There’s no doubt air pollution is a killer, causing more than 3 million deaths worldwide each year. But the top culprits behind the deadly air may come as a surprise.
Particles from small-scale energy use, mainly household fires for cooking and heating, are the leading cause of air-pollution deaths in many areas of Asia, researchers report in the Sept. 17 Nature. But in the northeastern United States, Russia and Europe, agricultural fumes from livestock and fertilizer were the deadliest types of air pollution, the researchers found. In these areas, small-scale energy use and agriculture beat out the more expected suspects: traffic and power plant pollution.
A different group of researchers found that recent reductions in Amazon forest fires save hundreds of lives each year on the continent. The finding appears September 16 in Nature Geoscience.
Though not all researchers are convinced by the researchers’ estimates, the findings may help guide new strategies on reducing pollution. In China, for instance, some policymakers battle smoggy days by regulating traffic, says coauthor of the Nature study Johannes Lelieveld, an atmospheric chemist at the Max Planck Institute for Chemistry in Mainz, Germany. “But this has actually done very little” to reduce pollution in some places, he says, because the true source of smog is home energy use.
Lelieveld and colleagues combined population and health data, satellite observations of atmospheric particles and a computer simulation of particles circulating and reacting in the atmosphere. The researchers then estimated the sources of those particles in each part of the world, attributing pollution to seven main categories, including agriculture, forest fires, power plants and traffic.
Drawing on earlier health studies linking air pollution exposure to risk of death, Lelieveld and colleagues estimated that air pollution causes 3.3 million deaths worldwide each year. Previous estimates pegged the death toll at around 3.2 million.
Next, by systematically removing one source of pollution at a time from their simulation, the researchers estimated each source’s deadliness. They found that small-scale energy use and agriculture were the two leading killers globally, accounting for 1 million and 660,000 deaths, respectively.
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The researchers also estimated the regional health impacts from each of the seven sources. For instance, in India and Vietnam, cooking and heating fires accounted for up to 60 percent of air-pollution deaths.
Looking forward, Lelieveld and colleagues estimate that if there are no new constraints on pollution emissions worldwide, deaths could double by 2050. That’s a worst-case scenario, Lelieveld says.
The study in Nature Geoscience looks backward, providing a glimpse of the potential benefits of reducing pollution. The authors used satellite data, a computer simulation and ground pollution observations to estimate the lives saved since 2004 by reducing fires. Leaders in Brazil worked to regulate and reduce those fires, which clear the way for cropland. The study, led by researchers at the University of Leeds, in England, found that reducing fire-fueled pollution in the atmosphere may have spared up to 1,700 lives each year in South America.
Some of the death figures may be overestimates, says UCLA environmental health scientist Michael Jerrett.Calculations that both studies used to assess death risks from air pollution assume that all particles are equally toxic. So the agriculture estimate, for instance, would hold true only if the nitrogen-based fumes from fertilizer and livestock urine are really as deadly as the pollution from, say, coal burning or traffic, Jerrett says. Some toxicological studies suggest that fumes from agriculture cause few health effects, he notes, although researchers still debate the topic.
Environmental health expert George Thurston of New York University School of Medicine agrees that the findings should be interpreted with caution. Most of the health data that both research groups used come from the United States. In a September 15 study in Environmental Health Perspectives, he and colleagues confirm the link between air pollution and death, using a decade of health data from 500,000 adults in the United States. But it’s unclear whether the mixes of air pollution particles in the United States and in other parts of the world are equally harmful, he says.
Editor’s Note: This article was updated September 25, 2015, to correct the description of areas where agriculture is the leading cause of air pollution-related deaths.