Delhi and Mumbai have topped a list of 10 Indian cities by breaching the safe levels for ground-level ozone while Calcutta has ranked sixth, the Central Pollution Control Board recently informed the National Green Tribunal (NGT).

“The Delhi-NCR and Mumbai Metropolitan Regions reported higher exceedances of ozone concentration compared to other regions,” the CPCB said.

The pollution regulator was responding to the NGT query in a suo motu case based on a news report about rising ground-level ozone pollution in Indian cities.

The CPCB had analysed safety limits of ground ozone levels at 178 monitoring stations across 10 cities in 2023.

A total of 25 stations of the 57 in Delhi, and 22 of 45 in Mumbai reported ozone levels beyond the safe limit of 2 per cent for eight hours.

Out of all 178 monitoring stations, the safe limit was breached at 65.

In Calcutta, the safety limit was violated at 1 of the 10 stations.

The fresh report is based on a 2 per cent exceedance benchmark compared with the old report, which was based on a 5 per cent exceedance count. A lower exceedance threshold provides a more sensitive measure of ozone pollution and its impact on human health.

The CPCB said the elevated ozone levels might be attributed to emissions from the transport sector, power plants and industrial activities.

“Additionally, ozone precursors may also originate from natural sources, including biogenic emissions of volatile organic compounds, soil-based emissions of nitrogen oxides, wildfire-induced emissions of carbon monoxide, and methane emissions from the biosphere,” the CPCB said. “The tropospheric or ground-level ozone is formed primarily from complex non-linear photochemical reactions between two major classes of air pollutants, volatile organic compounds and nitrogen oxides.”