Impacts of the 2023 wildfire smoke episodes in Ontario, Canada on asthma and other health outcomes: quasi-experimental evaluation

Background — During the 2023 wildfire season, Ontario, Canada, had unprecedented wildfire smoke, but the health impact on the population is unknown. We aimed to quantify the acute impact of the wildfire smoke on respiratory and cardiovascular outcomes across Ontario.

Methods — We conducted a quasi-experimental study by leveraging the timing of 2 consecutive wildfire smoke episodes in June 2023. Heavy wildfire smoke blanketed much of Ontario on 2 occasions, in early June and again in late June, causing severely degraded daily air quality. Following the epidemiologic triangulation framework, we collected health data on emergency department visits for 4 outcomes (asthma-related causes, other respiratory causes, ischemic heart disease, and non-cardiorespiratory causes) from Ontario’s real-time syndromic surveillance system and the National Ambulatory Care Reporting System. We also employed different epidemiologic methodologies, including interrupted time-series and case–crossover analyses.

Results — After the initial heavy wildfire smoke in early June 2023, daily asthma-related visits increased substantially across Ontario, peaking at a 23.6% increase (95% confidence interval 13.2%–34.9%) at a 1-day lag and lasting up to a lag of 5 days after the start of the smoke episode. The later episode of heavy smoke, despite causing higher exposures, had a reduced effect on asthma-related visits. We did not detect any effect on other outcomes in either episode. These findings were consistent across different methodologies and data sources. Post hoc analysis revealed that asthma-related visits were briefly elevated after the wildfire smoke among children (40% higher), but we observed a more sustained effect among adults (48% higher, lasting 1 week).

Interpretation — The 2023 wildfires substantially increased asthma-related emergency department visits in Ontario, with age and timing of exposure being important factors influencing the impact. As wildfires emerge as one of the fastest-growing environmental risk factors globally, future research should identify and evaluate measures to effectively mitigate the acute health impacts of wildfire smoke.