Kolkata, a city marked under India’s National Clean Air Programme (NCAP), continues to grapple with hazardous air despite national efforts, due to the unchecked burning of biomass and solid waste. A new study by the Bose Institute has revealed a sharp correlation between ultrafine particulate matter (PM2.5) and rising toxicity levels, identifying waste burning as a key contributor. The research, led by Prof. Abhijit Chatterjee and his team, studied the oxidative potential (OP) of PM2.5—referring to its capacity to produce reactive oxygen species (ROS) in human lungs. These ROS can overwhelm the body’s natural antioxidant defences, leading to oxidative stress and triggering severe respiratory and cardiovascular ailments. The findings show a non-linear pattern in the toxicity curve. Below PM2.5 concentrations of 70 µg/m³, toxicity levels remain steady. However, between 70–130 µg/m³, there is a sharp spike in OP values. Beyond 130 µg/m³, the toxicity plateaus, regardless of the rising particle load. Using Positive Matrix Factorization, the team traced PM2.5 sources and found that biomass and solid waste burning are the primary contributors to the toxicity load. While other NCAP-regulated sources such as vehicular emissions, road dust, and construction waste have seen reductions, biomass burning continues to release ultrafine toxic particles unchecked. Launched in 2019 by the Ministry of Environment, Forest and Climate Change (MoEFCC), NCAP aims to reduce PM levels by 40% by 2026 (compared to 2017 levels), focusing on 131 non-attainment cities. While the programme has led to positive air quality changes, especially during winter 2024–25, Kolkata’s air continues to suffer from the legacy of biomass emissions, the study noted. The Bose Institute, an autonomous body under the Department of Science and Technology, acts as the nodal agency in Kolkata under NCAP and serves as a national knowledge partner. The team’s research, now published in the journal Science of The Total Environment, has prompted local civic bodies to take stricter surveillance and enforcement measures against waste burning activities. With India spending billions of rupees on clean air missions—NCAP itself had an outlay exceeding Rs 6.37 billion as of 2022—the study urges for more targeted efforts in addressing pollutant sources that disproportionately affect public health. As cities like Kolkata strive to meet national ambient air quality standards, the need to eliminate high-toxicity sources like biomass burning remains urgent for achieving sustainable clean air goals.