Air pollution has become the world's single biggest environmental health risk, linked to around 7 million – or nearly one in eight deaths in 2012 – according to the World Health Organisation (WHO).

The new figures are more than double previous estimates and suggest that outdoor pollution from traffic fumes and coal-burning, and indoor pollution from wood and coal stoves, kills more people than smoking, road deaths and diabetes combined.

Around 80% of the 3.7 million deaths from outdoor pollution came as a result of stroke and heart disease, 11% from lung diseases and 6% from cancers. The vast majority were in Asia, with 180,000 in the Americas and Europe combined, said the WHO.

Indoor air pollution led to 4.3 million deaths, of which 34% were caused by strokes, 26% heart diseases and 12% respiratory disease in children. Only 19,000 of these deaths were in rich countries, with the vast majority being in low- and middle-income countries. Because many people are exposed to both indoor and outdoor air pollution, the WHO said deaths attributed to the two sources cannot be added together.

"The risks from air pollution are now far greater than previously thought or understood, particularly for heart disease and strokes," said Maria Neira, director of WHO's department for public health, environmental and social determinants of health. "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."

South-east Asia, said the WHO, is now the most polluted region in the world, with 3.3 million deaths linked to indoor air pollution and 2.6 million deaths related to outdoor air pollution. This reflects the explosive growth of cities and industrial development in China and India, as well as continuing deep poverty in rural areas.

A woman breastfeeds her baby while another prepares food on an open fire indoor in Kagera, Tanzania. Women had higher levels of exposure than men in developing countries. Photograph: Jochem Wijnands/Alamy

The new estimates are based not on an significant increase in pollution, but on improved knowledge of the links between air pollutants and heart diseases and cancers, in addition to known links with respiratory diseases. A 2008 WHO report estimated that outdoor pollution led to about 1.3 million deaths, while about 1.9 million people were killed by indoor pollution. A Lancet study last year suggested that the surge in car use in south and east Asia killed 2.1 million people prematurely in 2010. Last year, WHO's cancer agency classified air pollution as a carcinogen, linking dirty air to lung and bladder cancer.

"Cleaning up the air we breathe prevents non-communicable diseases as well as reduces disease risks among women and vulnerable groups, including children and the elderly," said Dr Flavia Bustreo, WHO assistant director general of family, women and children's health. "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."

Martin Williams, professor of air quality at the environmental research group, King's College London, said: "This is an important study, and although the majority of attributable deaths occur in south-east Asia and the western Pacific, air pollution impacts on mortality and health are still a significant public health problem in Europe, including the UK."

Air pollution is increasingly linked with ill health and deaths in rich countries as traffic emissions rise. In the US, air pollution causes about 200,000 early deaths a year, with emissions from cars and trucks causing 53,000 and power generation 52,000, according to MIT's environment laboratory. California suffers most from air pollution, with 21,000 early deaths.

In Europe, poor air quality is the top environmental cause of premature deaths in the EU, causing more than 100,000 premature deaths a year and costing from £300bn-£800bn a year in extra health costs, said Janez Potočnik, the EU environment commissioner.

Air pollution causes 29,000 early deaths a year in the UK and similar numbers in France and Germany.