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In 2023, Canada’s most destructive wildfire season on record, asthma-related emergency department (ED) visits in Ontario substantially increased, but the numbers differed greatly by age and timing of the exposure, data indicated. 

There were two major rounds of fires in June of that year, which gave researchers, led by Hong Chen, PhD, a scientist with the Environmental Health Science and Research Bureau of Health Canada and Public Health Ontario, an unusual opportunity to compare ED visits between the two fires. They used data from 30 public health units in Ontario, which cover 95% of the population. The two episodes triggered air-quality advisories and shut down outdoor recreational activities.

The data were published on May 5 in CMAJ.

Asthma Visits Increased

Using a standard interrupted time-series analysis, the researchers found that after the first heavy smoke, daily asthma-related visits peaked at a 23.6% increase from the eight weeks before, and effects lasted for up to 5 days after the start of the smoke episode. The second round of fires did not show an increase in asthma-related visits despite entailing higher exposures to fine particulate matter (PM2.5).

“We did not observe a similar increase in asthma-associated ED visits in the second episode, compared with the first,” Chen told Medscape Medical News. The study wasn’t designed to find out why that was, but “it may have to do with some extended protection from the first to the second episode,” said Chen. “This could include the protective effect of preventive medications prescribed during the first episode, increased supply and use of medication, or behavior changes.” For instance, patients may have used puffers or inhalers more or stayed inside more and used air filters or masks more often, he added.

A post hoc analysis of associations by age showed a brief spike in daily asthma visits among children (up to 40% higher) but a more sustained increase among adults (48% higher, lasting 1 week) during the first fires. No similar increases were observed for any age group during the second round.

Messages for physicians include the need to emphasize precautions when wildfires erupt and to urge patients to stay inside as much as possible, securely shut windows, use air filters and high-grade masks, and limit strenuous activity when air quality is poor, Chen said.

An important strength of this study is its use of diverse methodologies and data sources, the authors wrote. They add that Ontario’s population was not used to heavy wildfire smoke and was unlikely to have adopted preventive measures or behaviors that could confound associations.

The researchers also checked ED visits for other outcomes before and after both fires, including other respiratory causes, ischemic heart disease, and noncardiorespiratory causes. The noncardiorespiratory causes served as a control outcome as researchers did not expect those to show associations with wildfire smoke. They did not detect any effect on outcomes other than asthma in either wildfire episode.

The finding may suggest that the study period was not long enough to give a clear picture of the other outcomes, and that’s something future research may better determine, said Chen.

But he called the results a “wake-up call” and said that high spikes of asthma emergencies are not inevitable in wildfires. “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,” the authors wrote.

Forecasting Systems Crucial

The different outcomes between the first and second fires suggest potential public behavior changes and perhaps prove the importance of the Air Quality Health Index and smoke advisories, Eric Lavigne, PhD, a senior epidemiologist and research scientist with Health Canada Ottawa, told Medscape Medical News. Maybe by the time of the second fire, he said, “the public was acclimatized to what they should do when there’s smoke outside.”

Forecasting systems including wind patterns are crucial in predicting who likely will be affected and with what level of PM2.5, he said. “This [forecasting] could be beneficial in making sure populations are aware of risks that are coming, not necessarily just responding when air quality is bad.”

Physicians could also remind patients to check the Air Quality Health Index and make sure that patients who use medications for asthma renew their prescriptions, he said.

Lavigne said though this research was well conducted and offers insights on acute illness, he hopes that future long-term studies may assess whether wildfires may have eventual effects in cardiovascular disease and in conditions such as mental health.

“We are worried about that and wondering whether [increased smoke] is affecting those who are frail and might already have mental health conditions,” he said. Studies have shown a connection between increases in urban air pollution and increases in ED visits for patients with mental health conditions.

“Air pollution might affect some level of inflammation in the system, and it can also cross the blood-brain barrier,” he noted. “Extreme heat is also known to be a driver of increased ED visits for mental health. Perhaps smoke could be one as well.”

The study was funded by Health Canada. Chen and Lavigne reported having no relevant financial relationships. 

Marcia Frellick is a Chicago-based healthcare journalist who regularly covers primary care, pediatrics, cardiology, oncology, and infectious disease.