A new study says that emissions from farms outweigh all other human sources of fine-particulate air pollution in much of the United States, Europe, Russia and China. The culprit: fumes from nitrogen-rich fertilizers and animal waste that combine in the air with industrial emissions to form solid particles—a huge source of disease and death. The good news: if industrial emissions decline in coming decades, as most projections say, fine-particle pollution will go down even if fertilizer use doubles as expected. The study appears this week in the journal Geophysical Research Letters.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. The particles 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.