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The surprising driver of air pollution in the California Central Valley

By Hope Cupples, MESM '20

“Everyone has the right to clean air”— this anti-tobacco slogan is one of many that leads the charge against the multi-billion-dollar tobacco industry. But in places like the California Central Valley, this slogan is being leveraged as a call to action to fight climate change. The American Lung Association published the “State of the Air” report this past April, which compiles data from the U.S. Environmental Protection Agency to rank the cities with the highest levels of air pollution. The report once again found that cities in the Central Valley landed at the top of the list.[i]

So why is the air so dirty in the Central Valley? The combined effect of poor geographic location, industrial agriculture, and burning of fossil fuels creates an unfortunate air pollution sink that traps harmful toxins. Regulatory actions to improve air quality have primarily focused on reducing pollution from vehicles and other forms of transportation. Currently, the California Air Resources Board emission inventory states that transportation is the main contributor to nitrous oxide compounds (NOx) pollution, while soil emissions are considered negligible. However, research shows that agricultural soils are a leading source of NOx pollution throughout the state, with significantly high emission rates in the Central Valley.[ii]

These nitrous oxide compounds are released by soils and are increased by agricultural processes of fertilization and irrigation. NOx compounds act as precursor molecules to ozone. When elevated in the upper atmosphere, ozone protects us from the sun’s radiation. However, at ground level, ozone can be extremely dangerous when inhaled. Estimates of NOx production in California have shown that agricultural soil emits 161,100 metric tons of NOx per year, approximately 79% of total NOx soil emissions. Across all sectors of the state (including transportation), agricultural soils account for 20-32% of total NOx emissions.[iii]

Moreover, these soil emissions contribute greatly to the overall quality of the air in the Central Valley. The air quality is so poor that the health of Central Valley residents is greatly impacted. In fact, the International Agency for Research on Cancer has classified outdoor air pollution as a Group 1 carcinogen, which means that air pollution is known to be a cause of lung cancer.[iv] Additionally, in the San Joaquin Valley, one in six children have been diagnosed with asthma, which is the highest level in the state.[v]

Not only do residents of the Central Valley suffer from the health impacts of poor air quality, but there is also a known correlation between socioeconomic status and the distribution of air pollution.[vi] Those in a lower socioeconomic status tend to have a higher incidence of preexisting disease due to food insecurity, psychological stressors, educational disadvantages and lack of access to adequate health care.[vii] This disproportionate impact on communities in the Central Valley is a pressing environmental justice issue that needs to be addressed.

To help combat the injustices facing the residents of the Central California and reduce ozone pollution, policies need to mitigate the contributions of NOx emissions from industrial agriculture. As climate change worsens, there will be increased droughts and heat waves which exacerbate NOx emissions. Furthermore, these emissions will continue to rise as the demand for higher food production increases with a growing population. The surprising impacts of agriculture on air quality will hopefully lead to better mitigation of air pollution sources as residents of the Central Valley continue the fight for fresh, clean air.



[ii] Almaraz, Maya, et al. “Agriculture Is a Major Source of NOx Pollution in California.” Science Advances, American Association for the Advancement of Science, 1 Jan. 2018,

[iii] Almaraz, Maya, et al. “Agriculture Is a Major Source of NOx Pollution in California.” Science Advances, American Association for the Advancement of Science, 1 Jan. 2018,

[iv] Loomis, Dana, et al. "The carcinogenicity of outdoor air pollution." The lancet oncology 14.13 (2013): 1262-1263.

[v] Hajat A, Hsia C, O’Neill MS. Socioeconomic disparities and air pollution exposure: a global review. Curr Environ Health Rep 2015;2:440–450.

[vi] “County Level Statistics.” Central California Asthma Collaborative,

[vii] Adler NE, Stewart J. Health disparities across the lifespan: meaning, methods, and mechanisms. Ann N Y Acad Sci 2010;1186:5–23.


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