Fumes from nitrogen-rich fertilizers and animal waste combine in the air with combustion emissions to form solid particles in the air. These aerosols outweigh all other human sources of fine-particulate air pollution in much of the United States, Europe, Russia and China, according to new research.

The culprit: fumes from nitrogen-rich fertilizers and animal waste combine in the air with combustion emissions to form solid particles, which constitute a major source of disease and death, according to the new study.

The good news is if combustion emissions decline in coming decades, as most projections say, fine-particle pollution will go down even if fertilizer use doubles as expected, according to the new study published in Geophysical Research Letters, a journal of the American Geophysical Union.

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.

Aerosols can penetrate deep into lungs, causing heart or pulmonary disease. A 2015 study in the journal Nature estimates they cause at least 3.3 million deaths each year globally, and a recent study in Geophysical Research Letters found they cause over 500,000 annual deaths in India alone.

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.