Canadian Wildfire Smoke and Asthma Syndrome Emergency Department Visits in New York City

The wildfires in Canada in 2023¹ deteriorated air quality locally and distantly. For example, the smoke of wildfires in Quebec drifted into New York City (NYC), hundreds of kilometers away, and caused increased ambient fine particulate matter (PM_2.5) on June 6 to 8, 2023. Wildfire PM_2.5 has been shown to affect respiratory health, cardiovascular health, birth outcomes, and mental health.² However, previous work has mostly focused on populations residing near and affected directly by wildfires.^3,4 We examined the association between the 2023 Canadian wildfires and asthma syndrome emergency department (ED) visits in NYC.

Deidentified asthma syndrome ED visits were obtained from the NYC syndromic surveillance system,⁵ which records information on patients’ visits, age, and residential zip code each day from all 53 EDs in the city. Asthma syndrome includes ED chief complaint mention of asthma, wheezing, or chronic obstructive pulmonary disease and was identified by a text-processing algorithm.⁵ Ambient PM_2.5 concentrations from 10 local monitoring stations were obtained from the US Environmental Protection Agency. This study used public surveillance data and was exempt from further ethical review by the Yale University institutional review board.

We defined the wildfire smoke wave as at least 2 consecutive days with daily mean PM_2.5 exceeding the maximum level (56.8 μg/m³) during a baseline period of January 2021 to May 2023, which was June 6 to 8, 2023. To control for the day-of-the-week effect, we used 2 adjacent, nonsmoke periods with the same days of the week as the smoke wave as the reference period: May 30 to June 1 and June 13 to 15. To estimate the association between the smoke wave and asthma syndrome ED visits, we calculated the incidence rate ratio (IRR) and 95% CIs of visits between the smoke wave and reference period. We also conducted subgroup analyses by age groups and boroughs and sensitivity analyses using alternative reference periods (May 30-June 1 and June 13-15 separately, or May 23-25 and June 20-22) and excluding visits with missing zip code (3.8% [30/783] and 5.1% [55/1089] during smoke wave and reference period). The short study period helped mitigate potential population-level confounding. Analyses were conducted using the fmsb package in R version 4.2.3 (R Foundation for Statistical Computing) and statistical significance was defined as a 95% CI not crossing 1.

During the smoke wave, citywide daily mean PM_2.5 levels reached 100.9 μg/m³ (reference, 9.0 μg/m³) and asthma syndrome ED visits increased to 261 per day (reference, 181.5 per day), both peaking on June 7, 2023 (Figure and Table). In contrast, daily mean temperature was generally similar during the smoke wave and the reference period (22.6 vs 24.8 °C).

Figure.  Daily Fine Particulate Matter Concentration Before, During, and After the 2023 June Wildfire Smoke Wave in New York City

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NYC indicates New York City; PM_2.5, fine particulate matter.

Table.  Asthma Syndrome ED Visits by Age Groups and Boroughs in New York City Before, During, and After the Wildfire Smoke Wave in June 2023^a

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	Population	No. of ED visits			IRR (95% CI)
		Before smoke wave (May 30-June 1)	Smoke wave (June 6-8)	After smoke wave (June 13-15)
New York City
All age groups	8 335 897	523	783	566	1.44 (1.31-1.58)
Age 0-4 y	516 826	19	28	31	1.12 (0.71-1.78)
Age 5-17 y	1 225 377	87	112	83	1.32 (1.04-1.67)
Age 18-64 y	5 334 974	344	562	394	1.52 (1.36-1.70)
Age ≥65 y	1 258 720	73	81	58	1.24 (0.94-1.63)
Borough
Bronx	1 379 946	146	215	156	1.42 (1.20-1.70)
Brooklyn	2 590 516	173	247	192	1.35 (1.15-1.59)
Manhattan	1 596 273	97	138	93	1.45 (1.17-1.81)
Queens	2 278 029	69	128	89	1.62 (1.28-2.05)
Staten Island	491 133	13	25	6	2.63 (1.45-4.78)

We estimated significant increases in asthma syndrome ED visits during the smoke wave compared with nonsmoke periods in NYC overall (IRR = 1.44; 95% CI, 1.31-1.58) and all of its 5 boroughs (Table). We also estimated significant increases in visits among groups aged 5 to 17 years (IRR = 1.32; 95% CI, 1.04-1.67) and 18 to 64 years (IRR = 1.52; 95% CI, 1.36-1.70). Similar IRRs were estimated using the alternative reference periods (May 30-June 1, 1.50 [95% CI, 1.34-1.67]; June 13-15, 1.38 [95% CI, 1.24-1.54]; and May 23-25 and June 20-22, 1.52 [95% CI, 1.39, 1.67]) or excluding visits with missing zip code (1.46; 95% CI, 1.33-1.60).

Despite being located hundreds of kilometers away, the Canadian wildfires were associated with increased asthma syndrome ED visits in NYC during the June 2023 smoke wave. This association was acute (highest in the second day of the smoke wave) and affected individuals aged 5 to 64 years and all boroughs. Study limitations include the ecologic study design, study area restricted to NYC, examination of only one acute outcome based on syndromic surveillance, and lack of accounting for changes in population activity patterns or 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.