Ground-Level Ozone Hits Highest October Peak in Delhi Since 2020

Ozone: The Invisible Threat

< ozone (O₃) at ground level is not directly emitted by vehicles or factories; it is formed in the atmosphere when precursors—nitrogen oxides (NOₓ), volatile organic compounds (VOCs) and carbon monoxide—react in sunlight. During clear-sky conditions (as often occur in October) and with abundant precursors, ozone can climb rapidly and pose significant health risks. Experts caution that ozone, being a powerful oxidant, irritates airways, aggravates asthma, and increases respiratory hospitalisations—even when particulate matter (PM₂.₅/PM₁₀) levels may not be at extreme peaks.

What the Data Shows

Between 1 October and 16 October 2025, Delhi recorded 35 hours during which ozone concentrations exceeded 90 µg/m³. By contrast, during the same period in 2024 there were only 4 hours of such exceedance; in 2023 and 2022 there were one hour and none respectively.

Ozone was found to be the **lead pollutant** on 10 of the first 17 days of October—a significant shift. Typically, in cooler months, particulate matter governs pollution alerts, but this year ozone briefly dominated. Analysts attribute this to clearer skies (which favour photochemical reactions), high traffic volumes, and VOC loads from multiple sources.

Causes: A Triple-Threat Scenario

Experts point to three overlapping factors contributing to the ozone spike in Delhi-NCR:

• Transport emissions: With post-festive travel, commuter traffic remains high. Vehicles release NOₓ and VOCs which serve as precursors for ozone. Mr Sunil Dahiya of Envirocatalysts commented: “Transport, stubble burning and even fire-crackers are precursors of VOCs … the ozone levels are higher this October than in previous years mainly due to those emissions.”
• Stubble burning and biomass emissions: Although the peak of crop-burning ends by late October, emissions from burning fields in adjacent states (Punjab, Haryana) still contribute NOₓ/VOC loads which drift into Delhi-NCR under prevailing winds. These help amplify the photochemical formation of ozone.
• Festive pollution & firecrackers: The Diwali window and associated fire-cracker bursting release large amounts of VOCs and other un-burnt hydrocarbons into the atmosphere. Coupled with clearer skies and transitions in wind patterns, this creates favourable conditions for ozone build-up.

Health Implications and Public Impact

While much of Delhi’s air-pollution focus has traditionally centred on particulate matter (PM₂.₅/PM₁₀) and its acute effects, the surge in ozone raises different concerns. Ozone acts as a lung irritant and can exacerbate chronic respiratory conditions such as asthma, bronchitis, and COPD. Short-term exposure to high ozone levels has been associated with increased hospital admissions for respiratory ailments. The think-tank Centre for Science and Environment (CSE) warned that elevated ozone levels across a broader area signal that the “oxidative load” on lungs is rising.

One practical worry is that because ozone peaks when skies are clearer, people may underestimate danger—many assume “clear” skies equal “clean” air, which isn’t the case. The subtle nature of ozone exposure means that many vulnerable populations (children, elderly, people with lung disease) may not take precautions in time.

Why This Is a Warning Sign

This episode is concerning on multiple fronts:

• The fact that ozone—not particulate matter—led the pollution index indicates a shift in the pollutant mix. Many mitigation measures emphasise dust control or PM emissions; ozone requires a different strategy.
• October is typically a transitional month—between monsoon and winter dust seasons. The fact that ozone has spiked this early suggests that the window of “better air” in autumn is shrinking.
• The underlying conditions (traffic, stubble burning, biomass/celebration emissions) are systemic and recurring. Unless policy interventions pivot to address these VOC/NOₓ sources, such spikes may become more frequent and intense.

What Should Be Done: Mitigation & Adaptation

Given the nature of ozone, responses need both source-control and public-health adaptation:

• Strengthen precursor control: Measures to reduce vehicle NOₓ emissions—such as stricter vehicular emission tests, faster transition to electric/hybrid vehicles, and real-time diesel vehicle registrations—are crucial.
• Regulate VOC sources: Beyond vehicles, VOCs come from fire-crackers, informal biomass burning, paint solvents, petrol pumps and food stalls. Authorities should expand regulatory oversight and monitoring to these less-visible sources.
• Expand real-time monitoring & forecasting: Publicising ozone levels and health alerts can help citizens make informed decisions—such as reducing outdoor exertion on high-ozone days.
• Public-education campaigns: Many citizens misunderstand clear skies as “safe air”. Campaigns must explain that ozone, not just PM, poses serious risks. Also advise vulnerable groups on timing outdoor activities, avoiding high-traffic zones during peak hours, and indoor air-purification where feasible.
• Cross-border coordination: Since stubble burning emissions in Punjab/Haryana affect Delhi ozone, regional coordination for crop-residue management, incentives for alternatives to burning, and wind-trajectory forecasting are needed.

Policy Implications & Urban Air Management

This episode signals that Delhi-NCR’s air-management strategy must evolve from a singular focus on particulate matter and dust to a more nuanced chemistry-aware framework. Officials at CPCB and state pollution-control boards will have to integrate ozone forecasting and precursor emission inventories into their air-action plans.

It also aligns with global shifts: as countries reduce PM through filters, dust control and energy transitions, secondary pollutants like ozone and nitrogen-dioxide begin dominating urban air-quality risk. Delhi’s spike may be an early indicator of this trend. Policy-makers would do well to anticipate and act accordingly.

Voices from Experts

Mr Dahiya of Envirocatalysts noted: “Ozone was a very hazardous gas … the only option left is to prevent and control precursor gaseous pollutants, particularly during the clear-sky days.”

The CSE pointed to a broader risk: in June it flagged a rising number of high-ozone days in Delhi-NCR, warning that regulatory focus remains skewed. The recent data confirms those warnings.

What Happens Next & Outlook

With winter approaching, Delhi must brace for cumulative pollution burdens: dust, cold-trap conditions, stagnant airs and potential fire-cracker and crop-burning emissions all stacking up. If ozone formation correlates with clear-sky, high-solar-radiation days, authorities might see new peaks before winter sets in fully.

The immediate focus will be on public alerts and short-term advisories. But medium-term challenges remain: shifting the emissions-profile of the city, rethinking vehicular policies, and integrating multi-pollutant management. Without such a shift, Delhi risks recurring “silent” pollution waves—not driven by visible smog but by toxic chemical undercurrents like ozone.

Final Thoughts

Delhi’s spike in ground-level ozone is a wake-up call. For too long, attention has centred on visible smog and particulate matter. Ozone moves unseen and hits the lungs unannounced. The crisis this October may still be manageable—but if it becomes a regular precursor to winter pollution peaks, Delhi’s “air-problem” will enter a more complex, insidious phase. The health of millions of residents demands that the city’s pollution-strategy evolve accordingly.