Scientists say finding is alarming, and shows that harm caused by air pollution to the lungs, heart and brain has been underestimated.

Air pollution has become a major contributor to stroke for the first time, with unclean air now blamed for nearly one third of the years of healthy life lost to the condition worldwide.

In an unprecedented survey of global risk factors for stroke, air pollution in the form of fine particulate matter ranked seventh in terms of its impact on healthy lifespan, while household air pollution from burning solid fuels ranked eighth.

Valery Feigin, director of the National Institute for Stroke and Applied Neurosciences at Auckland University of Technology, said that while he expected air pollution to emerge as a threat, the extent of the problem had taken researchers by surprise.