This week, air pollution forced some 4,000 schools to close in New Delhi, as India’s capital suffers through an air quality nightmare. Now, here’s more bad news on the pollution front: the country is passing China as the world’s biggest emitter of deadly man-made sulfur dioxide (SO2).

According to a University of Maryland-led study published in Nature on Thursday (Nov. 9), China’s SO2 emissions have fallen 75% since 2007, while India’s emissions have increased 50% in the same period. That puts India on track to overtake China, the world’s largest SO2 emitter since 2005—if it hasn’t already.

Using NASA satellite data, as well as data from primary emitters (such as power plants), researchers from the US and Canada estimated SO2 emission ranges for India and China from 2005 to 2016. Their estimates showed China’s SO2 emissions to be far lower now than earlier projections had estimated—possibly as much as four times lower—while India’s was more in line with what had been estimated. They concluded that the different trajectories were largely due to national policies related to coal-fired power plants, the major contributors to SO2, and to strict emissions controls implemented by China.

The estimates for India for 2016 ranged between 9.5 and 12.6 (pdf, p.2) megatons, while the estimates for China ranged between 7.5 and 11.6 megatons.

Image: Quartz

Yet, the number of people living close to substantial SO2 pollution still remains lower in India than in China. Substantial is defined in the paper as people living with an SO2 concentration of around 14 micrograms per cubic meter (μg/m3), compared to the WHO’s 20 μg/m3 for a 24-hour mean. Some 33 million people in India live with substantial SO2 pollution, compared with China’s 99 million, according to the study.

Researchers say they chose the 14 μg/m3 concentration because at that level the toxic gas could already affect human health (pdf, p. 4).

China has been particularly aggressive in recent years in trying to curb its notorious air pollution. It has built its coal plants to higher emissions standards, and also tried to wean homes and industry off coal. This winter it plans to heat four million homes with natural gas instead, and has asked steel producers in major producing hubs to reduce by one-third (paywall) their coking coal production.