How ground-level ozone is making Delhi's air more deadly for residents

Ground-level ozone breached safe limits daily across Delhi-NCR this summer, exposing millions to serious respiratory risks and worsening the city's air quality

You can’t see it or smell it, but ground-level ozone, often referred to as “bad ozone,” is increasingly becoming a dangerous threat lurking in the air of India’s cities, and it is taking a serious toll on public health. According to an analysis by the Delhi-based think tank Centre for Science and Environment (CSE) based on Central Pollution Control Board (CPCB) data, each day between March 1 and April 25 this year, ground-level ozone pollution exceeded safe limits across Delhi-National Capital Region (NCR), sparking fresh concerns about a growing public health threat. CSE found that daily ozone levels breached the eight-hour average standard of 100 micrograms per cubic metre without exception during this period.

 

 

Hotspots identified by CSE include Nehru Nagar, Najafgarh, Major Dhyan Chand National Stadium, and Sri Aurobindo Marg, areas where residents face higher-than-average exposure to this invisible pollutant.

 

Unlike the "good ozone" high up in the Earth’s atmosphere that shields us from harmful ultraviolet rays, ground-level ozone is a harmful pollutant formed when emissions from vehicles, factories, and other sources react under the heat of the sun. As temperatures rise and cities expand, experts warn that ozone pollution is quietly fuelling a public health crisis.

 

According to CSE, "Ground-level ozone is not like other pollutants directly emitted into the atmosphere. It is formed through complex reactions involving nitrogen oxides and volatile organic compounds in the presence of sunlight. Primary sources include vehicle exhausts, industrial emissions, and certain natural processes." 

Health effects of ground-level ozone pollution in Delhi-NCR 

Talking about the health impact of ozone gas, Dr Pratibha Dogra, senior consultant, pulmonology and sleep medicine at Marengo Asia Hospitals, Gurugram, said to Business Standard, "Ground-level ozone is a highly reactive gas that irritates the respiratory system, leading to inflammation of the airways, reduced lung function, and increased vulnerability to infections."

 

"Even short-term exposure can cause coughing, throat irritation, chest tightness, and shortness of breath. For people with asthma, Chronic Obstructive Pulmonary Disease (COPD), or bronchitis, elevated ozone levels can severely worsen symptoms."

Who is most vulnerable to ozone pollution in Indian cities? 

While ground-level ozone affects everyone, Dr Dogra noted that certain groups are especially vulnerable:

• Children under five years
• Older adults with pre-existing heart or lung conditions
• Individuals with asthma or other chronic respiratory diseases
• Outdoor workers and athletes
• Pregnant women, as exposure may be linked to adverse birth outcomes

"In rapidly urbanising, traffic-heavy areas, this means millions are exposed daily to harmful ozone levels that quietly damage their lungs and overall health. From construction workers to children playing outdoors, many are breathing in air that is silently harming them every day," she said. 

Short-term and long-term health impact of ozone pollution 

The health impacts of ground-level ozone are not just temporary. Dr Dogra highlighted that short-term exposure can cause:

• Coughing and throat irritation 
• Wheezing and chest tightness 
• Difficulty in breathing during physical activity 
• Increased fatigue

 

But the long-term effects are even more alarming. "Repeated exposure to ozone can accelerate lung function decline, contribute to chronic respiratory diseases, and even raise the risk of heart problems," Dr Dogra warned. Chronic inflammation from ozone pollution may also age the lungs faster and make individuals more vulnerable to environmental allergens and infections. 

How to protect yourself from ground-level ozone pollution? 

The good news? There are simple steps you can take to minimise your risk, especially on high-alert ozone days:

• Stay indoors during peak hours (afternoon to early evening) when ozone levels are highest 
• Limit strenuous outdoor activities like running or heavy workouts 
• Use air purifiers and keep windows closed to reduce indoor pollution 
• Monitor your local Air Quality Index (AQI) and plan outdoor time during early mornings or late evenings 
• Wear a pollution mask — though not perfect against ozone, it can help reduce overall inhalation of pollutants 
• People with chronic conditions should stick to prescribed treatments and have an emergency action plan ready

 

"Public awareness and preventive behaviour are key," stressed Dr Dogra. "At the same time, we need stronger emission controls to reduce the root causes of ozone formation." 

Ozone pollution trends in Delhi, Mumbai and other Indian cities 

Delhi-NCR is not alone. CPCB’s monitoring of ten major metropolitan cities between 2023 and 2024 found that the Delhi-NCR and Mumbai regions experienced more frequent ozone exceedances compared to other cities.

 

Ozone precursors, nitrogen oxides and volatile organic compounds (VOCs), originate mainly from motor vehicles, industrial emissions, coal and wood burning, waste incineration, and even certain biogenic sources like green spaces and crops. In the presence of sunlight, these precursors form ozone, turning sunny urban days into a breeding ground for pollution.

 

As Indian cities continue to expand and heat up, experts warn that ground-level ozone pollution, if left unchecked, could intensify into a larger health crisis, impacting millions more in the years to come.  For more health updates and wellness insights, follow #HealthWithBS