The Nagpur-based National Environmental Engineering Research Institute found an increased concentration of nitrogen oxides and carbon monoxide in the city’s atmosphere during a survey (1970 to 2000). According to an emissions estimate calculated by Neeri, transport emissions increased from 1,825 tonnes per annum in 1970 to 25,550 tonnes per annum in 1990. Diesel’s contribution to PM2.5 in Kolkata is the highest among all Indian metros: 61 per cent. Post-2000, the city had a large number of autorickshaws which increased the vehicular pollution manifold due to the use of katatel, a highly polluting mixture of solvent and kerosene, much cheaper than conventional fuels.