Ozone: The Secret Pollutant Creeping Into Metropolitan Cities This Winter

Delhi has earned notoriety for being among the most polluted cities in the world. In any given week in August, the air quality index for Delhi ranged from between 101 (unhealthy for sensitive groups) and 34 (good) on one day to between 182 (unhealthy) to 59 (moderate) on another. 

Within the country, the top most polluted places vary on a day-to-day basis.

The one thing that connects them though is the fact that they are all industrial areas.  But, before we come to that, a very brief note on the Air Quality Index (AQI) which serves as a crucial indicator of pollution levels in the atmosphere—an index that is often misunderstood and oversimplified—is required.  

AQI is a composite measure that indicates the overall quality of air at any given time based on the concentration of various pollutants. These include particulate matter (PM 2.5 and PM10), ground-level ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2) and carbon monoxide (CO). 

However, AQI is not a static measure. It fluctuates based on atmospheric conditions like temperature, humidity, wind speed, and pressure. These conditions influence the dispersion, transformation, and concentration of pollutants in the air, making AQI dynamic. For instance, temperature changes can trap pollutants close to the ground, leading to sudden spikes in AQI levels.

AQI Vs Ground-Level Ozone Monitoring

In India, AQI reports often emphasize particulate matter (PM2.5 and PM10) due to its direct health impacts and visibility, while other components like ground-level ozone receive less attention.

Ground-level ozone is particularly concerning because it is a secondary pollutant—it is not directly emitted but forms in the atmosphere through reactions between nitrogen oxides and volatile organic compounds.  High concentration of ground-level ozone can lead to respiratory problems, exacerbate asthma and reduce lung function say doctors. 

A recent analysis by the Centre for Science and Environment (CSE) has pointed out how there has been a dramatic increase in ground-level ozone in smaller metropolitan areas.

Ozone does not just build up in an area, but with wind, it drifts, resulting in a wide geographical spread. Monitoring ozone is also difficult.

A senior officer at the Karnataka State Pollution Control Board explained how monitoring ground-level ozone while measuring AQI is difficult. AQI data is averaged out and sent every few hours, the officer explained. However, ozone measurement can only be done real time because of its transient nature. Therefore, the officer said, it can be monitored mainly on the online monitoring stations that work real time, round the clock.

The CSE report indicates that in 10 cities in the country (including Bengaluru), ground-level ozone levels have exceeded permissible limits during the summer of 2024.  Delhi-NCR was the most affected with 176 days of high levels during the study period (between April and mid-July), with Mumbai-MMR and Pune following at 138 days each. In the south of the country, the number of high-ozone days were lower—Hyderabad saw 86 days, Bengaluru 59 days and Chennai, nine. 

Further, lower levels of ground-level ozone can be explained by the fact that it moves pretty quickly. Though ozone is formed with catalysts like nitrogen oxide which is prevalent in highly polluted areas, it gets mopped up in these polluted areas as it continues to react with the catalysts, the CSE report says.

Regional Pollution Hotspots

However, the ozone that drifts to cleaner areas does not find these catalysts to react with and therefore remains there, creating a hotspot.

The perception that industrialisation is the primary source of air pollution in India’s urban centres is somewhat outdated, says Nagesh Hegde, a Bengaluru-based environmentalist.

“Many industries have been relocated to peripheral towns and industrial zones to mitigate their impact on urban air quality. For example, in the case of Bengaluru, a major tech hub, industries have moved to areas like Doddaballapur and Tumkuru,” he explains.

While the direct impact of industrial emissions on urban air quality may have decreased, the overall pollution landscape has shifted. The emissions from these relocated industries still contribute to regional pollution levels and can affect nearby urban areas.

This, says the CSE report, is especially true of ground-level ozone. It shows that high-end and green neighbourhoods are the worst affected in these metropolitan cities simply because the ozone has moved from polluted areas and remains in the relatively cleaner ones where there are no catalyst gases to dissipate it. 

The need, therefore, says Hegde, is to actively monitor ozone levels. AQI monitoring focuses only on particulate matter, or dust that we inhale, he points out.

Need To Monitor Ground Level Ozone

While that is important, the need to alert citizens of ozone levels is absolutely critical. He says people exposed to ground-level ozone can find it difficult to breathe due to inflamed airways and can be susceptible to various infections. 

Ground-level ozone monitoring is popular in large parts of Europe and the United States.

Ozone alerts must become as common place as AQI level information, Hegde asserts.

The CSE report also seeks a policy change in monitoring. It suggests that production of gases that help form ozone ought to be controlled through control of emissions from vehicles, industry, open burning, and other sources while controlling PM2.5 levels simultaneously. 

This is easier said than done. When a problem is out of sight, it is usually out of mind. So, this conversation will only get started when the monsoon ends and winter brings it haze of noxious fumes and deadly air. Till then, enjoy the fresh air!

(The author is an independent journalist based out of Bengaluru. Views are personal)