Canadian Wildfire Smoke Increased Asthma-Related ED Visits in the U.S.

— New York saw an 82% increase in such visits on the day with the highest recorded pollution

A photo of pedestrians on the streets of Manhattan under an orange sky and haze from Canadian wildfire smoke.

Exposure to smoke originating from the Canadian wildfires led to a rise in asthma-associated emergency department (ED) visits in the U.S., with a large spike seen in New York during days with the highest recorded pollution, CDC data showed.

Asthma-related ED visits across the U.S. were 17% higher than expected during the 19 days with wildfire smoke that occurred from April 30 to August 4, reported Cristin McArdle, PhD, of the CDC's Epidemic Intelligence Service and the National Center for Environmental Health, and colleagues in the Morbidity and Mortality Weekly Report.

A simultaneously published report also showed that daily mean concentrations of particulate matter with aerodynamic diameter ≤2.5 µm (PM2.5) were highest on June 7 for all regions in New York, except the Adirondacks, leading to an 81.9% increase in asthma-associated ED visits statewide compared with the mean number of ED visits during June 1-5, according to Haillie Meek, DVM, of the New York State Department of Health and CDC's Epidemic Intelligence Service, and colleagues.

Reached for comment, Alison Lee, MD, of the Icahn School of Medicine at Mount Sinai in New York City, suggested that high-risk groups consider protective measures on days where the air quality is worse.

"If you fall into a sensitive group, wearing a well-fitted mask -- an N95 or KN95 mask -- can help filter out some of those particulates," she told MedPage Today. "Staying indoors and making sure that you have good air filtration is also important on those days where the air quality is poor."

Lee noted as well that Wildfires can no longer be thought of as isolated events, and they can cumulatively begin to affect healthy populations as well.

"These fires have been burning almost continuously," she said. "So then we're talking about a situation where it's not bad air quality on one day, but rather repeated events, multiple-day or multiple-week exposures to poor air quality. I think then you'll have the potential to see health impacts on otherwise healthy groups."

With the possibility of wildfires being more common, more research needs to be conducted on the potential health impacts, including long-term effects, she added, and clear guidelines regarding the Air Quality Index (AQI) should be released.

U.S. Study

The report by McArdle and team showed that the HHS-defined regions most affected by the wildfire smoke were Regions 2 (New Jersey, New York, Puerto Rico, and the U.S. Virgin Islands), 3 (Delaware, District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia), and 5 (Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin). Region 3 had five wildfire smoke event days, while regions 2 and 5 had four days each.

Wildfire smoke event days are defined at the HHS region level as "when at least one Environmental Protection Agency (EPA) air quality monitor in the region measures ambient 24-hour average PM2.5 concentrations ≥35.5 µg/m3, corresponding to the EPA AQI value of 101." An AQI of 101-150 is considered to be "unhealthy for sensitive groups," 150-200 is "unhealthy," 200-301 is "very unhealthy," and anything above 301 is considered "hazardous."

Region 2 had the biggest increase in asthma-associated ED visits, with higher-than-expected visits occurring for all age groups on 2 days from June 6-8, representing 364 excess visits. Region 3 had 179 excess asthma-associated ED visits across all age groups, and region 5 had 172 excess visits. When broken down by age, patients ages 18-64 experienced the greatest number of excess visits across all three regions.

Other regions experiencing higher-than-expected asthma-associated ED visits included Regions 4 (Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee), 8 (Colorado, Montana, North Dakota, South Dakota, Utah, and Wyoming), and 10 (Alaska, Idaho, Oregon, and Washington).

New York Study

Narrowing in on New York, a total of 1,310 asthma-associated ED visits occurred from June 1-14, as recorded in the New York State Department of Health's Electronic Syndromic Surveillance System, which includes the 134 EDs in New York excluding New York City. The largest region-specific increases in asthma-associated ED visits occurred in the Eastern Lake Ontario (179.1%), Central (132.8%), and Upper Hudson Valley (86.4%) regions.

There were increases in asthma-related ED visits across several age groups, with an increase of 197.6% in those ages 10-29, an increase of 89% in those ages 50-69, 77.1% in those ages 30-49, and 76.5% in those 70 and older. Children ages 0-9 years saw a 7.4% decrease in visits.

McArdle and colleagues highlighted the role of climate change in the risk of wildfire smoke exposure. Research has indicated that climate change is already impacting wildfires, with increased temperatures, more dry seasons, and longer wildfire seasons, leading to situations like the Canadian wildfires becoming more common.

According to Lee, "until we act to impact climate change, these wildfires are going to continue to be concerns we will have to face. ... I think we really need to think about how we can push our politicians and our representatives to try to tackle climate change. Until we do that, this wildfire smoke is just going to get worse."

  • author['full_name']

    Elizabeth Short is a staff writer for MedPage Today. She often covers pulmonology and allergy & immunology. Follow


McArdle, Meek, and Lee reported no conflicts of interest.

One co-author reported relationships with the European Respiratory Society and the Career Mentoring Committee of the American College of Epidemiology.

Primary Source

Morbidity and Mortality Weekly Report

Source Reference: McArdle CE, et al "Asthma-associated emergency department visits during the Canadian wildfire smoke episodes -- United States, April-August 2023" MMWR 2023; DOI: 10.15585/mmwr.mm7234a5.

Secondary Source

Morbidity and Mortality Weekly Report

Source Reference: Meek HC, et al "Asthma-associated emergency department visits during a wildfire smoke event -- New York, June 2023" MMWR 2023; DOI: 10.15585/mmwr.mm7234a6.