Authors: Manu Mehta, Shivani, Khushbu Dahiya, Somya Garg and Ranu Gadi, Indian Institute of Remote Sensing, ISRO Dehradun

Aerosols are well known for their impact on the climate as well as public health. Numerous studies have outlined these dual effects of aerosols, which highlight the urgent need for effective mitigation techniques and additional research. Due to air pollution, India, a country with a large population and diverse climate, faces grave challenges. The situation could be more vulnerable in Indian megacities with an increasing load of population load and associated anthropogenic activities.

Studying the effects of air pollutants on heat stress—a crucial measure of thermal comfort—is crucial due to the relationship between these pollutants and climatic variables, especially temperature and humidity. The need for adaptation methods to reduce health risks is highlighted by projections showing an increase in extreme heat stress days, particularly in sensitive and coastal regions (Koteswara Rao et al., 2020; Maurya et al., 2024). A thorough investigation into the co-variability of air pollution and heat stress across India, especially the densely populated pockets, is therefore required.

Our recent study (Mehta et al., 2025) provides an in-depth analysis of air pollutants (PM₂.₅, PM₁₀, NO₂, O₃, SO₂, CO, NH₃, and NO) and heat stress across Delhi, Mumbai, Chennai, Bengaluru, and Hyderabad over eight years (2016–2023). The employment of Wet Bulb Globe Temperature (WBGT) as a heat stress variable provides regionally specific information about human comfort under different pollution levels and weather conditions. Through this study, we have tried to develop an understanding of the trends of air pollutants and heat stress, their diurnal patterns, as well as their correlations. Understanding these interactions is essential for addressing the dual problems of air pollution and heat stress in crowded megacities at danger from climate change, as urbanization and human emissions increase.

Figure 1: Seasonal diurnal plots of air pollutants and WBGT for the five megacities; (a-e) during winter; (f-j) during pre-monsoon; (k-o) during monsoon; (p-t) during post-monsoon seasons for Delhi, Mumbai, Chennai, Bengaluru and Hyderabad respectively (Mehta et al., 2025)

The findings reveal a complex relation between urban pollution levels and thermal stress patterns, with significant seasonal and diurnal variations across the Indian megacities. Hyderabad displayed unique diurnal heat stress patterns, with afternoon WBGT lower than other times of the day, contrasting with the typical pattern observed in other cities. The diurnal variability of air pollutants suggests targeting peak pollution hours for mitigation strategies like traffic management and emission controls to reduce pollutant levels effectively. Seasonal correlations further indicated that higher pollutant concentrations often coincide with lower WBGT during winter, particularly in Delhi and Hyderabad, underscoring the influence of meteorology and urban morphology on pollution-heat dynamics. While an overall decreasing trend in particulate matter was observed, indicating the early impact of emission control strategies, rising WBGT trends, especially in Mumbai and Hyderabad, suggest growing thermal discomfort and vulnerability.  As Indian cities continue to grow, integrating such multi-parameter assessments into urban planning and policy will be critical for safeguarding public health and sustainability.

References

• Koteswara Rao, K., Lakshmi Kumar, T. V., Kulkarni, A., Ho, C. H., Mahendranath, B., Desamsetti, S., … & Sabade, S. (2020). Projections of heat stress and associated work performance over India in response to global warming. Scientific Reports, 10(1), 16675. https://doi.org/10.1038/s41598-020-73245-3
• Maurya, H. K., Joshi, N., & Suryavanshi, S. (2024). Projected changes in the heatwave’s characteristics and associated population exposure over India under 1.5–3° C warming levels. Stochastic Environmental Research and Risk Assessment, 1-18. https://doi.org/10.1007/s00477-024-02695-2
• Mehta, M., Shivani, Dahiya, K., Garg, S. and Gadi R., (2025). Diurnal variability and trends of air pollution in association with heat stress over five Indian megacities. Environmental Monitoring and Assessment, 197, 571 (2025).  https://doi.org/10.1007/s10661-025-14040-3