As much as 41 percent of smog-causing nitrogen oxide emissions in California are coming from heavy fertilizer use in the state’s Central Valley, according to a new report released by UC Davis researchers.

The fertilizers simulate soil microbes that can convert nitrogen to nitrogen oxide, a potent greenhouse gas and pollutant linked to asthma, other breathing problems and heart disease.

The rural Central Valley region grows more than half of U.S. vegetables, fruits, and nuts.