High incidence of ground-level ozone

A new nation-wide analysis by Centre for Science and Environment (CSE) says that in the summer of 2024, the levels of ground-level ozone in the air went up in the 10 major metropolitan areas of India.

These areas and cities include Bengaluru Metropolitan Area (Karnataka), Chennai Metropolitan Area (Tamil Nadu), Delhi-NCR, Greater Ahmedabad (Gujarat), Greater Hyderabad (Telangana), Greater Jaipur (Rajasthan), Kolkata Metropolitan Area (West Bengal), Greater Lucknow (Uttar Pradesh), Mumbai Metropolitan Region (Maharashtra) and Pune Metropolitan Region (Maharashtra). Says Anumita Roychowdhury, executive director, research and advocacy, CSE: “Ground-level ozone, a highly reactive gas, has serious health consequences. Those with respiratory conditions, asthma and chronic obstructive pulmonary disease – as well as children with premature lungs and older adults – are at serious risk. This can inflame and damage airways, make lungs susceptible to infection, aggravate asthma, emphysema, and chronic bronchitis and increase the frequency of asthma attacks leading to increased hospitalisation.”

The 2020 State of Global Air report states that age-standardised rates of death attributable to ground-level ozone is among the highest in India.The seasonal eight-hour daily maximum concentrations have recorded one of the highest increases in India between 2010 and 2017– about 17 per cent.

Ozone needs special attention -- ground-level ozone is not directly emitted from any source. It is produced from complex interaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) that are emitted from vehicles, power plants, factories, and other combustion sources and undergo cyclic reactions in the presence of sunlight to generate ground-level ozone. VOCs can also be emitted from natural sources, such as plants.

CSE points out that as the National Clean Air Programme (NCAP) gears up for its second phase, “its reform agenda has to address the multi-pollutant crisis and the combined threat from PM2.5, ozone, nitrogen oxides and other gases. Global experience shows that there is usually a trade-off -- as particulate pollution gets reduced, the problem of NOx and ground-level ozone increases. This requires significant tightening of the regulatory benchmark for ozone to address the toxic emisisons from industry, vehicles, households and open burning”. “Moreover, ozone not only builds up in metropolitan areas but also drifts long distances to form a regional pollutant that makes both local and regional action necessary,” she adds.

According to Avikal Somvanshi, programme manager, Urban Lab, CSE, who has led the CSE study, “in 2024, we have found that the geographical spread of the problem is much wider than what we had seen in the lockdown summer of 2020 in most metropolitan areas. This time’s toxic build-up has lasted longer in locations affected by the problem. Even the smaller metropolitan areas have witnessed rapid increase. In metropolitan areas in the south and western coastal belt, the problem is not limited only to the summer months.”

Inadequate monitoring, limited data and inappropriate methods of trend analysis have weakened the understanding of this growing public health hazard. The complex chemistry of ground-level ozone makes it a difficult pollutant to track and mitigate. Due to its toxic nature, ozone’s national ambient air quality standard has been set for only short-term exposures (one-hour and eight-hour averages), and compliance is measured by the number of days that exceed the standards. This requires early action.

Monitoring of ozone and method of its trend analysis need to improve to inform policy: the compliance method as set by the CPCB requires that the ozone standards are met for 98 per cent of the time in the year. It may exceed the limits on two per cent of the days in a year, but not on two consecutive days of monitoring. Says Kaur: “In other words, there should not be more than eight days in a year when the ozone standard is breeched, and none of those allowed exceedances can be on two consecutive days.”

Somvanshi explains: “The standard practice of the CPCB to average out the data of all stations in the city to determine daily AQI does not work for ground-level ozone as it is a short-lived and hyper-localised pollutant. A city-wide average concentration level over an extended time frame does not indicate the severity of the problem and health implications from local build-up and exposure for people living in hotspots.”

Ground-level ozone exceedance has been reported in all 10 metropolitan areas, with Delhi being the most affected: This year, so far, Delhi-NCR has recorded 176 days of exceedances during the study period. Mumbai-MMR and Pune are tied at the second spot with 138 days of exceedance each. Jaipur recorded an exceedance on 126 days; Hyderabad, on 86 days; Kolkata-KMA, 63 days;Bengaluru, 59 days; Lucknow, 49 days; and Ahmedabad, 41 days. The least number of exceedance was recorded in Chennai: a mere nine days.

Critical to act immediately

India must immediately refine the action strategy for combined control of particulate pollution, ozone and its precursor gases like NOx to maximise the co-benefits of the action plan. We need stringent control of precursor gases from vehicles, industry, cooking on solid fuels, open burning and other combustion sources while reducing PM2.5. Simultaneously, the country needs to develop a robust public information and dissemination system to alert the public about ozone exceedance – wherever ozone build-up is

happening. (CSE)