How Did Canadian Wildfires Impact Emergency Department Visits for Asthma Syndrome in New York?


New research on the effects of the Canadian wildfires on asthma syndrome in patients sheds light on the downstream effects of ecological disaster on regions outside of those affected.

The sharp increase in asthma syndrome emergency departments in New York City in June of 2023 is acutely associated with the concurrent Canadian wildfires, according to a new research letter, with all boroughs impacted and the rates spiking the most in the second day of the smoke wave.1

This new data resulted from a study conducted to assess the most significant repercussions of the 2023 wildfires in Canada for those not directly affected by the fires, given the apparent impacts on air quality in both nearby and more distant regions.2 This research was led by Kai Chen, PhD, from the Department of Environmental Health Sciences at Yale School of Public Health.

Chen and colleagues noted that from June 6 - 8, 2023 when smoke from the wildfires in Quebec reached the city—several hundred kilometers removed—this led to greater levels of fine particulate matter (PM2.5) observed in the atmosphere.3 Additionally, previous data had shown that wildfire-generated PM2.5 can adversely impact cardiovascular health, respiratory health, birth outcomes, and even mental well-being.

“However, previous work has mostly focused on populations residing near and affected directly by wildfires,” Chen and colleagues wrote. “We examined the association between the 2023 Canadian wildfires and asthma syndrome emergency department (ED) visits in NYC."

Background and Findings

The investigators began their research by collecting data from New York City’s syndromic surveillance system, utilizing de-identified data from asthma syndrome emergency department visits. The syndromic surveillance system of the City’s Department of Health and Mental Hygiene (DOHMH) is known to compile data on the ages of patients, visits, and residential zip codes of all 53 emergency departments found in the City.

The patients’ asthma syndrome was noted by the research team through the use of a text-processing algorithm. Additionally, the team received the ambient PM2.5 concentrations seen in the City from 10 local monitoring stations of the US Environmental Protection Agency (EPA).

The investigators defined a ‘wildfire smoke wave’ as a specific period with a minimum of 2 consecutive days in which the daily mean PM2.5 levels were shown to have exceeded a specified threshold which was, in this case, 56.8 μg/m3. The team compared patients’ asthma syndrome-related emergency department visits at the time of these waves with reference periods, with the team controlling for day-of-the-week effects.

In order to look into the association, the research team calculated the incidence rate ratio (IRR) and 95% confidence intervals (CIs). Then they performed subgroup analyses for various boroughs and age ranges , with sensitivity analyses being conducted by the team with alternative reference periods.

The investigators also used R version 4.2.3 for their analysis of the data, and statistical significance was then set by the team at a 95% CI not crossing 1.

Overall, the investigators ended up finding that the Canadian wildfires—despite their considerable distance from New York City—were shown to be linked to a substantial rise in visits to emergency departments in the City for asthma syndrome cases during the smoke wave occurring in June 2023. The research team pointed out that the association was shown to be most striking on the second day of the wave, adding that it impacted individuals in the age range of 5 - 64 years over all of the city's boroughs.

However, the investigators also noted the study’s specific limitations, explaining that its ecological study design focused entirely on New York City and examined only a single acute outcome from syndromic surveillance data. Additionally, the team explained that they did not consider alterations in the patterns of population activity or patient movement.

“Given the findings and that wildfires have become more frequent and larger in recent years as a result of a warming climate, timely communication about limiting wildfire smoke exposure is needed to protect vulnerable populations,” they wrote.


  1. Chen K, Ma Y, Bell ML, Yang W. Canadian Wildfire Smoke and Asthma Syndrome Emergency Department Visits in New York City. JAMA. Published online September 21, 2023. doi:10.1001/jama.2023.18768.
  2. Natural Resources Canada. National wildland fire situation report. Accessed July 5, 2023.
  3. Reid CE, Brauer M, Johnston FH, Jerrett M, Balmes JR, Elliott CT. Critical review of health impacts of wildfire smoke exposure. Environ Health Perspect. 2016;124(9):1334-1343. doi:10.1289/ehp.1409277.
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