Delhi faces a daunting challenge of cutting pollution level by 65 per cent to meet the clans air standards, an environment think tank claimed on Friday.

The NGO, Centre for Science and Environment (CSE), analysed the annual air quality data recorded by the Central Pollution Control Board (CPCB).

According to the CPCB data, the three-year average of PM 2.5 (particulate matters with less than 2.5 microns width) levels during 2016-2018 was 25 per cent lower than the 2011-2014 baseline (three-year average).

"However, CSE cautions that even after this reduction and stabilisation, Delhi faces the daunting challenge of 65 per cent reduction from the current baseline to meet the clean air standards for PM 2.5," the NGO said in a statement.

The CSE, which released new results from its ongoing analysis of air quality trends in Delhi, however, said, "After the initiation of comprehensive clean air action plan and graded response action plan, there are early signs of stabilisation and bend in pollution curve, increased number of days in cleaner air quality index categories, and change in the pattern of smog episodes."

It also said the stabilisation has been possible because of multi-sector intervention to clean up the vehicle fleet and fuels, tightening of industrial pollution norms and phasing out dirty industrial fuels of pet coke, furnace oil and coal.

It also listed shutting down of all coal power plants in the city, action on brick kilns and hotspot areas, and dust control in construction sites.

"While this level of action has helped in stabilizing the problem, much harder decisions and aggressive action at a scale is needed to achieve 65 per cent cut in PM2.5 to meet the clean air targets," said Anumita Roychowdhury, executive director-research and advocacy, CSE.

As many as 122 cities are preparing and implementing clean air action plans under the National Clean Air Programme (NCAP), which aims at reducing PM10 and PM2.5 (air pollutants) levels in the country by 20-30 per cent by 2024.

The NCAP aims to attain the annual average air quality standard in 122 non-attainment cities.

PM2.5 pollutants can go deep into the respiratory tract and reach the lungs. Exposure to them can cause various short-term health effects such as eye, nose, throat and lung irritation.