In the wake of a severe North American heatwave, wildfires continued to char miles of wilderness and shroud city skies on Monday, September 12.

In Canada, California, and the Pacific Northwest, wildfires raged across the landscape as mid-size to large-scale conflagrations.

The immediate cause for poor air quality is obvious. Multiple wildfires are racing across the region’s dry landscape, the largest including the Cedar Creek fire in Oregon (87,734 acres), Mosquito in Northern California (46,587 acres), Ross Fork fire in Idaho (37,233 acres), and the Bolt Creek and White River fires in Washington state (7,660 and 5,803 acres respectively) (1)(2)(3)(4)(5).

Wildfire smoke degraded air quality across multiple states and provinces. Air quality monitors registered unhealthy to hazardous measurements of PM2.5 concentrations. Smoke carries airborne particulate matter measuring 2.5 diameters or smaller, referred to as PM2.5. When we breathe PM2.5, it enters our lungs and can cause respiratory and cardiac illness.

Less apparent, as cooler temperatures finally prevail, is that the heat wave itself helped kindle, fuel, and intensify the fires that continue to smolder throughout the region. As heat waves and wildfires become more extreme and common in dry regions like the western U.S. and Canada, air quality issues will persist when wildfire smoke lingers long after the heat abates.

Widespread poor air quality ranging from moderate to hazardous detected by air quality monitors in the Pacific Northwest and Canada. Source: IQAir.

Wildfires send Lake Tahoe smoke, poor air quality

A vicious heat wave ended over the weekend in California, cooling temps down from highs rising well above 100 degrees Fahrenheit. But fires like the Mosquito fire east of Foresthill have abundant dry brush to feed them, leading to firefighters battling 11 separate wildfires in the state (6).

Cities east of Sacramento, like Auburn and Georgetown, continued to struggle with concentrations of PM2.5 within the cities measuring up to 376 micrograms per cubic meter of air (µg/m3) on Monday.

Concentrations of PM2.5 measured over 251 µg/m3 are in the hazardous range according to the U.S. air quality index (AQI). Hazardous air quality may prompt emergency condition alerts, as it can affect all people in a region. However, people sensitive to poor air quality are often the worst affected.

Wildfire smoke made poor air quality far more pervasive in Northern California after the heat wave ceased. In South Lake Tahoe, smoke brought concentrations of PM2.5 that measured 171 µg/m3.

The last time regional air quality reached hazardous levels was on August 30, 2021, when the city evacuated ahead of the Caldor fire.

Air quality in San Francisco, more distant from the fire than South Lake Tahoe, was impacted on Monday. The city’s average concentrations of PM2.5 at 11 a.m. measured 26 µg/m3, in the moderate range. San Francisco’s annual average PM2.5 concentration in 2021 was 8.2 µg/m3.

Neighboring Nevada also suffered poor air quality. Air quality monitors in Reno measured unhealthy concentrations of PM2.5 of 137 µg/m3, while Carson City averaged 330 µg/m3.

Hazardous air quality found along the shores of Lake Tahoe and in the eastern foothill communities of the Sierra Nevada. Source: IQAir.

Wildfires in Washington state harm air quality

Wildfires in Washington state, many burning in the Cascade mountain range, are creating unhealthy to hazardous air quality in towns near the fires and east of them. The air quality west of the Cascades, including the Seattle area, was mostly moderate on September 12.

In the mountain town of Leavenworth, Washington, the average measured PM2.5 concentration at midday was 366 µg/m3. At the same time, an average PM2.5 concentration of 206 µg/m3 was measured around the city of Spokane in eastern Washington.

Last September, the average concentration of PM2.5 in Leavenworth was 10 µg/m3. Spokane’s average concentration of PM2.5 in September of 2021 was 13 µg/m3.

Fires in the Cascade mountain range harm air quality in Seattle while hazardous smoke descends into eastern Washington. Source: IQAir.

Poor air quality in Spokane, Washington and Coeur d’Alene, Idaho. Source: IQAir.

Montana and Idaho air quality: unhealthy to hazardous

Multiple largely uncontrolled fires continued to impact Idaho air quality as wildfire smoke rolled across the state border into Montana’s mountain valleys.

As of Sunday, Idaho was struggling with the most wildfires in the country and the second most acreage burned (7). Poor air quality shrouded Idaho from the Panhandle to Boise.

Measured concentrations of PM2.5 in Lewiston were 186 µg/m3 – unhealthy – while air quality monitors in Coeur d'Alene detected very unhealthy concentrations of PM2.5 of 233 µg/m3.

In Montana, Butte, Bozeman, and Missoula measured concentrations of PM2.5 of 48, 73, and 143 µg/m3 respectively.

Multiple fires enveloped Idaho’s skies in smoke. Source: IQAir

Bend air quality becomes hazardous

As of September 12, there were 19 active wildfires reported by the Oregon Department of Emergency Management. The massive Cedar Creek fire, which has been burning since August 1, took a turn for the worse over the weekend resulting in its containment numbers falling from 12% to 0%. Smoke from the fire has blanketed Portland, Oregon, and its surrounding areas (8)(9).

In Bend, around 40 miles northeast of the Cedar Creek fire, the measured midday PM2.5 concentration on September 12 was 165 µg/m3. In September 2021, the city air quality was poor as well – enduring an average monthly PM2.5 concentration of 46 µg/m3.

The Cedar Creek fire created hazardous air quality conditions in Bend, Oregon on Monday. Source: IQAir.

While Canada’s west burns, smoke threatens residents

193 active wildfires in British Columbia, Canada sent roiling wildfire smoke across the province over the weekend. The wildfires and smoke sparked evacuations and air quality warnings (10).

Vancouver air quality, like other coastal cities in the neighboring western United States, was not as poor as the air quality measured in interior cities. The city’s average concentration of PM2.5 at 11 a.m. September 11 measured an unhealthy 147 µg/m3. By 1 p.m. on Monday, air quality was moderate at 33 µg/m3. Even so, an earlier air quality warning issued by Environment Canada was extended due to transboundary smoke from the United States.

Interior cities, including Kamloops, Vernon, Kelowna, and Castlegar all endured unhealthy concentrations of PM2.5 as late as 1 p.m. on Monday.

Rural Canada’s air quality was unhealthy on Monday. Source: IQAir.

The takeaway

Heat waves affect air quality by elevating ozone levels, and when accompanied by stagnant air and no rain, pollutants don’t disperse as easily. Though a break in a heat wave is good news for everyone suffering under oppressive temperatures, the longer-term consequences of extreme heat and dry ground cover mean that wildfires outlast heatwaves. Wildfire smoke ensures that air quality stays dangerously poor long after temperatures return to normal.

If air quality becomes hazardous and skies are smoky, avoid outdoor exercise and activity. Don’t open windows and dorun an air purifier to breathe better indoors.