The pollution is produced by fertilizers and highlights how agriculture is also a significant contributor to air pollution. Corn alone is responsible for killing 4,300 Americans each year.

Turns out, maize can be a silent killer.

We all know that air pollution kills people. We also have a general idea of what causes air pollution: burning fossil fuels is chief among the causes, wildfires and waste also major issues, but agriculture is often overlooked.

“You think air quality and you think coal plants, and you think dirty diesel trucks,” says Jason Hill, an engineering professor at the University of Minnesota and the study’s lead author. “Certainly both of those are major contributors to reduced air quality, but corn production? Yes, that too.”

A few months ago, Hill published another study which found that agriculture is responsible for around 16,000 air pollution-related deaths a year in the US alone. Now, he and his colleagues have turned their eyes towards corn, the most popular US crop.

“We show that reduced air quality resulting from maize production is associated with 4,300 premature deaths annually in the United States, with estimated damages in monetary terms of US$39 billion (range: US$14–64 billion). Increased concentrations of fine particulate matter (PM_2.5) are driven by emissions of ammonia — a PM_2.5 precursor — that result from nitrogen fertilizer use,” the study reads.

The team created a life-cycle assessment of corn crops, which includes all stages of production. They found that 86% of corn-related pollution was generated directly on farmland, most of which can be traced directly to the use of synthetic nitrogen fertilizers. Nitrogen is essential for corn, but because the plant can’t produce its own nitrogen, it draws it from the soil via symbiotic bacteria. In time, the nitrogen is drained from the soil and must be replaced artificially.

The most popular nitrogen fertilizer is ammonia, which converts into nitrate once it enters the soil — a compound that plants can absorb. But some ammonia also escapes the soil and rises into the atmosphere as a gas. Once it does this, it contributes to the fine particulate matter that can cause serious issues to human health. The scientists traced the pollution to its source thanks to the fact that fine particulate matter doesn’t tend to travel much.

If you want to help with this issue, you shouldn’t focus on eating less corn — but rather on eating less beef. Each year, around 90 percent of the corn we grow goes to feeding livestock and producing ethanol, and the US government heavily subsidizes corn production for these purposes. The country is also the world’s largest exporter of corn.

Researchers want to carry out this type of study for different types of crops, to ultimately “the air quality impacts of all the foods we consume.”

The study was published in Nature Sustainability.