According to a new report by the World Health Organization, air pollution is the cause of 7 million deaths a year worldwide, and is the single largest environmental health risk in the world today.
The staggering number — one in eight of all deaths, globally — is more than double previous WHO estimates of those killed by air pollution. WHO says that there is a stronger link between pollution and cardiovascular diseases like stroke and heart disease, and between air pollution and cancer, than previously thought. 
Certain cities and countries around the world have reputations as pollution hotspots. China has a well-documented and dangerous air pollution problem, which experts are recently referred to as a “nuclear winter.” In February, Beijing had zero visibility for nearly a week and the sun was blocked to the extent that it threatened the nation’s food chain. Paris is fast becoming the Chinese capital’s air pollution rival; earlier this month, the city opened up public transportation free of charge in an effort to curb air pollution from traffic. 
And in 2013, it was announced that air pollution alone killed 4,460 people in Tehran over the course of a year, following a decision by the city to shut down for five days in a bid to keep cars off the road. Iran’s air pollution problem was partly caused by the filthy fuel the country has had to conjure up after strict sanctions imposed by the U.S. in 2010 on refined gasoline. 
The WHO says that improved technology used to measure the effects of human exposure to pollutants has led to better estimates of the number of deaths. Exposure to indoor and outdoor air pollution contributes to respiratory diseases, including acute respiratory infections and chronic obstructive pulmonary disease, WHO reports. A reduction in air pollution will potentially save millions of lives and prevent against noncommunicable diseases.
WHO estimates that 4.3 million deaths were caused by indoor air pollution in 2012 in households that cooked over coal, wood and biomass stoves. There are an estimated 2.9 billion people who use wood, coal or dung as their main cooking fuel, which exposes them to a raft of pollution and potential diseases.
Low- and middle-income countries in South-East Asia and Western Pacific Regions suffered the largest burden of air pollution in 2012, with 3.3 million deaths linked to indoor air pollution, and 2.6 million deaths linked to outdoor air pollution. But cities in the Eastern Mediterranean Region had the highest mean levels of air pollution from 2003-2010.
“Poor women and children pay a heavy price from indoor air pollution since they spend more time at home breathing in smoke and soot from leaky coal and wood cook stoves,” Dr. Flavia Bustreo, WHO Assistant Director-General Family, Women and Children’s Health, said in a statement.
Exposure to outdoor and indoor air pollution, which killed an combined 7 million people, was the cause of similar health risks, including ischemic heart disease (coronary artery disease), stroke and lung cancer.  Acute lower respiratory infections in children was also one of the main diseases brought on by air pollution.
“The risks from air pollution are now far greater than previously thought or understood, particularly for heart disease and strokes,” says Dr Maria Neira, Director of WHO’s Department for Public Health, Environmental and Social Determinants of Health, in a statement. “Few risks have a greater impact on global health today than air pollution; the evidence signals the need for concerted action to clean up the air we all breathe.”
For more detailed information on individual countries, this interactive map from WHO lets you compare the data.