India may have overtaken China as the world’s largest emitter of sulphur dioxide, says a study carried out by researchers at the National Aeronautics and Space Administration and the University of Maryland in the United States.

China’s sulphur dioxide emissions declined by 75% since 2007, while India’s emissions increased by 50%, says the Nasa study published in the journal Scientific Reports.

Sulphur dioxide, which is produced mostly when coal is burned to generate electricity, causes acid rain, smog and many health problems.

The report comes at a time when Delhi and several parts of North India is engulfed in a toxic smog. Air pollution levels have been over 30 times the World Health Organisation standards for daily exposure in Delhi and parts of its neighbouring states.

China and India are the world’s top consumers of coal, which contains up to 3% sulphur. However, the two countries are “on opposite trajectories for sulfurous pollution”. “The rapid decrease of sulphur dioxide emissions in China far exceeds expectations and projections,” said researcher Can Li. “This suggests that China is implementing sulphur dioxide controls beyond what climate modelers have taken into account.”

Although China seems to have brought sulphur dioxide emissions under control, “haze remains severe in China, indicating the importance of reducing emissions of other pollutants”, the report says. It noted that in India, approximately 33 million people now live in areas with substantial sulphur dioxide.

“Continued growth in emissions will adversely affect more people and further exacerbate morbidity and mortality,” the study concludes.