Around seven million people died in 2012 as a result of exposure to air pollution, accounting for one in eight deaths around the globe, the World Health Organisation said yesterday.
The figure has doubled over eight years. WHO officials explained that the latest estimate was more comprehensive than those in the past.
New methods were developed to measure outdoor air pollution in rural areas and more diseases like stroke and heart disease were found to be caused by air pollution, said Dr Carlos Dora, co-ordinator for interventions for healthy environments with the WHO's Department of Public Health and Environment.
He said the figures might not show whether the problem had worsened, but he added: "The problem is at large and should be a priority."
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The WHO estimates there were 3.7 million deaths from outdoor air pollution in 2012 from urban and rural sources worldwide, while indoor air pollution was linked to 4.3 million deaths. Some people were exposed to indoor and outdoor air pollution.
The WHO report combined various university studies that traced millions of people in different parts of the world exposed to different levels of air pollution. Their health was monitored along with various risk factors like smoking, diet and activity levels.
Strokes and ischaemic heart disease - both cardiovascular diseases - were the top two causes in both deaths caused by indoor and outdoor pollution. The rest of the causes were mainly lung diseases.
Rural outdoor air pollution was measured for the first time by new methods such as taking satellite pictures and using ground level monitors.
Deaths from indoor air pollution were mainly linked to the practice of 2.9 billion people living in homes cooking over coal, wood and biomass stoves.
The WHO did not produce figures for China. In January a former health minister, Chen Zhu, wrote an article claiming that air pollution caused 350,000 to 500,000 premature deaths a year in mainland China.
Professor Wong Tze-wai, of the Chinese University of Hong Kong 's school of public Health, said although the city's air pollution was mainly outdoor, indoor pollution such as from smoking, poor kitchen ventilation and harmful fumes released from furniture should not be overlooked.
In Hong Kong last year, 3,100 deaths were estimated to be related to air pollution, according to the Hedley Environmental Index developed by the University of Hong Kong.
A Department of Health spokesman said it had all along worked with the Environment Bureau and Environmental Protection Department by providing professional input from a public health perspective.
In January Chinese University was asked to conduct a 15-month study to develop a suitable way to quantify the health and economic impacts of air pollution.