Unseen and odorless, microscopic particles of air pollution wafting overseas and across continents kill some 380,000 people each year, according to a new study.
Exhaust from diesel engines, sulfur from coal-fired power plants, and desert dust swirl into an insidious cocktail of of tiny particles that can spend weeks airborne.
The most harmful are the smallest, less than 2.5 microns in diameter; when inhaled they can irritate the lungs or pass directly into the bloodstream and damage arteries.
Scientists and regulators know this is a major public health problem, especially in developing countries. But less clear is the effect that air pollution generated in regions like China and Southeast Asia has on far-off lands — say, North America.
Particulate pollution born overseas that floats into Canada, Mexico and the United States accounts for 6,600 premature deaths each year, Junfeng Liu of Princeton University and a team of researchers found.
Similarly, their study, published in the journal Atmospheric Environment suggests that a dust plume from Africa and a fog of pollution from Europe converge on the Indian subcontinent, condemning nearly 200,000 people to early deaths.
Globally, the team estimates that some 380,000 people die prematurely as the result of particulates emigrating from foreign lands.
"It's clear that this is having an impact on health," said Denise Mauzerall of Princeton University, a co-author on the study. "If we want to develop a strategy to deal with anthropogenic pollution, we should consider the health aspect as well as climate implications."
Mauzerall said that regulating diesel exhaust would be particularly beneficial. Diesel engines emit both black carbon, which absorbs sunlight and warms the atmosphere and microparticles.
But Richard Derwent, an independent air pollution specialist based in Newbury in the United Kingdom questions the importance of focusing on transcontinental pollution.
Most pollution tends to stay local; The team's study shows that in all cases, less than 20 percent of a regions' total pollution comes from foreign sources.
"If I was a policymaker in Europe, and I look at this and see that 2 percent of my pollution is coming from North America, am I going to immediately start negotiations with the U.S. or Canada?" he said. "I'm not."
"Domestic emissions dominate," Mauzerall agreed. "But there is enough long range transport to have an effect — it's small, but not negligible."
What's more, the difficulty of studying microparticles on a global scale means that the team had to treat natural dust sources as equally toxic to people as smoke from a coal-fired power plant. This is a gray area for science, as researchers are still uncertain which types of particles do the most damage.