Air pollution kills more than 5.5 million people each year, new research has shown.
Most of the deaths occur in China and India, two of the world's fastest-growing economies, say scientists.
International researchers conducted estimates of indoor and outdoor air pollution levels in the two countries and calculated their impact on health.
The data was compiled by the World Health Organisation's Global Burden of Disease project.
Results show that China and India together account for 55 per cent of all the deaths caused by air pollution worldwide. Some 1.6 million people died as a result of poor air quality in China, and 1.4 million in India in 2013.
Professor Michael Brauer, from the University of British Columbia in Canada, said: "Air pollution is the fourth highest risk factor for death globally and by far the leading environmental risk factor for disease.
“Reducing air pollution is an incredibly efficient way to improve the health of a population.”
The findings were presented at the annual meeting of the American Association for the Advancement of Science(AAAS) taking place in Washington DC.
Power plants, industrial manufacturing, vehicle exhaust and burning coal and wood were all named as sources of small particles that lodge in the lungs and can endanger health.
In China, burning coal was the biggest contributor to air pollution. This alone was responsible for around 366,000 Chinese deaths in 2013.
A major source of poor air quality in India was the practice of burning wood, animal dung and other forms of biomass for cooking and heating.
Millions of families, including some of India’s poorest, were regularly exposed to high levels of particulate matter in their own homes.
Over the past half century, North America, Western Europe and Japan have made great strides to combat air pollution by using cleaner fuels and more efficient vehicles, limiting coal burning, and imposing restrictions on electric power plants and factories, the researchers pointed out.
Dan Greenbaum, president of the Health Effects Institute, a Boston-based non-profit organisation that sponsors efforts to analyse air pollution from different sources, said: "Having been in charge of designing and implementing strategies to improve air in the United States, I know how difficult it is. Developing countries have a tremendous task in front of them.
“This research helps guide the way by identifying the actions which can best improve public health.”
PA