Susheel Kumar said air pollution levels in Delhi are a serious concern and need to be tackled quickly. He clarified that the pollution watchdog disagreed with the World Health Organization's recent report which estimated that PM2.5 (fine, respirable particles) was the highest among 1,600 global cities.

"Although the actual average in Delhi, as measured by our monitoring stations, are lower than the values in the WHO report, the variation does not undermine the fact that particulate matter (PM10 and PM2.5) has been a matter of concern," he said. "The situation becomes more critical with municipality and complexity of sources and unfavourable meteorological conditions, especially low winds and mixing height during winter months," Kumar added.

Prominent sources of PM emissions in Delhi according to him include road dust re-suspension, vehicular exhaust emissions, and industries including power plants. "It appears that the pace of growth has offset the benefits of mitigation actions," Susheel Kumar said.

The WHO data had revealed annual mean PM2.5 level is 153 micrograms per cubic metre and PM10 level is 286 microgram per cubic metre-far above the limit. But, according to CPCB data for 2013, the PM10 concentration in Delhi was 237 and PM2.5 was 112. The permissible limit for the two, according to Indian standards, is 60 and 40, respectively, as against the WHO standard of 20 and 10, respectively.