We know that fumes belched out by power plants and vented from cars contribute to the shocking levels of pollution seen across many of the world’s busiest cities, which leads to millions of premature deaths. But it seems that these sources may actually be outweighed by yet another cause, as a new study documents how farming contributes more fine-particulate air pollution across much of the U.S., Europe, and China, than all other human sources. The results are published in Geophysical Research Letters. 

As plants require nitrogen to grow, which is normally found in the soil, farmers will frequently spray their fields with nitrogen-rich fertilizers to try and encourage stronger growth from their crops. One of the by-products of this is ammonium, which also comes from the breakdown of livestock poop, is then carried by the wind long distances where it can enter cities and industrial areas. When it is mixed with emissions from the combustion of fossil fuels, such as from power plants and cars, it combines with nitrogen oxides and sulfur compounds to form solid particles, known as particulates, which are then inhaled.

This can then lead to long-term health impacts. Worldwide, it is thought that between three and five million people die prematurely due to illnesses caused due to this air pollution. Aside from causing breathing problems as the particles get caught in the lungs, particularly of the young and elderly, the long-term effects can include lung cancer, cardiovascular disease, and in many cases, death. Air pollution is becoming such an issue that some cities are taking action by banning cars on certain days, and other countries have even declared it a public health emergency.

But the researchers stress that they are not against the use of fertilizers needed to feed a growing population, and posit that there are other ways of dealing with the problem. “This is not against fertilizer – there are many places, including Africa, that need more of it,” explains Susanne Bauer, from Columbia University's Center for Climate Systems Research and NASA's Goddard Institute for Space Studies. “We expect population to go up, and to produce more food, we will need more fertilizer.”

Instead, if we simply remove the pollutants from combustion with which the fumes from fertilizer combine to form the harmful particulates, we could cut the level of pollution even if the use of fertilizer doubles, as is expected to occur. If coal power plants continue to shut, and we continue on the trend of removing diesel vehicles from cities, and potentially shifting to electric instead, then the level of fine-particulates would drop correspondingly.