• Posted May 5, 2025

Wildfire Smoke Flooded ERs With Asthma Cases

Asthma attacks caused emergency room surges during the 2023 Canadian wildfires, with smoke-filled air sending hundreds to the hospital daily, a new study says.

Daily asthma-related visits to Ontario hospitals increased nearly 24% during an early-June 2023 wildfire that blanketed the area with smoke, researchers reported today in the Canadian Medical Association Journal.

“The unprecedented wildfires of 2023 are a wake-up call that wildfires — a persistent feature of Canada’s landscape — are becoming more intense and prolonged in a changing climate, affecting millions of people,” concluded the research team led by Hong Chen, an assistant professor with the University of Toronto Dalla Lana School of Public Health.

However, a second wave of wildfire smoke that descended on Ontario later that same month did not result in an increase in asthma-related ER visits, researchers found.

It’s likely that asthma patients caught unprepared for the first smoke event had taken precautions to help them get through future episodes, researchers speculated.

“Ontario’s population was unaccustomed to heavy wildfire smoke episodes and was therefore unlikely to have adopted anticipatory measures or behaviors” prior to the first episode, researchers wrote.

The second time around, they likely had access to asthma medications and had taken steps to avoid the smoky air, such as staying indoors and using air filters, researchers said.

For the study, researchers analyzed data on ER visits to Ontario hospitals during the first and second smoke waves, which caused “some of the world’s worst air quality,” they wrote.

An unusually dry spring in 2023 led to 155 simultaneous fires across Quebec in early June, including a 1.2-million-acre monster fire in southern Quebec.

“These fires displaced thousands of people in Canada, destroyed property, and blanketed vast areas of North America with smoke,” researchers wrote.

ER records revealed daily increases of 11% to 24% in asthma-related visits during the first wave of smoke that passed over Ontario, continuing up to 6 days after the event ended, researchers said.

The smoke’s effect on people with asthma had diminished by the time the second wave came in later June, even though the air had been even worse, researchers found. 

“As wildfires emerge as one of the fastest-growing environmental risk factors globally, future research is needed to better understand the dose–response relationship, especially in relation to moderate wildfire exposure, and to identify and evaluate measures to effectively mitigate the acute health effects of wildfire smoke,” concluded the researchers.

More information

The U.S. Centers for Disease Control and Prevention has more on the health effects of wildfire smoke.

SOURCE: Canadian Medical Association, news release, May 5, 2025