In a study published in the journal Nature, they found that outdoor air pollution already kills about 3.3 million people a year worldwide. The majority of those deaths are in Asia where residential energy emissions, such as those from heating and cooking, have a major impact.

And that toll could double over the next 35 years, the researchers warned, unless clean-up measures are taken.

"This is an astounding number," said Jos Lelieveld of the Max Planck Institute for Chemistry in Germany, who led the research.

"In some counties air pollution is actually a leading cause of death, and in many countries it is a major issue."

Air pollution deaths are most commonly from heart disease, strokes or a lung disease called chronic obstructive pulmonary disease (COPD). It is also linked to deaths from lung cancer and acute respiratory infections.

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.

So for this study, Lelieveld's team combined a global atmospheric chemistry model with population data and health statistics to estimate the relative contribution of different kinds of outdoor air pollution, mainly from so-called fine particulate matter, to premature deaths.

- NDTV