You've probably heard statistics about how our diet affects the health of the planet. Like how a beef hamburger takes considerably more water and land to produce than a veggie burger or that around a quarter of global greenhouse gas emissions stem from food production. In fact, there are websites that can calculate the carbon footprint of specific foods.
But you may not have considered how the food we eat contributes to the quality of the air we breathe.
Air pollution is the largest environmental health risk factor in the United States, and agriculture contributes in a number of ways. Fertilizer application, gas use, pesticide production and dust kicked up from tilling all affect air quality. But the sort of accounting done for the carbon footprint of foods hasn't been done for their air pollution footprint.
That changed Monday with a study published in Nature Sustainability. It modeled how the production of a single crop, corn, contributes to air pollution in the United States. The researchers found that corn production accounts for 4,300 premature deaths related to air pollution every year in the United States. Ammonia from fertilizer application was by far the largest contributor to corn's air pollution footprint.
"If you want to do anything about air pollution, you need to know the cause," says Susanne Bauer, a climate modeler at the Earth Institute at Columbia University, who was not involved in the study. "This is an important study because instead of just saying air pollution kills people, it's detailing the specific contributions of different parts of the corn production process to air pollution."
Corn is the largest agricultural crop in the United States. But you may not immediately associate air pollution with the endless fields of rolling green in Iowa or Illinois. That's a mistake, according to study author Jason Hill, an engineering professor at the University of Minnesota.
Previous work has found that agricultural practices — like fertilizer production, running tractors and tilling land — account for about 16 percent of all human-caused air pollution of a type called PM2.5. Atmospheric particulate matter with a diameter smaller than 2.5 micrometers (for comparison, a human hair is about 50 micrometers wide) is classified as PM2.5, and exposure has been associated with cardiovascular problems, respiratory illness, diabetes and even birth defects.
Hill and his colleagues wanted to understand how the various stages of corn production fit into this picture. "Does most of the impact from corn production happen on the farm or upstream in the supply chain — the locations that feed activities on the farm?" asks Hill.
To find out, they used a detailed life cycle model that describes the different stages of the corn supply chain. They analyzed every stage of the process — fertilizer production, fuel and electricity use, transportation and distribution, on-farm activity — and calculated which contribute to PM2.5 emissions.
The researchers fed the model with publicly available data on corn yields and manure and synthetic fertilizer usage from corn-producing counties in the U.S., giving them an estimate of the total emissions caused by corn production.
However, not all the emissions it takes to produce a bushel of corn in, say, Stearns County, Minn., are emitted there. "Growing corn in Minnesota results in emissions in Florida, where phosphate fertilizer is produced," among other places, says Hill. Their model accounts for where the fuel, fertilizer and other upstream processes occur.
With these data in hand, the researchers modeled how corn production emissions contribute to air pollution on a county-by-county basis and mapped those data onto census data to understand who's exposed to these emissions.
Finally, they used existing research on the health effects of different exposure levels to estimate how many premature deaths per year (out of an estimated 102,000 deaths from domestic human-caused emissions in the U.S.) could be tied to emissions from corn production.
The study estimates that "4,300 premature deaths can be attributed to corn production," says Hill. "That's about a quarter of all [air pollution-related] agricultural deaths. That's significant."
The vast majority, 86 percent, of PM2.5 emissions happen on farms, with the rest occurring elsewhere in the supply chain, according to the study. The main culprit?
"Ammonia from fertilizer application accounted for about 70 percent of attributable deaths," says Hill. The nitrogen in fertilizers helps fuel plant growth, but not all of it can be used by the plant. "Some of that nitrogen washes into waterways, and some of it gets released into the atmosphere as ammonia," says Hill.
Once ammonia hits the atmosphere, it reacts with other particles, mostly nitrogen and sulfur oxides, to form PM2.5, according to Hill.
Hill estimates that air quality-related health damages cost $39 billion annually. The top five corn-producing states — Iowa, Illinois, Nebraska, Minnesota and Indiana — bore 54 percent of the deaths associated with air pollution caused by corn production.
But in areas with lower yields or that were closer to major cities, the per-bushel health damages of corn actually exceed its market price.
"That suggests that in many parts of the U.S." — parts of the Eastern Seaboard, the South and near some large, urban areas — "producing corn is more damaging than the economic benefit it provides," says Hill.
Bauer was surprised at how low the premature death numbers were. "I think if you did this study in Europe, for example, where higher densities of people live closer to farms, you'd get much higher numbers," she said.
The total contribution of corn to air pollution is likely higher, according to Hill, as this study didn't consider what is done with the corn after it gets produced. Much of the corn produced in the U.S. goes to feed livestock, which has its own contributions to air pollution.
Hill says that reconsidering where corn is grown or changing fertilizer practices could mitigate the harmful consequences of corn production. Injecting nitrogen-based fertilizers into the ground, for example, leaks less ammonia into the atmosphere than surface application.
More broadly, Hill hopes his work can help people make more informed decisions about what we eat.
"What we eat affects not only our health but the health of others," he says. "Of course we need to eat. But is corn, or the livestock we feed corn, the best thing to eat?"
Jonathan Lambert is an intern on NPR's Science Desk. You can follow him on Twitter: @evolambert.