Each year, around 15 million people globally suffer from strokes and it is the fifth leading cause of death in the U.S. Scientists have long known that behavioral factors like diet, smoking, and exercise all contribute to those statistics, but now, a new study published in the journal Lancet Neurology names another leading contributor: air pollution.

The study, funded by public health groups in the U.S. and New Zealand, found that environmental and household air pollution was associated with a third of global strokes in 2013. The number was significantly higher in developing countries (34 percent) than it was in developed countries (10 percent). From 1990 to 2013, strokes associated with environmental air pollution — the particulate matter that’s released by burning biomass and running cars — increased by over a third. The global burden of stroke from environmental air pollution is almost as bad as smoking.

“Our study is the first to demonstrate a large and increasingly hazardous effect of air pollution on stroke burden worldwide,” researcher Valery Feigin told The Guardian.

However, as Feigin noted, the health burden from pollution could be even higher than researchers know because researchers only looked at fine particulate matter. Fossil fuels emissions tend to stress the cardiovascular system even more.

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