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

Research by the US-based Health Effects Institute (HEI) showed that air pollution caused more than 4.2 million early deaths worldwide in 2015, making it the fifth highest cause of death, with around 2.2 million deaths in China and India alone.

The institute, which has also launched an online database showing the global impact of pollution on health, 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 like asthma.

China and India, the world's two most populated nations, each accounted for 1.1 million deaths, the findings showed, but China is now 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.

China's authorities have also been reluctant to draw direct links between air pollution and mortality, with the country's 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 domestic 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.