Air pollution killed about 7 million people in 2012, making it the world's single biggest environmental health risk, according to the World Health Organization.

The toll, a doubling of previous estimates, means one in eight of all global deaths in 2012 was linked to polluted air and shows how reducing pollution inside and outside of people's homes could save millions of lives in the future, according to the United Nations health agency.

Air pollution deaths are most commonly from heart disease, strokes or chronic obstructive pulmonary disease. "The evidence signals the need for concerted action to clean up the air we all breathe," says Maria Neira, head of the WHO's environmental and social public health department.