Billowing clouds of wildfire smoke descended on city skylines across western Canada and the United States on May 19, 2023. Air quality monitoring stations in multiple Canadian provinces and U.S. states reported poor air quality ranging from “unhealthy” to “hazardous.”

Air quality in Edmonton, Regina, and Calgary was heavily impacted by wildfire smoke as early as Wednesday, May 17. By Friday, shifting winds had carried smoke further south into Helena, St. Louis, and Chicago.

By 7:15 AM, Denver, Colorado was the second most polluted major city in the world. Wildfire smoke in Denver could be seen and smelled by residents, where it clouded skies and reduced visibility (1).

The U.S. air quality index (AQI) for Denver was 155. An AQI of 0 to 50 is considered “good” according to the index. However, no amount of air pollution is truly safe.

Air quality was unhealthy in Denver, Colorado on May 19, 2023. Denver was the second most polluted major city in the world at 7:15 AM. Source: IQAir.

Unhealthy air quality was widespread across Canada and the United States. Source: IQAir.

Wildfire smoke can travel for hundreds or even thousands of miles from their source, crossing continents and oceans to pollute distant city skylines.

Wildfires in Canada fueled by springtime heat wave

There are multiple active wildfires in Canada, primarily in British Columbia, Alberta, Saskatchewan, and the Northwest Territories. There are 58 uncontrolled wildfires, while another 40 are “being held,” meaning firefighters are controlling the fires and they’re unlikely to spread (2).

The wildfires coincide with a heat wave in western Canada. The heat wave creates conditions that promote the fires spreading and growing.

The heat wave pattern is being described by forecasters as similar to extreme weather events that broke temperature records in 2021 (3). Human-induced climate change is increasing the intensity, frequency, and length of heat waves.

Heat waves worsen air quality by contributing to ozone creation and by drying out vegetation. Tinderbox conditions in remote, difficult to access wildlands then fuel wildfires and bush fires.

Unhealthy air quality was widespread across Canada and the United States. Source: IQAir.

Some air quality monitors closest to the wildfires – including Fort St. John, British Columbia and Peace River, Alberta – measured concentrations of PM2.5 well in the hazardous range.

PM2.5, particulate matter measuring 2.5 microns in diameter or smaller, is the most common airborne component of wildfire smoke. PM2.5 is so tiny that it can enter the bloodstream after inhalation, and is linked to cardiac and respiratory diseases, including asthma.

Air quality is hazardous when it falls in the range of 301 or greater. An AQI of 301 is the equivalent of 250.5 micrograms per cubic meter of air (µg/m3) – and the health consequences are profound. Everyone exposed to hazardous air quality is at high risk of negative health consequences, including strong eye, skin, and throat irritation, triggered asthma symptoms, acquired breathing and lung diseases, and hospitalization.

Smoke from the wildfires poured south this week, blanketing Canadian cities and spreading hundreds of miles south into the U.S.

Smoke reaches from Rockies to Great Lakes

Wildfire smoke prompted air quality alerts across the U.S. in several states, including Minnesota, Nebraska, Colorado, and South Dakota (4)(5)(6)(7).

Cities in Montana like Helena and Butte were subjected to air quality in the unhealthy range as early as Wednesday, May 17. Butte measured an hourly average concentration of PM2.5 at 108 µg/m3 at 7 PM, May 18. Helena measured an hourly average concentration of 124 µg/m3 of PM2.5 at 4 PM, May 18.

Montana cities, including Helena, made up 5 of the 10 most polluted U.S. cities at 9 AM, May 19.

Oregon, Wyoming, Colorado, and Kansas – all states impacted by the wildfire smoke – rounded out the list.

Poor air quality hung over city skylines in Montana, Idaho, and Washington State on May 19. Source: IQAir.

Elevated concentrations of poor air quality were measured by air quality monitors across the U.S. Midwest. Source: IQAir.

The wildfire smoke’s reach extended across the Great Plains and could be seen as far east as Chicago, Illinois and St. Louis, Missouri.

The takeaway

If wildfire smoke is threatening your air quality, take steps to protect yourself.

• Download an air quality app and be alerted when air quality worsens.
• Close doors and windows to keep smoke out of your home.
• Run an air purifier for wildfire smoke.
• If you have to go outside, wear a KN95/FFP2 mask.