Asthma-Related Emergency Department Visits Rose Following Canadian Wildfires

An increase in asthma-related emergency department visits took place across Ontario in the aftermath of the Canadian wildfires in early June of 2023.1,2

These findings on asthma-related emergency department visits were highlighted in a recent publication in the Canadian Medical Association Journal titled "Impact of the 2023 wildfire smoke episodes in Ontario, Canada, on asthma and other health outcomes: an interrupted time-series analysis." It was led by such investigators as Hong Chen, PhD, a scientist with Health Canada, ICES, and Public Health Ontario.

“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,” Chen and colleagues said in a statement.1

The investigative team carried out a quasi-experimental analysis with the aim of assessing the health impact of 2 successive wildfire smoke events that took place in June 2023 in Ontario, Canada. The wildfire episodes, which took place in early and late June, were shown to have resulted significant air quality deterioration across the province.

This was highlighted as Ontario had experienced some of the most hazardous air conditions across the world, with the wildfires burning in neighboring Quebec pointed to as the major cause.

Chen and colleagues thus sought to assess visits that took place at emergency department within a timeframe spanning from 8 weeks prior to the first major smoke event in early June through to the end of July 2023. This would be 4 weeks following the second smoke episode. They gathered data from 30 public health units, encompassing 95% of Ontario’s population.

Using an epidemiologic triangulation approach, the research team assessed visits at emergency departments for 4 specific health outcomes: asthma-related issues, ischemic heart disease, other respiratory conditions, and non-cardiorespiratory causes. For their data sources, the team utilized Ontario’s real-time syndromic surveillance system and the National Ambulatory Care Reporting System.

The investigators' analysis applied multiple analytic strategies, with interrupted time-series and case-crossover methodologies being implemented. Overall, Chen et al. found that a marked surge in asthma-related emergency department visits across the province took place after the early June smoke exposure period.2

The peak department visit increase was observed by the investigative team to be 23.6% (95% CI: 13.2%–34.9%), taking place with a single-day lag.2 Additionally, the team pointed to elevated rates that persisted for up to 5 days in the post-exposure period.

In 1 notable finding, the investigators concluded that the second smoke event in late June resulted in an even greater level of air pollution, yet its effect on asthma-related visits was shown to have been less pronounced. Chen and colleagues did not observe statistically significant changes for the other health outcomes they had evaluated during either smoke event.

Their results were found to be consistent across various analytical methods and data sources. In a post hoc analysis, age-specific patterns also emerged for the investigators. For example, asthma-related visits to emergency departments rose briefly by 40% among children but had a more sustained increase of 48% among adults. This increase lasted approximately 1 week following the initial smoke event.

“The biggest impact of wildfire smoke is on acute respiratory morbidity, where its effects are consistently greater than those of air pollution from other sources,” Sarah Henderson, PhD, from the BC Centre for Disease Control, stated in a related commentary.3 “Less clear associations have been shown for a much wider range of acute health outcomes, including cognition, diabetic control, and mental health, among many others.”

References

1. ED visits for asthma spiked during 2023 Canadian wildfires. Canadian Medical Association Journal. May 5, 2025. https://www.eurekalert.org/news-releases/1082211. Date accessed: May 6, 2025.

2. Chen H, Kaufman JS, Chen C, et al. Impact of the 2023 wildfire smoke episodes in Ontario, Canada, on asthma and other health outcomes: an interrupted time-series analysis. CMAJ 2025 May 5;197:E465-77. doi:10.1503/cmaj.241506.

3. Borchers Arriagada N, Horsley JA, Palmer AJ, et al. Association between fire smoke fine particulate matter and asthma-related outcomes: systematic review and meta-analysis. Environ Res 2019;179:108777.