India has the world’s second highest rate of premature mortality due to outdoor air pollution after China, with Delhi faring the worst among cities by accounting for 19,000 such deaths in 2010 — the number is expected to rise to 31,000 by 2025.

Kolkata’s 2010 figure of 13,500 is expected to double to 26,000 while Mumbai’s 10,000 is set to rise to 17,000 by 2025, the study said.

“As a global average, the per capita mortality by air pollution is about five per 10, 000 persons per year. In the urban environment this can be several times higher,” Jos Lelieveld of the Max Planck Institute of Chemistry, Germany, who led the study, said. “We calculated that 75% of the premature mortality by air pollution occurs in Asia, with 1.4 million people/year in China and 650,000 in India.”

Calculating the health and mortality effects of outdoor air pollution on a global scale is not easy, partly because air quality is not monitored in every region and the toxicity of particles varies depending on their source.

Air pollution deaths are most commonly from heart disease, strokes or a lung disease called chronic obstructive pulmonary disease. Strokes and heart attacks are responsible for nearly 75% of air pollution-related mortality — somewhat more than 25% is related to respiratory diseases and lung cancer.

Of the 644,993 premature deaths in India due to outdoor pollution in 2010, the study found 50% were attributable to residential energy, 14% to power generation, 7% to industrial and biomass burning and 5% to vehicular pollution.

The study linked premature mortality to different emission source categories of atmospheric pollutants. Overall, outdoor air pollution, mostly by fine particulate matter and to a lesser extent by ozone, leads to 3.3 million premature deaths a year worldwide.

This estimate of the magnitude of premature mortality by air pollution is consistent with the values given in the latest global burden of disease study.