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According to a new analysis by the Centre for Science and Environment (CSE), parts of the Delhi-NCR region witnessed ground-level ozone readings exceeding the national standards on 87 out of 92 days in the summer period between March and May. The worst affected parts in the area are New Delhi and South Delhi neighbourhoods.
Based on Central Pollution Control Board (CPCB) data on ozone levels from 58 stations across Delhi-NCR, the CSE analysis also noted that although the spatial spread — number of stations exceeding the standard across the core NCR — of ground-level ozone has been lower this year, its duration has increased.
“This summer, at the stations which reported exceedance the rolling 8-hr average stayed above standard for 4.9 hours on average, which is up from 4.6 hours observed last summer.,” the assessment noted.
Moreover, the region is seeing a rare phenomenon where ozone levels remain elevated hours after sunset — ground-level ozone should ideally become negligible during the night. Another issue is that the pollutant, which once used to be prominent only during the summers, has become a yearlong problem.
What is ground-level ozone?
Also known as tropospheric ozone, ground-level ozone is “a colourless and highly irritating gas that forms just above the Earth’s surface (up to 2 miles above the ground).,” as per Environment and Climate Change Canada (ECCC). Notably, it’s not directly emitted into the air but rather produced when two primary pollutants react in sunlight and stagnant air. These two primary pollutants are nitrogen oxides (NOx) and volatile organic compounds (VOCs). Therefore, ground-level ozone is called a “secondary” pollutant.
“NOx and VOCs come from natural sources as well as human activities. About 95 per cent of NOx from human activity comes from the burning of coal, gasoline and oil in motor vehicles, homes, industries and power plants. VOCs from human activity come mainly from gasoline combustion and marketing, upstream oil and gas production, residential wood combustion, and from the evaporation of liquid fuels and solvents.,” ECCC noted.
Ground-level ozone is likely to breach safety standards on hot summer days in urban areas, but can also reach unhealthy levels during colder months. The United States Environmental Protection Agency (EPA) mentioned on its website that the pollutant can also travel long distances due to wind and affect rural areas also.
What are the harmful effects of ground-level ozone?
The CSE analysis said as ground-level ozone is a highly reactive gas, it has serious health consequences. “Those with respiratory conditions, asthma, chronic obstructive pulmonary disease, and particularly 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.,” it mentioned.
The pollutant can also affect sensitive vegetation and ecosystems, including forests, parks and wildlife refuges. Significantly, it can harm sensitive vegetation during the growing season too, EPA explained.
What is the situation of ground-level ozone in India?
In recent years, ground-level ozone has become a serious public health issue in India, as per the CSE analysis. It quoted the 2020 State of Global Air report, which stated that age-standardised rates of death attributable to ground-level ozone are among the highest in the country “and the seasonal 8-hour daily maximum concentrations have recorded one of the highest increases in India between 2010 and 2017– about 17 per cent.”
