New Delhi, Dec. 17: Air pollution over Delhi kills at least 20 people every day and triggers an estimated six million asthma attacks each year, a study has suggested.

Researchers at IIT Delhi say their study of the capital’s air pollution loads also holds lessons for India’s other metros on why efforts to improve city air quality sometimes don’t work.

The study, published last week in the journal Environmental Development, used a mathematical model to estimate the health impacts of air pollution from sources such as vehicle exhaust, diesel generators and brick kilns.

Using the concentrations of air pollutants observed over the city during 2010, the study estimated that air pollution contributes to anything from 7,350 to 16,200 premature deaths each year. (See chart)

Government air pollution readings show that the average levels of pollutant particles less than 10 micrometres in size (PM10) over Delhi increased two-and-a-half-fold to 260 micrograms per cubic metre between 2001 and 2010. This average value is four times the level prescribed under national standards and 13 times the level stipulated by the World Health Organisation.

The IIT Delhi study suggests that reducing pollution from a single source will not pull down PM10 concentrations to levels below the standard limit of 60 micrograms per cubic metre.

“A single clean-up strategy won’t work — that’s a lesson for other metros,” said Sarath Guttikunda, a project scientist at the Transport Research and Injury Prevention Programme at IIT Delhi and the study’s principal investigator.

A decade ago, the Delhi government initiated a diesel-to-CNG conversion plan but, Guttikunda said, the focus was almost entirely on public transport. “But diesel-burning continues for things other than buses. And there are many other sources of pollution,” Guttikunda said.

Guttikunda and co-author Rahul Goel, a PhD research scholar at IIT Delhi, have generated an emissions inventory that quantifies air pollution from vehicles as well as diesel generator sets, power plants, road dust, industries and the burning of city waste.

The study suggests that emissions from vehicles account for about 13 per cent of the PM10, 53 per cent of the oxides of nitrogen, 51 per cent of the volatile organic compounds and 18 per cent of the carbon monoxide in the air.

Diesel generator sets contribute 4 per cent of the PM10, 25 per cent of the oxides of nitrogen, 14 per cent of the volatile organic compounds, and 7 per cent of the carbon monoxide.

Guttikunda and Goel estimated the health impacts through a model that computes mortality and illness at different levels of concentrations of air pollutants.

Delhi is also girdled by hundreds of brick kilns that feed the construction industry across the National Capital Region. The study has estimated that brick kilns release 11 per cent of the PM10 and 12 per cent of the carbon monoxide.

The city’s colossal waste is another source of air pollution. Delhi’s three active landfills have a combined capacity of 5,000 tonnes of garbage per day, but the city churns out 9,000 tonnes a day. A part of the garbage is burned at landfill sites and elsewhere, and accounts for 7 per cent of the PM10.

“These findings highlight the importance of examining pollution sources beyond traffic emissions,” said Bhola Ram Gurjar, associate professor of civil engineering at IIT Roorkee, who was not associated with this study.

“What the trends in Delhi tell us is that multiple strategies are needed to address air pollution,” Guttikunda said. The researchers have called for a combination of measures that include technology and behavioural interventions.