Delhi may well have reduce air pollution levels in the last few years, but new challenges are threatening to undo the good work in the absence of sustained initiative in this direction, said the participants at a lecture on air pollution today. They pointed out that lesser-known toxins like Benzene, Ozone and poly-aromatic compounds are emerging as new headaches in the post-conversion era in the capital. "Bucking the general trend, nitrogen dioxide levels are rising inthe city," warned Anumita Roy Chowdhury of the Centre for Science and Environment.