SHANGHAI (Reuters) - China and India accounted for more than half of the total number of global deaths attributable to air pollution in 2015, researchers said in a study published on Tuesday.

The U.S.-based Health Effects Institute (HEI) found that air pollution caused more than 4.2 million early deaths worldwide in 2015, making it the fifth highest cause of death, with about 2.2 million deaths in China and India.

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The institute's study, the first of its kind, was based on the Global Burden of Disease (GBD) project, a database backed by the Bill & Melinda Gates Foundation that tracks the role that behavioral, dietary and environmental factors play in deaths across 195 countries.

New evidence and methodologies mean that the estimate is significantly higher than the figure published by the World Health Organization last year, which put the number of global air pollution-related deaths in 2012 at 3 million, HEI said.

The institute, which has also launched an online database showing the global impact of pollution on health (https://www.stateofglobalair.org), said 92 percent of the world's population lives in areas with unhealthy air.

Air pollution has been linked to higher rates of cancer, stroke and heart disease, as well as chronic respiratory conditions such as asthma.

China and India, the world's two most populous countries, each accounted for 1.1 million deaths, the findings showed, but China is pushing ahead when it comes to taking action, HEI president Dan Greenbaum told Reuters.

"(India) has got a longer way to go, and they still appear to have some ministers who say there is not a strong connection between air pollution and mortality in spite of quite a lot of evidence," he said.

A spokesman for India's environment ministry could not be reached for comment, but minister Anil Madhav Dave said last week that "there is no conclusive data available" on the link between pollution and mortality, media reported.

China's environment ministry did not respond to a request to comment on whether the estimate of 1.1 million deaths was accurate.

Though China has launched a campaign to improve air quality, authorities have been reluctant to draw direct links between air pollution and mortality, with the health ministry saying it had "no data" linking smog to higher incidences of cancer.

"It is currently too early to draw conclusions about the extent of the impact of smog on health, especially its long-term impact on the body," a ministry spokesman told media during a press briefing in January.

In a long-term national healthcare plan published last October, the government acknowledged the link between health and pollution, and pledged to assess the precise impacts as well as boost environmental monitoring capabilities.

By David Stanway

(Additional reporting by Mayank Bhardwaj in New Delhi; Editing by Richard Pullin, Robert Birsel)