Fumes from nitrogen-rich fertilizers and animal waste are being blamed for being sources of fine-particulate air pollution in much of the United States, Europe, Russia and China, according to a new study.

The new study was published in Geophysical Research Letters, a journal of the American Geophysical Union, and it blamed the fumes combining with combustion emissions to form solid particles that can lead to disease and death.

The silver lining to the study was that fine-particulate pollution is projected to decrease in the coming decades, which will help air quality, even if fertilizer use increases.

The American Geophysical Union explained that agricultural air pollution comes mainly in the form of ammonia, which enters the air as a gas from heavily fertilized fields and livestock waste. It then combines with pollutants from combustion—mainly nitrogen oxides and sulfates from vehicles, power plants and industrial processes—to create tiny solid particles, or aerosols, no more than 2.5 micrometers across, about 1/30 the width of a human hair.

Many regional studies, especially in the United States, have shown agricultural pollution to be a prime source of fine-particulate precursors, but the new study is one of the first to look at the phenomenon worldwide and to project future trends. The study’s results show more than half the aerosols in much of the eastern and central United States come from farming.

“This is not against fertilizer—there are many places, including Africa, that need more of it,” said Susanne Bauer, an atmospheric scientist at Columbia University’s Center for Climate Systems Research and NASA’s Goddard Institute for Space Studies in New York and lead author of the study. “We expect population to go up, and to produce more food, we will need more fertilizer.”

The fact that agricultural emissions must combine with other pollutants to make aerosols is good news, according to Bauer. Most projections say tighter regulations, cleaner sources of electricity and higher-mileage vehicles will cut industrial emissions enough by the end of this century that farm emissions will be starved of the other ingredients necessary to create aerosols, she said.

“You might expect air quality would decline if ammonia emissions go up, but this shows it won’t happen, provided the emissions from combustion go down,” said Fabien Paulot, an atmospheric chemist with Princeton University and the National Oceanic and Atmospheric Administration who was not involved in the study. That means pollutants other than ammonia should probably be targeted for abatement, he said.