In Delhi, which was for two consecutive years in 2014 and 2015 ranked as the most polluted city in the world, people could live nine years longer if it met WHO standards. And, six years longer if it met the country’s own national standards.

Residents of Kolkata and Mumbai could live roughly 3.5 years longer if the accepted particulate matter levels conformed to WHO standards, the tool show.

According to WHO, reducing annual average particulate matter (PM10) levels from 70 to 20 micrograms per cubic metre (µg/m3) can cut air pollution-related deaths by around 15 per cent.

A WHO factsheet of September 2016 states that the annual acceptable mean for PM2.5 has been set at 10 µg/m3 and for PM10 at 20 µg/m3. The national guidelines in India have set the annual acceptable mean for PM2.5 at 40 µg/m3 and PM10 at 60 µg/m3.

Indians could live more than a year longer on average, or a combined more than 1.6 billion life years, if the country reduces pollution to comply with its national standards, the AQLI reveals. According to official figures, the average life expectancy in India is 67.3 for males and 69.6 for females.

The AQLI “translates particulate pollution concentrations into their impact on lifespans” and “complements” the frequently used Air Quality Index (AQI) that does not directly map the effects of air pollution on health. So far, “studies tend to rely on data tracking people’s exposure over a short time period” but this particular tool tried to capture lifetime exposure.

The AQLI is based on data from a pair of studies — Chen et al. 2013 and Ebenstein et al. 2017 — published in the Proceedings of the National Academy of Sciences (PNAS).

The 2017 study New Evidence on the Impact of Sustained Exposure to Air Pollution on Life Expectancy from China’s Huai River Policy finds “that a 10 µg/m3 increase in airborne particulate matter (PM10) reduces life expectancy by 0.64 years…”

The estimate is derived from the quasi-experimental variation in PM10 generated by the Huai policy, which provides free or heavily subsidised coal for indoor heating during the winter to cities north of the river but not to the south, according to the AQLI.

For the study, the researchers collected air pollution data for six main pollutants in 154 cities in the north and south from 1981 to 2012 with the main subject being PM10. They then linked the particulate pollution data to mortality data from 2004 to 2012 for which they used the Chinese Center for Disease Control and Prevention’s (CDC) Disease Surveillance Points (DSP) survey, a high-quality nationally representative survey through a coverage population of more than 73 million people at 161 locations.

“High levels of air pollution are a part of people’s lives in India, just as they were in the US, England, Japan and other countries in the past. The last several decades have seen tremendous progress in many of these countries, but this progress did not happen by accident — it was the result of policy choices,” wrote Micheal Greenstone, director of EPIC and one of the authors of the 2017 study.

“As India navigates the dual and conflicting goals for economic growth and environmental quality, the AQLI provides a tool to make the benefits of policies to reduce air pollution concrete,” wrote Greenstone, who is also Milton Friedman Professor in Economics.

EPIC India director Anant Sudarshan told The Indian Express that air pollution discussions at the government level are rarely motivated in terms of health effects. “It is not explained to people why we need to meet a standard. The government needs to speak the language of health, and explain to people that not meeting a standard would mean living less number of years,” he said.