Twenty six-year-old boatman Suraj manages at least 10 trips up and down the famous ghats of Varanasi on a lucky day in the tourist season. But these trips use up at least 10 litres of diesel, a major air pollutant. His is among the 600 diesel boats running on the River Ganga in Varanasi, which adds up to the pollution levels in a city that - according to official figures of the Central Pollution Control Board - had the most toxic air across India last year.