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California air pollution often conjures images of smoggy Los Angeles skies, but farms—not city streets—might be the actual culprits behind the state’s air pollution.

A study out Wednesday in Science Advances looks at the role fertilized croplands play in the state’s nitrogen oxides (NOx) emissions. As it turns out, farms emit some serious levels of NOx—so much so that the state’s NOx emissions could be 51 percent higher than current estimates.

That’s bad news because NOx can help create the smog California cities used to be known for, which can in turn trigger respiratory illnesses. It’s really bad stuff. For the most part, this pollution has been associated with industrialization and vehicles, but this study is showing a more overlooked source: farms. In California, much of this pollution lands in the Central Valley because, well, that’s where the agriculture is at.

This sorta’ sucks for California because the state has been actively trying to reduce its NOx pollution through stringent regulations on heavy-duty vehicles, but it’s been ignoring these fertilizers. Farms with heavy application of nitrogen-rich fertilizers end up emitting NOx through microbial processes. Only about half of the fertilizer that goes on the soil ends up in the crops, explains Maya Almaraz, a co-author of the study who is a researcher at the University of California at Davis. The other half is lost—either as runoff or to microbes that eat it and then convert it into this NOx gas.

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Estimates of NOx emissions from California soils. Image Courtesy of Science Advances

This translates to more than 355 million pounds of nitrogen emitted from California soils as NOx each year, according to the new study. And that means these fertilized soils make up a potential 32 percent of the state’s total NOx emissions—and it’s all flying under the radar. The state doesn’t regulate fertilizer with air pollution in mind. It could, though, as the study lays out. California could require farmers to use different fertilizing methods, decrease the number of nitrogen applications, or simply monitoring groundwater to adjust nitrogen levels accordingly. The goal, Almaraz said, is to enforce regulations in a way that doesn’t hurt profits and yields—but does protect health.

“The Central Valley is unique in that they already struggle with these issues of poor air quality,” Almaraz told Earther. “And it affects the people who live there, so we see this as a great opportunity to minimize those emissions also protect people’s health in that region.”

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Researchers aren’t clear if this pollution translates into health impacts, but that’s next on their agenda. They’ve assembled their team and will begin looking at how this NOx pollution could be hurting residents’ health through the next year. Stay tuned.