New Delhi: Delhi saw 176 days in summer 2024, between 1 April and 18 July, where ground ozone levels exceeded national air quality standards, according to a new report by the Centre for Science and Environment (CSE), a New Delhi-based think tank. Released Tuesday, the CSE report on ground-level ozone is based on a nationwide analysis which found that 10 metropolitan cities saw an increase in daily maximum levels of ozone between 2020 and 2024.
The report found that ozone levels also increased in non-metropolitan areas compared to last year, and persisted beyond summer months which, it said, was a concerning trend.
Ozone pollution is a very specific phenomenon — the presence of the gas in the stratosphere is considered healthy, but ground-level ozone causes health issues for humans. The CSE report explained how ground-level ozone is not released by a particular source, but forms through the “complex interaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs)”, thereby making it difficult to both track and mitigate ground-level ozone.
VOCs like benzene, formaldehyde and ethanol are produced by various sources including industries, vehicles, air fresheners and even some plants and trees.
“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 hospitalizations,” Anumita Roychowdhury, executive director, research and advocacy, CSE, said in a press release.
The report also found that between April and July this year, Delhi, Mumbai, Bengaluru, Lucknow, Kolkata, Pune, Hyderabad, Ahmedabad, Chennai and Jaipur recorded more than 100 days of ground-level ozone exceeding the national ambient air quality standards set by India.
This trend has also been reported by the State of Global Air Report 2020, which found that India witnessed a 17 percent increase in seasonal daily maximum ozone concentrations between 2010 and 2017. Additionally, India also has one of the highest death rates from ozone pollution. A 2020 Lancet study also found that ambient ozone pollution deaths increased by 152 percent in India between 1990 and 2019.
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Ozone pollution: measurement & mitigation
The issue with ozone pollution is not just its harmful health effects but the current incapacity of pollution monitoring systems to accurately understand it. The Central Pollution Control Board’s (CPCB’s) method of calculating ground ozone levels includes averaging the data from all stations in a city. The report flagged how this is problematic because ozone, unlike particulate matter, is short-lived and a highly 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,” Avikal Somvanshi, study lead and programme manager, Urban Lab, CSE, said in the press release.
The current ozone compliance method by the CPCB is that 98 percent of the days in a year, the ozone pollution should be within the standards set by the CPCB. The ozone levels should also not exceed CPCB standards for two consecutive days in a year. However, to accurately measure this, the calculation method should move beyond city-wide averages, the report said.
Another important issue is that the CPCB monitors have a cap on the amount of ozone concentration they can measure. The report pointed out how the CPCB monitors do not measure ozone levels beyond 200 micrograms per cubic metre, since at times the Delhi Pollution Control Committee monitors have shown higher levels of ozone concentration.
This cap could seriously affect the measurement of ozone pollution and hinder understanding of peak ozone concentrations, the report pointed out. For instance, the one-hourly maximum standard of ozone is 180 micrograms per cubic metre. If the CPCB monitors do not record ozone levels beyond 200 micrograms per cubic metre, it will be difficult to assess “short-term exposure impacts” of ozone.
Year-wide ozone levels and hotspots
While the report mainly looked at ozone concentrations during the summer months of April to July, they also found an uptick in the number of days when ozone levels were higher than the standard levels during winter months in many places. Although fog and lower temperatures of winters usually hinder the formation of ground-level ozone, the report said that ozone levels exceeded in many New Delhi stations. Similarly, night-time ozone levels are normally negligible because the presence of sunlight is needed to form ground-level ozone. But again, all stations across the country that breached ozone standards reported night-time breaches also. Mumbai was the highest with 171 nights, followed by Delhi at 161 nights. Ozone concentrations beyond 100 micrograms per cubic metre between 10 pm to 2 am were considered an exceedance by the CSE report.
The exact metrics of the concentration of ozone in metropolitan areas is also an interesting phenomenon because the report observed that while ozone forms in areas with high nitrogen dioxide, it persists in areas with low nitrogen dioxide and low PM2.5 concentrations. Most of the ozone hotspots are in green areas with low levels of nitrogen dioxide, indicating that ozone spreads beyond its origin in highly polluted areas.
“This is consistent with the science that ozone drifts and gathers in areas where comparatively lesser amount of gaseous pollutants are present to further mop it up. Thus, relatively cleaner areas including urban peripheries can have comparatively higher ozone concentration,” Roychowdhury said in the release.
This fact also calls into question the current mitigation measures by the government for ground ozone pollution. While the National Clean Air Programme largely tackles PM2.5 and PM10 pollution, the report urges focus on the ‘multi-pollutant crisis’ that results in ozone pollution. Policy framework and mitigation to tackle ozone pollution should start with bringing about changes in the way it is measured, and also understanding the hyperlocal impact of ground-level ozone, the report said.
“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,” Roychowdhury said.
(Edited by Radifah Kabir)
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