Fertilized soils may be contributing up to 40 percent of the state’s nitrogen oxides emissions.

Despite its progressive environmental policies, the state of California actually has the worst air quality in the nation, according to a 2017 report from the American Lung Association. More specifically, California’s Central Valley, which produces one third of the country’s vegetables and two thirds of its fruits and nuts, is home to some particularly nasty air, thanks to its bathtub-like topography, which traps air pollution in the region. Just in the past month, the Central Valley saw, by some measures, its worst period of polluted air in nearly 20 years, which researchers have attributed primarily to smoke from wildfires that ravaged areas of Southern California, as well as the typical culprits—vehicle emissions and residential wood-burning.

All this pollution has not been good for Californians’ health. On a global scale, the World Health Organization estimates that air pollution causes one in eight premature deaths, and is associated with an increased risk of asthma (with children being particularly vulnerable), cancer, and birth defects. The state alone loses tens of billions of dollars every year in health-related costs of air pollution.

California has traditionally tried to tackle the problem by regulating and limiting vehicle emissions, among other measures. But, a new study led by researchers at the University of California-Davis found the state may be vastly underestimating a major source of air pollution: fertilizer.