The Central Pollution Control Board (CPCB) has claimed significant improvement in the air quality of Delhi since the tightening of auto emission norms. Compared to 1995 levels, sulphur-dioxide level has dropped by 4 to 40 per cent, nitrogen oxide by 4 to 13 per cent, particulate matter by 6 to 17 per cent, carbon monoxide by 3 per cent and lead by 11 to 60 per cent, depending on the areas in which the levels were measured.The report has been sent by CPCB chief Dilip Biswas to the environment and forests ministry and the Environment Pollution(Prevention and Control) Authority, a body set up by the Supreme Court for the improvement of environment in the National Capital Region(NCR).