Air quality throughout nearly all of California’s Central Valley posed a threat to breathable air and human health for nearly a week in mid-November. The region also experienced dense tule fog, a radiation fog common to the valley during cooler winter nights.¹

From Sacramento to Bakersfield and from San Francisco to the foothills of the Sierra Nevada mountain range, air quality consistently fell in the “Unhealthy for Sensitive Groups” air quality index range from Thursday, November 11 through Sunday, November 14. Regional air quality on the morning of Monday, November 15 mostly fell in the “Moderate” range, though several major cities still measured air quality as “Unhealthy for Sensitive Groups.”

Poor air quality in the Central Valley is a well-established fact of life. During that same week, several environmental groups filed a federal lawsuit against the U.S. Environmental Protection Agency (U.S. EPA).² The suit, filed on November 10, 2021, argued that the EPA failed to step in and require that California meet air quality standards in the San Joaquin Valley – and had failed to do so for many years.

The San Joaquin Valley comprises much of the Central Valley south of Sacramento.

Pictured: Air quality across California’s Central Valley on Friday, November 12, was “Unhealthy for Sensitive Groups.” Source: IQAir

Measuring air quality in the region

Air quality is visualized through a range of numbers between 0 and 500 according to the U.S. AQI (air quality index). Hazardous air quality can measure above 500. These numbers can be tied back to PM2.5 concentrations measured by micrograms of PM2.5 per cubic meter, or µg/m3.

PM 2.5, particulate matter measuring smaller than 2.5 microns in diameter, can cause heart and lung disease.^3,4 After being inhaled, PM2.5 and even tinier ultrafine particles may enter the bloodstream and endanger every bodily organ.

The interactive IQAir air quality map assigns air pollutant concentrations a number and color that visually represent air quality categories. Green dots display air quality stations that are measuring “Good” air quality. Yellow shows “Moderate” and orange stations are recording air quality that is “Unhealthy for Sensitive Groups.”

Any concentration of airborne pollutants presents a potential hazard to human health. But these designations can be helpful for determining the level of health risk. For instance, when air quality is deemed “Unhealthy for Sensitive Groups,” the general public and individuals with poor air quality sensitivities may be more likely to experience lung irritation and difficulty breathing.

Pictured: Air quality image of Sacramento, California taken November 12, 2021. Source: IQAir

Pictured: Air quality in California’s Central Valley east of San Francisco on November 12, 2021. Source: IQAir

Pictured: Air quality in Modesto and Fresno, California reached “Unhealthy” levels on November 12, 2021. Source: IQAir

Environmental injustice in California

More than one lawsuit has been filed against the EPA regarding poor air quality in the San Joaquin Valley. A 2001 lawsuit placed the EPA under court supervision over a lack of progress in resolving poor air quality.⁵

The more recent lawsuit noted that California hadn’t provided a plan to meet national clean air standards in multiple years – 1997, 2006, and 2012. The EPA made this determination in 2018. The EPA is required to enforce a plan within two years of similar findings unless the agency approves submitted revised plans. Per the lawsuit, the agency has failed in enforcement since January 2021.

Major cities across Central California impacted

The Central Valley is home to many large cities with populations ranging from tens to hundreds of thousands of people in each. When the full rural population is accounted for, it’s estimated there are 6 million people or more living in the region as of 2020.⁶

Some of the major Californian cities recently affected by poor air quality included:

• Antioch
• Concord
• Fresno
• Madera
• Merced
• Stockton
• Visalia

Cities close to San Francisco and the East Bay measured air quality found in the “Moderate” range – better air quality than that found in the “Unhealthy for Sensitive Groups ” range, but still not safe for human health.

Why is Central Valley air quality so poor?

Some air quality problems may be an anticipated concern in a major American metropolitan area like Los Angeles, which recently experienced worse than usual air quality. However, the cities of the Central Valley and San Joaquin Valley experience similarly poor air quality without the same degree of intensely centralized urbanization.

Though the population is more disbursed than in the Los Angeles Basin, there are several related factors contributing to the region’s poor air quality.

Like Los Angeles, vehicle traffic is heavy in the Central Valley, with emissions creating PM2.5, nitrogen oxides and volatile organic compounds (VOCs). Industrial activity in and around cities emits airborne particulate pollutants. Energy production plays a role in adding to PM2.5, particularly towards the southern end of the valley.

In contrast to California’s metropolitan regions, the Central Valley is home to many farms. The region provides 8 percent of the United States’ agricultural output and 40 percent of its fruits, nuts, and other table foods.⁷ That intensive agricultural activity, accompanied by crop burning, pesticides, machinery emissions, and wind-blown dust, results in more PM10 and high concentrations of nitrogen oxides (NO_X) and nitrogen dioxide (NO₂). Extensive use of fertilizer can elevate levels of NO_x in the air.

NO₂, when combined with VOCs and sunlight, can result in ground-level ozone, the primary component of smog. The Central Valley is noted for its high levels of summertime ozone.

Temperature inversions can play a role in poor air quality as well. When a layer of cool air is trapped at the surface level of a valley, pollutants are also stuck at ground-level. As temperatures rise, the inversion may end, allowing easier disbursement of trapped pollutants and their exit from the region.

Tule fog, resulting in sharply reduced visibility, is a pollution-related problem in the Central Valley.⁸ The density of this radiation fog can be increased by air pollutants, according to a 2019 research article published in the Journal of Geophysical Research: Atmospheres. The paper indicated that long-term trends in fog density in the region are being driven by water-attracting particle pollutants forming fog droplets.

Finally, increasing wildfires in the ringing mountains can add to poor air quality through drifting wildfire smoke, as occurred in 2021 during the Dixie Fire in Northern California and the KNP Complex Fire in Sequoia and Kings Canyon National Parks.

The takeaway

Environmental justice can be slow in coming as citizens in Jakarta and London recently discovered.⁹ But continued pressure and environmental activism can result in accountability and legal action being taken to counter air pollution, as also happened in those cases.

If air quality is poor where you live, be prepared to take action to protect yourself.

• Use indoor and outdoor air quality monitors so you can see whether pollution is an immediate concern in your environment.
• While on the go, have an air quality app installed on your smart phone to receive real-time air quality updates.
• If you must go outdoors, put on an air pollution mask.
• Turn on your air purifier to prevent outdoor airborne contaminants from impacting your indoor air.