The annual death toll from outdoor air pollution could double to 6.6 million globally by 2050 without new antipollution measures, a new study suggests. But policymakers seeking to reduce the death toll will need to clamp down on a wide array of potentially hard to control pollution sources—including household furnaces and agricultural activities—that are expected to play a growing role, researchers report today in Nature.

The study marks a solid step toward clarifying exactly how major sources of air pollution contribute to premature death around the world, says Aaron Cohen, an epidemiologist at the Health Effects Institute, a nonprofit research organization in Boston, who wasn’t involved in the study. That information will prove useful to policymakers, he suggests.

Existing estimates have been hampered by gaps in air pollution data, particularly in the developing world, and a lack of knowledge about how specific air pollution sources contribute to the risk of disease and death.

To get a clearer picture, researchers led by Jos Lelieveld of the Germany-based Max Planck Institute decided to take a global look at outdoor air pollution, which the World Health Organization (WHO) estimates is responsible for almost 3.5 million premature deaths annually. (WHO estimates indoor air pollution accounts for an additional 3.5 million.)

Using a computer model that fused air pollution and atmospheric chemistry data, they estimated what annual average levels of ozone (a key smog ingredient) and fine particulates smaller than 2.5 microns (PM2.5) were in 2010 within 100-km-by-100-km grid squares across the world. Then they forecast what the levels of both pollutants would be in 2050, assuming policymakers implemented no new controls.

Next, the researchers estimated how many premature deaths the pollution caused in each square. To do that, they used a set of equations—recently updated based on the most recent epidemiological research—describing how exposure to air pollution affects a person’s risk of dying from various diseases. These “exposure response relationship” equations enabled the researchers to calculate how fine particles and smog would affect the risk of a range of diseases, including heart attacks, strokes, lung cancer, and pulmonary disorders.

In a final step, they estimated the fraction of deaths in each square attributable to a specific pollution source, including automobiles, power plants, in-home energy generation, and farm activities such as burning crop residues.

Overall, the researchers concluded that, in 2010, 3.3 million people died prematurely from outdoor PM2.5 and ozone pollution. That number echoes recent WHO estimates. But the more troubling finding, the researchers say, is that the annual death toll would rise to 6.6 million by 2050 without new controls.

The deadliest outdoor pollution source—accounting for 31%, or about 1 million, of premature deaths in 2010—is residential energy use, such as furnaces. And the bulk of these deaths would occur in Asian countries such as India and China, the researchers concluded, where households often use soot-emitting stoves and furnaces powered by wood. These emissions could be tricky to clamp down on; for instance, persuading residents of India to adopt cleaner technologies has proven difficult, Lelieveld says, in no small part because of cultural and family traditions.

The second deadliest source of pollution in 2010 was agriculture, accounting for about 20%, or more than 600,000, of the premature deaths in 2010, the researchers say.

“I was surprised” by that result, Lelieveld said. “What you tend to think is that [air pollution comes from] mostly traffic, and maybe industry.” But agricultural activities such as animal husbandry and fertilizer use generate ammonia, which can be converted to fine particles in the air, he explained. Agriculture is the leading source of outdoor pollution–related premature mortality in the eastern United States, Europe, and in countries such as Russia, Japan, and Turkey, the researchers found.

Other pollution sources, including the power sector, industry, biomass burning, and vehicle traffic, each made smaller contributions to the death total, the study concluded.