Just as one immense California wildfire was nearly brought to heel, a second wildfire complex erupted over acres of old growth forest mountainsides.

In the Six Rivers National Forest, a complex fire – two or more wildfires in close proximity – straddling both Humboldt and Trinity Counties was sparked by lightning strikes on August 5, 2022. By Monday, August 16, fir timber litter and dead hardwood fueled six individual wildfires spanning 19,272 acres (1, 2).

As firefighters largely contained the 60,392-acre McKinney fire in Siskiyou County over the weekend, the struggle to contain the Six Rivers Lightning Complex fire began in earnest. And while the two wildfires are both near the Oregon border and around 70 miles apart, wildfire smoke has impacted West Coast air quality in dramatically contrasting ways.

Wildfire smoke clouds Redding

Wildfire smoke harms people’s lungs: this can even occur when we live far from a conflagration. Smoke carrying deadly PM2.5, tiny particle pollutants 2.5 microns in diameter or less, can cross oceans and continents. In 2021, California wildfire smoke tinted East Coast cities skies with orange and reddish air pollution.

Even when a wildfire is in a remote national forest, prevailing winds can disperse wildfire smoke to distant cities and towns, as was the case in Northern California on Monday morning.

Air quality monitors measured poor air quality from Redding to San Jose on August 15. Source: IQAir AirVisual.

Air quality was poor on the morning of August 15 from Redding to the South Bay area. The most polluted community was the small town of Weaverville, west of Redding. Four separate air quality monitoring stations measured concentrations of PM2.5 in the hazardous range. Concentrations of PM2.5 measured 430 µg/m3 (micrograms per cubic meter air) at 10:30 am on August 15 at the Odd Fellows Avenue, Weaverville.

Air quality was very unhealthy to hazardous in Redding, especially on the west side of the city nearest to the wildfire. On August 15 morning, concentrations of PM2.5 at one air quality monitoring station measured 287 µg/m3.

Air quality monitors measured poor air quality from Redding to San Jose on August 15. Source: IQAir AirVisual.

Hazardous concentrations of PM2.5 are a danger to everyone’s health. PM2.5 exposure has been linked to heart and lung disease, triggers asthma attacks, reduced lung function, and even death.

Wildfire smoke has severely worsened Redding’s air quality in previous years. Redding experienced a monthly average PM2.5 concentration of 85 µg/m3 in August 2021. At the time, the Dixie fire marred regional air quality and became the largest single wildfire in California history.

Wildfire smoke’s long reach

Wildfire smoke can be as unpredictable as it is dangerous.

Though triple the size of the Six Rivers Lightning Complex fire, the McKinney fire had impacted a much smaller geographic area. Winds had carried the smoke north just over the state border into nearby neighboring southern Oregon. At its worst, wildfire smoke created poor air quality in Medford, Grants Pass, and Ashland. By contrast, smoke originating in the Six Rivers National Forest reduced air quality nearly 300 miles away.

Wildfire smoke reduced air quality in the San Francisco Bay Area and Sacramento on Monday morning. While PM2.5 measurements didn’t rival the hazardous air quality found in Redding, any amount of air pollution – both short-term and long-term – is dangerous for human health.

The United States uses the U.S. air quality index (AQI) to help describe air quality. While the air quality across the region was characterized as in the “moderate” range, the health implications are anything but that.

An air quality monitor in Kelseyville reported that the hourly US AQI was 100, in the moderate range. But the PM2.5 concentration was 35 µg/m3, more than double the World Health Organization’s (WHO) recommended 24 hour average PM2.5 exposure guideline.

The WHO revised its guideline in 2021 based on overwhelming scientific evidence that exposure to even low concentrations of PM2.5 can result in poor health outcomes and increased mortality (3).

Poor air quality measured in the South and East Bay Area on August 15. Source: IQAir AirVisual.

Poor air quality measured in Sacramento on August 15. Source: IQAir AirVisual.

Sacramento and the Bay area does experience poor air quality from vehicle traffic and industrial pollution. However, the poor air quality measured on August 15 didn’t correspond with vehicle traffic patterns, indicating that wildfire smoke was the primary pollutant source. Wind direction carried smoke southeast from the border to distant regional cities including Modesto, Oakland, San Jose, and Stockton, almost 300 miles south of the Six Rivers National Forest.

San Francisco enjoyed better air quality than neighboring communities on Monday as winds pushed most of the smoke further east. But shifts in prevailing winds could put the city in danger to greater PM2.5 exposure.

The takeaway

More intense heatwaves fuel wildfires, resulting in greater risk of exposure to wildfire smoke.

Because of the great distances wildfire smoke can travel and other factors like wind direction and geography, we can’t precisely predict when or where we’ll be affected by smoke pollutants. Download the AirVisual app to monitor your local air quality.

The best protection against wildfire smoke is advanced preparation. Avoid going outdoors when the air is smoky and, even if it doesn’t seem to be, when air quality monitoring indicates that the air is polluted. If you must go outdoors, wear a face mask. Don’t leave windows open and consider running an air purifier.