While all norms are lax, those for on-road diesel vehicles are particularly lenient - smoke density norms are 50 HSU only for the Bharat Stage IV-compliant diesel vehicles. This clearly brings out the ineffectiveness of the programme.The smoke test was introduced in the 1960s to reduce the visibility problems due to diesel smoke. There can be a risk of misclassifying polluters - low smoke emissions can also mean high particulate emissions. But new generation diesel vehicles will come with advanced emissions control systems to reduce particulate and NOx. In countries like Brazil only 46 per cent of the diesel vehicles have working SCRs - rest have been disabled.

Source: Times of India February 21, 2017 15:00 UTC