Air pollution has become a curse for millions of city residents in Asia, posing a mounting risk to the very young and very old, pregnant women and people with heart and respiratory problems, experts say.
"The levels of pollution in parts of China, India and elsewhere in Asia are just astronomically high and the health impacts are dramatic," said Bob O'Keefe of the Health Effects Institute (HEI), a US non-profit research agency. "This is a threat that was really underestimated in the past," he said.
This week, Singapore is grappling with record levels of air pollution, unleashed by fires in neighbouring Indonesia.
In January, pollution in Beijing went off the scale of an air-quality monitor at the American embassy, and the city's hospital admissions increased by 20 per cent. In August last year, Hong Kong suffered its highest-recorded pollution levels, prompting the territory to urge vulnerable population groups to stay indoors.
HEI estimates, derived from an exceptionally detailed analysis called the Global Burden of Disease, say some 3.2 million people around the world died prematurely from outdoor air pollution in 2010. China and India together accounted for some 2.5 million of these deaths, sharing the tally roughly equally.
Cathryn Tonne, at the London School of Hygiene and Tropical Medicine, points the finger at so-called PM2.5 - particles measuring 2.5 micrometres across or less, or 30 times smaller than a human hair.
Mainly generated by the burning of coal and oil for power, and of diesel and petrol for transport, these are many times more perilous than PM10 particles, which are 10 micrometres across, Tonne and colleagues found in research into heart deaths in England and Wales.
"We found that for every 10 microgrammes per cubic metre in PM2.5, there was a 20 per cent increase in the death rate," Tonne said.