New Delhi: After the early 2000s, when Delhi’s air became dramatically clean after the introduction of CNG in public transport, it has once again turned into a deadly cocktail of various pollutants. High levels of particulate matter (PM) that fired the original ‘clean air’ campaign 15 years ago, are no longer the only problem confronting policy makers. Levels of ozone, a product of the breakdown of oxides of nitrogen (NOx) in sunlight — and a major cause of respiratory problems — have been going up over the years.