More than 850 wildfires are currently burning in Canada—and the smoke is reaching cities thousands of miles away, from Chicago to New York City. Next week, it’s expected to reach Europe. If the sky is a hazy shade of yellow where you live, here’s what you should know about the potential health impacts.
Even long-distance smoke is harmful
Tiny particles of smoke called PM 2.5—20 to 30 times smaller than the width of a human hair—can easily enter your lungs and even your bloodstream. The effects can range from reduced lung function to increased risk of heart attack and stroke. You might notice some impacts after a few days of exposure; others might show up later.
In a study that looked at the effect of Canadian wildfire smoke reaching Baltimore in 2023, smoky days were associated with an 18% increase in medical visits for heart and lung issues.
In California’s Bay Area in 2020, where smoke from fires across the state caused a record 30 days of air quality alerts (and temporarily turned San Francisco an eerie shade of orange), doctors saw a 43% increase in strokes and other cardiovascular issues.
Less is known about longer-term impacts, because fewer studies have been done yet. But one 2025 study showed that the risk of some effects, from arrhythmia to COPD, lasted at least three months after the smoke was gone.
In Montana, after a fire that caused six weeks of smoky air, researchers studied the impact on residents over the next two years. Changes in lung function started to show up a year after the fire and persisted, raising the risk of lung diseases like asthma and COPD.
The risk in cities
When smoke travels a long distance from wilderness to a city, it can create a new hazard. Beyond the problems from PM 2.5, the smoke can react with pollution from cars and power plants to cause more ground-level ozone. That reaction can happen days after the smoke leaves the fire. Elevated ozone levels can exacerbate asthma and cause other health issues.
“Not only are we having these larger populations exposed [to wildfire smoke], they’re larger urban populations that are being exposed—where there are a lot of underlying exposures from traffic and industry,” says Jennifer Stowell, a research scientist at Boston University’s Center for Climate and Health who studies the impact of wildfire smoke.
Smoke also changes over time, and some particles oxidize and become more dangerous to breathe as the smoke lingers in the atmosphere. One study in Europe found that smoke samples were twice as toxic five hours after they were released from a fire. With more time, they became four times as toxic.
The urban heat island effect, which makes cities hotter, can also trap pollution near the ground longer.
We need more research
As climate change makes wildfires larger and more likely, smoke exposure is changing, and studies will need to catch up.
In Canada, for example, the area burned each year is now roughly double what it was in the 1970s. A record area, the size of England, burned in 2023. Another 5.3 million hectares burned in Canada in 2024, and 2025 was the second-worst year on record. If you’re breathing in more smoke, over longer periods of time, it still isn’t clear what that might mean for your health in five or 10 years.
Researchers are now also beginning to study what’s inside wildfire smoke—the specific components of PM 2.5, which are very different if trees are burning versus houses filled with electronics, plastics, and other products that can release toxic compounds. New methods for modeling now make it possible to study those components.
“We also need more and better air quality monitors, and more funding to follow people who are exposed—before, during, and after fires,” says Stowell.
How to protect yourself
At a distance from a wildfire, the air might not necessarily smell like smoke, but could still be unsafe. The first step: keep track of your local air quality index. Weather apps typically list air quality, and air quality apps like IQAir or PurpleAir can give you more data.
The health risks are greater for children, pregnant women, older adults, and people with existing conditions like heart disease.
But it makes sense for everyone to limit time outside when the air quality index is greater than 150. That’s especially true for exercise outside. That’s “going to cause you to breathe harder and heavier, and you’re increasing your exposure because you’re just bringing more of that smoky air into your lungs,” says Stowell.
If you have air conditioning, set it to recirculate air. It’s also a good idea to use an air filter. If you have a condition like asthma, make sure that you have medication and N95 masks on hand. As smoky air becomes the new normal in the summer and fall, there’s also likely more that cities can do to help prepare.
“Something that I want to study in the future are interventions that would be high efficacy, but probably low cost or low effort to help protect other people,” says Mary Maldarelli, a fellow in pulmonary and critical care at the University of Maryland Medical Center, who started studying the impact of wildfire smoke after seeing her own patients struggle to breathe. “Things like whether we can distribute masks, or just warn people that there are bad days ahead and that you should stay inside. I think that there’s some of those low-cost interventions that could help.”