NEW DELHI • The mega-city of New Delhi has tried everything, from banning diesel-guzzling SUVs to taking about half the city's cars off the streets, in a fight against air pollution.
Officials may yet have to do much, much more, based on National Aeronautics and Space Administration (Nasa) satellite research.
The research depicts how much sunlight is blocked by airborne particles, providing a proxy for levels of pollution.
The data shows parts of the Indo-Gangetic Plain - stretching across northern India from eastern Pakistan on one side to Bangladesh on the other - suffer some of the planet's worst haze in October through January after monsoon rains end in September.
On average during those months, as much as 10 times more solar radiation was blocked over the plain from 2008 to 2014 compared with the United States, signalling substantial concentrations in the air of the tiny, toxic PM2.5 particles that damage health.
While New Delhi is heaving with millions of vehicles, exhaust-pipe emissions in cities in the densely populated plain are just part of a complex picture, according to Dr Pawan Gupta, a research scientist at Goddard Earth Sciences Technology and Research in Maryland.
"During the post-monsoon season, the Indo-Gangetic Plain is one of the most heavily polluted regions," Dr Gupta said.
The burning of vegetation, for instance, by farmers or for cooking, as well as a winter climate and a topography that traps pollutants, all add to the problem, he said.
A ribbon of 17 cities across or near the plain dominates the World Health Organisation's ranking of the 30 metropolises with the worst air.
The chain starts with Peshawar in Pakistan and heads south and east in a shallow arc through New Delhi and on to Narayanganj in Bangladesh.
Hemmed in by the Himalayas, the region has what Dr Gupta calls a valley-like topography that helps trap pollutants when temperatures cool.
New Delhi-based Bhaskar Deol, founder of Mynergy Renewables, said much more needs to be done by the government, businesses and society to achieve clean air.
"International experience shows that comprehensive programmes tackling a wide range of pollutants can successfully solve the problem without sacrificing economic growth," he said.
The World Bank in 2013 estimated the annual cost of environmental degradation in India at US$80 billion (S$108 billion).