U.S. farms cost the economy more in health and environmental damage than they contribute to the economy, according to a recent study in the Proceedings of the National Academy of the Sciences (PNAS).

For 20 economic sectors, the study compares the cost of premature deaths from particulate air pollution to the value-added to the economy. Farms performed worst, driven down by animal agriculture.

“At the margin, we’re seeing that the damages from air pollution provided by farms are larger than the marginal value that the farms provide in economic terms,” said co-author Inês M.L. Azevedo, an associate professor in Stanford University’s Department of Energy Resources Engineering.

The study, conducted with researchers from Carnegie Mellon University, focuses on particulate pollution, including PM 2.5, which the EPA has identified as the cause of 90 percent of the 100,000 annual premature deaths in the U.S. from air pollution.

The study does not include health costs from consuming animal products, nor does it include the economic costs of other forms of pollution, such as greenhouse gas emissions and their impact on the climate.

For each sector of the economy, the researchers developed ratios of gross economic damages (GED) to value added (VA). A GED/VA ratio less than one means that value exceeds damage. A number greater than one means damage exceeds value:

“Whereas the GED/VA ratio was 0.72 for the group of crop-producing industries in 2014, it was 2.0 for animal production in that year,” the study states. “Preliminary calculations indicate that the GED/VA ratio within this group is highest for the poultry industry, with an estimated range of 3 to 7.”

Poultry litter emits particulate pollution and ammonia. Ammonia combines with other pollutants—sulfur dioxide and nitrous oxide, Azevedo said, to produce a secondary source of additional deadly PM 2.5.

Azevedo sees these findings as a call to assess the way the sector operates.

“This work is not advocating that we should shut down agriculture in the United States,” she said in a video released this month by Stanford. “It is providing the intuition that, at the margin, the damages are larger than the value provided. So we’ll need to think about the sector more seriously.”

In a commentary that appears in the same issue of PNAS, the economist Juan Moreno-Cruz says, “Agriculture emissions remain stubbornly high and exhibit a flat trend, making agriculture one of the most polluting industries per dollar in the United States right now.”

Utilities were in the same boat in 2008—producing more damage than value at the margin—but utilities have cleaned up their act somewhat thanks largely to the closure of coal burning power plants.

The study attributes some of the drop in pollution from utilities and manufacturing to the Great Recession.

“Right now it’s only animal production and agriculture that has damages much larger than the value added,” Azevedo said.

Transportation emissions have also dropped but remain a concern in less regulated parts of the sector: “The biggest sources of both NOx and primary PM2.5 damages within the transportation sector are from trucks and diesel combustion in marine and rail transportation,” the study says.

Azevedo adds: “Our results suggest that policymakers should consider targeting for their emissions reductions, if that is cost-effective, in transportation and in agriculture.”