Kolkata: A recent study on Kolkata’s Air Quality Index (AQI) has found that while particulate matter (PM2.5 and PM10) remains a major concern, the city’s air quality in 2025 was often driven by toxic gaseous pollutants such as nitrogen dioxide (NO₂) and ground-level ozone (O₃).
An analysis by ‘Respirer Living Sciences’ using its Atlas AQ platform showed that on 82 days in 2025, gaseous pollutants were the primary AQI drivers, indicating that air toxicity in Kolkata is increasingly multi-pollutant in nature.
“We often treat AQI as a proxy for particulate pollution, but the data tells a more complex story. Residents were exposed to health risks beyond what PM-centric narratives capture,” said Ronak Sutaria, founder and CEO of Respirer.
Of the 82 days when particulate matter was not the main AQI driver, nitrogen dioxide dominated on 68 days, ground-level ozone on 12 days, and carbon monoxide on two days. PM10 led the AQI on 166 days, while PM2.5 was dominant on 117 days.
Annual average NO₂ levels hovered close to or exceeded 40 µg/m³—the nationally prescribed safe limit—reflecting sustained combustion-related emissions. Ozone frequently crossed the 100 µg/m³ eight-hour standard, particularly during warmer months.
Spatial analysis showed traffic-heavy and industrial zones recording higher pollution loads, with Dunlop, Ultadanga, Moulali, Rabindra Sarobar, and the Howrah Bridge corridor emerging as major hotspots, especially for NO₂ and PM10.
Health experts warned that NO₂ exposure is linked to airway inflammation, aggravated asthma, and increased susceptibility to respiratory infections, particularly among children and the elderly. Ground-level ozone can irritate airways, reduce lung function, and worsen chronic respiratory conditions even when particulate levels are moderate.
“If cities continue to treat air pollution purely as a particulate problem, they risk missing a substantial part of the health burden,” Sutaria said.
Independent experts also flagged high carbon monoxide emissions from widespread biomass and waste burning in Kolkata. “Lower temperatures this year led people to burn waste, including plastics and electrical materials, for warmth. This emits carbon monoxide and carcinogenic polynuclear aromatic hydrocarbons, causing severe respiratory disorders,” said Abhijit Chatterjee, professor at Bose Institute.He added that these pollutants combine to form Peroxyacetyl Nitrate (PAN), contributing to winter morning haze observed this year, and said biomass and waste burning needs immediate attention.
Respirer’s analysis called for multi-pollutant air quality strategies, including improved public communication on gaseous pollutants, targeted emission controls, and time-
specific advisories.