked to early death. (The study looked for a connection between smoke exposure and cardiovascular problems, as well, but results were inconclusive.)

Of course, there are the immediate effects of breathing in smoke and soot, especially if you're close to the fire itself and the smoke is especially thick. Smoke inhalation symptoms can include a cough, shortness of breath, injury to the throat and lungs, and, in extreme cases, can cause oxygen to be cut off from the heart—which could be fatal.

But more long-term, poor air quality has also been linked to an increased risk of diabetes, kidney disease, fertility problems, and spikes in blood pressure. Some research suggests it may even be linked to neurodegenerative diseases like Alzheimer's.

Research shows that children, pregnant women, the elderly, and people with pre-existing respiratory or cardiovascular conditions are the most vulnerable to these and other health risks from air pollution. And the smaller the pollution particles are, the greater the danger.

“Wildfires can dramatically increase the levels of fine particulate matter, which are little aerosols that are so tiny they can travel through your lung tissues and other tissues in your body,” Jia Coco Liu, PhD, a research associate at Johns Hopkins University, told TIME after California fires in 2019. These particles—also known as PM2.5 because they have a diameter of less than 2.5 micrometers—are too small to see or smell.

Fine particulate matter tends to stay in the air longer and travel farther, compared to heavier components of smoke that settle to the ground more quickly, such as dust and ash. This makes them a concern not just for people affected by wildfire smoke, but also for people in neighboring cities, counties, or even states.

How to stay safe when fires are nearby

If you haven’t already, sign up for daily air-quality alerts at AirNow.gov, which will give you information about both visible air pollution and fine particulate matter. (These alerts can be helpful even when there’s not a threat of wildfire, because they also keep track of ozone levels and other types of urban or industrial pollution.)

Then, try to stay indoors on days when air quality is low—and definitely try to limit strenuous outdoor exercise or labor, which increases the amount of air you’re breathing in. “It might not be a good idea to go for an hour run outside if you know there are fires nearby,” Liu told TIME.

The US Environmental Protection Agency also recommends running household air conditioners, as long as the filter is cleaned and the “fresh air” setting—which brings in air from outside—is disabled. This will circulate air internally and keep much of the dangerous smoke out of your home.

While you’re indoors without access to fresh air, avoid smoking, cooking, burning things (like wood, stoves, or candles), and running the vacuum, which stirs up dust and particles that have settled in the carpet.

Many people reach for dust masks or surgical masks when air quality is low, but Horney says most store-bought products don’t offer much protection. “They need to be properly fitted, and they need to be the right type to keep out the smallest particulate matter,” she says. Experts recommend looking for a “particulate respirator” mask with the word NIOSH (National Institute for Occupational Safety and Health) and either N95 or  P100 printed on it, which are designed to block at least 95% of small (0.3 micron) particles. (Of course, particulate-filtering masks are in short supply these days due to the COVID-19 pandemic.)

Don't ignore health and safety warnings

In addition to monitoring air quality, it’s also important to know whether the fires near you are still spreading. “The situation changes so rapidly with wildfires; it’s not like a hurricane where you have days to prepare,” says Horney.

Have an evacuation plan and an emergency go-bag ready, says Horney, and stay tuned to local news and emergency broadcasts. And if you or a loved one has asthma or another chronic condition, make sure you have any medical supplies you might need.

Finally, if you’re returning to an area that’s been affected by wildfires, heed all safety warnings and be cautious about sifting through wreckage. “The fine particulate matter may still be floating around, but a lot of the larger particles are going to have settled,” says Horney. “If you’re walking around or cleaning up those areas, you’re going to redistribute some of those chemicals back into the air.”

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