Canadian Wildfire Smoke and Asthma Emergency Visits in the U.S., 2023

Summary

• Wildfires and their smoke are increasing across the United States, and so is concern about their health effects.
• From April 30 to August 4, 2023, smoke from wildfires in Canada reached most of the contiguous United States. On the 19 days counted as smoke days, emergency department (ED) visits for asthma were 17% higher than expected.
• The rise in asthma visits during and after smoke episodes can help health communicators, clinicians, policymakers and the public track and reduce exposure for people with asthma.

Why smoke matters

Wildfire smoke is a mix of gases and particles. The most worrying part is fine particulate matter (PM2.5) — particles generally 2.5 micrometers across or smaller — which can worsen heart, metabolic and lung conditions and send people to EDs and hospitals as exposure rises and falls. Much less is known about long episodes of very heavy smoke like 2023's, so a quick look at the health effects was needed to guide warnings.

How it was measured

• A smoke day: a day when at least one EPA air-quality monitor in a U.S. Department of Health and Human Services (HHS) region measured a 24-hour average PM2.5 of 35.5 μg/m³ or more — an Air Quality Index (AQI) of 101, the point where air turns "Unhealthy for Sensitive Groups" (AQI 101–150). Higher still come "Unhealthy," "Very Unhealthy" (201–300) and "Hazardous" (301 and above).
• Asthma visits: ED visits with asthma as the chief complaint, from CDC's National Syndromic Surveillance Program, which draws on about 6,000 EDs — 76% of eligible U.S. facilities; 4,317 were included.
• Expected visits: estimated by anomaly-detection methods from the previous 30 days, by region and age group (0–4, 5–17, 18–64, 65 and older). Excess visits are observed minus expected on smoke days.

What was found

• Excess asthma visits were more common on days when more monitors recorded smoke-level PM2.5.
• HHS Regions 2, 3 and 5 had the most smoke days, the highest PM2.5 — daily averages peaking at 204, 259 and 216 μg/m³ — the largest shares of monitors detecting smoke, and the most excess visits. Across smoke days, the share of monitors detecting smoke ranged from 0.5% to 69.0%.

• Region 2 had another smoke day, June 29, with no rise in asthma visits; Region 3 had none on June 28–29.
• Regions 1, 4 and 9 each had one smoke day; only Region 4 saw more asthma visits than expected. Region 7 had 4 smoke days with no increase. Region 8 had 3 smoke days and one day with 18 excess visits (ages 18–64); Region 10 had 4 light smoke days and 14 excess visits (ages 18–64).

Figure 1. HHS Region 2. Figure from the CDC report.

Figure 2. HHS Region 3. Figure from the CDC report.

Figure 3. HHS Region 5. Figure from the CDC report.

What it means

The smoke traveled hundreds of miles, causing multi-state emergencies, air-quality alerts and significant rises in asthma ED visits. Every HHS region except Region 6 had at least one smoke day. Excess visits rose with more smoke, and occurred in every age group, most often among adults 18–64. There was no information on whether people with asthma could reduce their exposure. Some excess visits also appeared on non-smoke days, mostly in children under 5 and aged 5–17, early in the period.

Health departments using syndromic surveillance to watch smoke's effects might start with asthma as an indicator, and could add heart and lung ED visits to gauge the harm more fully.

Limits: the report cannot directly tie the high AQI to the Canadian fires; the surveillance data are not nationally representative and are summarized by region, which can hide local patterns; only ED visits are counted, not primary or urgent care; and an AQI threshold of 101 may miss smoke in places where PM2.5 jumped sharply from a low starting level.

Protecting people

Smoke risk is rising with climate change, land management practices and the growth of homes along the wildland–urban interface — often in places not used to smoke. Syndromic surveillance gives some of the earliest warning signs. Communities should prepare for surges in care. Clinicians can counsel patients — especially people with asthma, chronic obstructive pulmonary disease or heart disease, children, older adults and pregnant people — to:

• follow current and forecast air quality;
• stay indoors and use air filtration;
• wear a properly fitted N95 respirator outdoors.

Track smoke with AirNow's Fire and Smoke Map and app, the Emergency Alert System, and the National Oceanic and Atmospheric Administration's Weather Radio.

Sources

Based on Cristin E. McArdle, Tia C. Dowling, Kelly Carey and others, "Asthma-Associated Emergency Department Visits During the Canadian Wildfire Smoke Episodes — United States, April–August 2023," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's count of smoke days for Regions 2, 3 and 5 is ambiguous, and its AQI bands overlap at 150; this page leaves those figures out.