On the morning of August 9, 2021, the Dixie Fire became the largest fire in California history. At 489,287 acres, the wildfire burned across four counties – Butte, Lassen, Tehama, and Plumas.^1,2

That same morning, measured air quality degraded in mountain towns across most of California’s Sierra Nevada mountain range. Drifting wildfire smoke also created unhealthy air quality in Salt Lake City, Denver, Santa Fe, and Cheyenne.

Denver and Salt Lake City were listed among the most polluted major cities in the world on August 9.

Pictured: Salt Lake City and Denver were respectively the fifth and seventh most polluted cities in the world on August 9, 2021. Source: IQAir

Air quality in the United States and Canada has been impacted by numerous western wildfires for weeks, resulting in air quality in the Eastern United States that has measured from “moderate” (51 to 100) to “unhealthy” (151 to 200) on the U.S. air quality index.

No amount of air pollution, even if it is measured as “good” or “moderate,” is safe for human health.

On August 9, most of the Eastern U.S. and Canada measured its air quality as “moderate”. Western cities ranged from “unhealthy for sensitive groups” (101-150) to “very unhealthy” (201-300) on the slopes of the Sierra Nevada mountains.

Extreme heat and dry conditions are intensifying the wildfire season in Canada and the western United Statesas a result of human-caused climate change. While major fires like the Bootleg Fire in Oregon and the Tamarack Fire in California had contributed to poor air quality in eastern North America late in July through early August, those fires were mostly contained by August 9.

Pictured: California’s Dixie fire, combined with other fires in the state, resulted in unhealthy air quality throughout California, Nevada, Utah, Colorado, Wyoming, and New Mexico. Source: IQAir

Meanwhile, the Dixie Fire has fed on dry conditions to pose a serious safety threat across four counties and a further health hazard as smoke was carried far from its source to neighboring counties and states.

California fires merge and grow

The Dixie Fire, which ignited on July 13, merged with the Fly Fire on July 22.

The fire destroyed the historic town of Greenville and 589 buildings.³ Three firefighters were injured and at least eight people were missing.⁴ The cause of the fire is still unknown, though drought, hot, dry conditions, and strong winds have played a role in sustaining the fire.

5,813 personnel were assigned to contain and extinguish the fire, which burned in Lassen Volcanic National Park, Lassen National Forest, Plumas National Forest, and along the shores of Lake Almanor.

Like nearly all of California, the region has endured serous drought conditions through the summer. Regional drought reached an intensity of “exceptional” by late July.⁵

Hazardous air in Sierra Nevada towns

Smoke traveled south and east from the Dixie fire, carrying a mixture of pollutantsincluding:

• carbon dioxide
• carbon monoxide
• nitrogen oxides
• volatile organic compounds (VOCs)

The primary pollutants found in smoke are PM2.5 (airborne particulate matter measuring less than 2.5 microns in diameter) and ultrafine particles (PM that measures less than .1 micron in diameter).

Pictured: “Unhealthy” air quality was measured from Medford, Oregon south to Redding and Chester California. Air quality in Reno and Carson City measured as “unhealthy for sensitive groups" as far south as small towns east of Bakersfield. Source: IQAir

Air quality near the fire, which has also been impacted by numerous smaller fires in Oregon and California, was some of the worst recorded in the United States on August 9. Cities with “unhealthy” air quality included:

• Redding, California
• Mount Shasta, California
• Reno, Nevada
• Medford, Oregon

Further south, Carson City, Nevada, and California towns ranging along the length of the border down to Ridgecrest, east of Bakersfield, recorded air quality considered “unhealthy for sensitive groups.”

Air quality near the fire itself in the small town of Chester was “hazardous.”

Smoke in Salt Lake Valley

The Salt Lake Valley in Utah was enveloped by intense smoke on August 6. Salt Lake City and its suburbs experienced reduced visibility from the combined smoke of the Dixie Fire and other West Coast fires converging on the valley.⁶

By August 9, the region continued to measure poor air quality ranging from “unhealthy for sensitive groups” to “unhealthy” in Park City and nearby Evanston, Wyoming.

Pictured: Air quality in Salt Lake City mostly measured as “unhealthy for sensitive groups,” with some towns recording “unhealthy” air quality. Source: IQAir

Hazy skies over the Rockies

All of Colorado’s major cities felt the effects of poor air quality on August 9. Cities that recorded air quality in the “unhealthy for sensitive groups” range included:

• Denver
• Boulder
• Fort Collins
• Colorado Springs
• Pueblo
• Grand Junction

Further north along the Front Range, Cheyenne and Laramie, Wyoming also recorded similar air quality.

Poor air quality was measured miles south into urban New Mexico, where both Albuquerque and Santa Fe were impacted by wildfire smoke. Wind and rains may help mitigate poor air quality in the state in coming days.⁷

Pictured: Air quality was recorded as “unhealthy for sensitive groups” from Laramie, Wyoming and south to the major cities of Colorado and New Mexico. Source: IQAir

The takeaway

The 2021 wildfire season isn’t over. Even as fires break state records, today’s record-holder may soon be overtaken by even greater conflagrations. Extreme weather conditions, ongoing drought, and dry ground cover can continue to fuel major fires in coming months.

Plan ahead in case wildfires and wildfire smoke threaten your safety and air quality.

• Leave immediately when given orders to evacuate. Fire can spread quickly, and smoke can reduce visibility during an evacuation.
• Keep an eye on airborne pollutants with an air quality monitor.
• Keep doors and windows closed.
• Avoid smoking or using gas or wood burning stoves to reduce a potential buildup of indoor PM2.5.
• Turn on an air purifier for wildfire smoke to help clean the air and filter particles when they enter the home.
• Minimize time spent outdoors.
• If going outdoors becomes essential, put on an air pollution mask.