Farming, not always as green as you think. Image: Brian Evans / Flickr

The largest source of deadly air pollution in many parts of the world isn’t cars or power plants—it’s farms. That’s the unsettling conclusion of a study conducted by researchers at Columbia University, who found that agricultural nitrogen emissions are a major contributor to fine particulate matter, tiny particles that cause heart disease and respiratory problems.

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The trouble starts when fertilizers and manure release ammonia, or NH3, into the air. As ammonia is swept downwind of farms, it encounters pollutants produced by vehicles and factories, including nitrogen oxides and sulfates. Through a series of chemical reactions, these molecules combine to generate tiny, gag-inducing particles that are less than 2.5 microns in diameter (often referred to as “PM 2.5").

PM 2.5 is notoriously dangerous—a recent study estimated that these particles are responsible for up 5.5 million premature deaths each year. Just last week, an analysis by the World Health Organization revealed that PM 2.5 and larger particles create unsafe air for more than 80 percent city dwellers, especially in the developing world.

But in the United States, Europe, Russia and China, folks living near farms may be at the greatest risk.

The role of farms as a source of local air pollution spikes has been studied for years. But the new study, published this week in Geophysical Research Letters, highlights just how widespread of an issue ammonia emissions are on a global scale. In much of the eastern and central United States, ammonia is responsible for over half of all aerosol precursors. In parts of Europe and China, farms are an even bigger contributor to foul air.

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There is, however, a silver lining to understanding the cause of air pollution: figuring out how to clean it up. Indeed, since it’s the combination of ammonia and industrial pollutants that produce particulate matter, air pollution near farms should start to fall as we transition to clean energy vehicles and impose stricter regulations on power plants.

That’s very good news, because it’s unlikely the world is going to wean itself off nitrogen fertilizers anytime soon—not if we hope to feed an extra two billion people by 2050.

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[Columbia Earth Institute News]