Shah et al. (2025) Spatiotemporal changes in heat stress exposure in India, 1981-2023

Identification

• Journal: Nature Communications
• Year: 2025
• Date: 2025-10-28
• Authors: Arpit Shah, Anish Sugathan, Deepak Malghan, Rockli Kim, S. V. Subramanian
• DOI: 10.1038/s41467-025-64840-x

Research Groups

• Centre for Public Policy, Indian Institute of Management Bangalore, Bengaluru, India
• Faculty of Arts and Sciences, Harvard University, Cambridge, MA, USA
• Strategy, Indian Institute of Management Ahmedabad, Ahmedabad, India
• Centre for Sustainability and Corporate Governance, Indian Institute of Management Ahmedabad, Ahmedabad, India
• Asia Centre, Stockholm Environment Institute, Bangkok, Thailand
• Division of Health Policy and Management, College of Health Science, Korea University, Seoul, South Korea
• Interdisciplinary Program in Precision Public Health, Department of Public Health Sciences, Graduate School of Korea University, Seoul, South Korea
• Harvard Center for Population and Development Studies, Cambridge, MA, USA
• Department of Social and Behavioral Sciences, Harvard T.H. Chan School of Public Health, Boston, MA, USA

Short Summary

This study provides a comprehensive analysis of the spatiotemporal evolution of heat stress exposure (HSE) across India's districts from 1981 to 2023, revealing a 3.3% increase in average HSE duration and significant variations by time of day, time of year, and region, with critical implications for public health and policy.

Objective

• To present a comprehensive analysis of the spatiotemporal evolution of heat stress exposure (HSE) across India’s districts from 1981 to 2023.
• To provide evidence for variation in the evolution of HSE by time of day, time of year, and across regions in India.
• To provide estimates of changes in outdoor occupational exposure from 2019-2023, including caste-based inequalities.

Study Configuration

• Spatial Scale: India's districts (722 districts), with underlying data at 0.1° spatial resolution.
• Temporal Scale: 1981-2023 (43 years) for climate data, with hourly granularity; 2019-2023 for occupational data, collected in successive waves.

Methodology and Data

• Models used: Universal Thermal Climate Index (UTCI) as the primary thermal comfort index. Heat Stress Exposure (HSE) defined as the number of hours per day when UTCI exceeds 32 °C. Robust Sen’s slope estimates for trend analysis.
• Data sources:
  • ERA5 dataset (hourly, 0.25° spatial resolution) for meteorological variables.
  • ERA5-Land dataset (hourly, 0.1° spatial resolution) for meteorological variables.
  • Consumer Pyramids Household Survey (CPHS) for outdoor occupational data (September 2019 - December 2023).
  • Updated district boundaries from Jain et al. (2024).

Main Results

• The average Heat Stress Exposure (HSE) duration across districts increased by 3.3% when comparing 1981-1995 and 2011-2023.
• Average daily HSE increased from 5.1 hours/day in 1981 to 5.6 hours/day in 2023 (a 10.8% increase), while average UTCI increased from 22.5 °C to 23.4 °C (a 3.6% increase).
• HSE has diverged across pixels, with areas that were hotter in 1981 experiencing larger increases in HSE.
• HSE predominantly occurs between 08:00 and 18:00, with increased levels observed across daytime hours.
• Daily minimum UTCI values increased faster (1.05 °C) than daily maximum UTCI values (0.26 °C), leading to a shrinking of the diurnal UTCI range across most districts.
• HSE increased in Quarters 1, 3, and 4 (6.5%, 2.6%, and 9.9% respectively), but not significantly in Quarter 2 (April-June), which already had high baseline exposure.
• Daytime HSE increased by 3.3%, while night-time HSE remained stable, primarily because night-time UTCI generally stayed below the 32 °C threshold.
• The frequency of HSE exposure events increased (from 276 to 280 per year), the interval between events declined (from 21.7 hours to 21.3 hours), and the average duration of events increased (from 7.96 hours to 8.05 hours).
• Outdoor occupational heat stress exposure increased from 2019 to 2023, with marginalized caste groups (Scheduled Castes and Scheduled Tribes) experiencing greater exposure than dominant groups.

Contributions

• Provides a long-term (1981-2023) and policy-relevant analysis of heat stress exposure at the district level in India, utilizing high-resolution (0.1°, hourly) spatiotemporal data.
• Introduces and analyzes Heat Stress Exposure (HSE) (UTCI > 32 °C) as a critical metric for public health, complementing average temperature changes.
• Offers a nuanced characterization of thermal stress evolution, including intra-day, intra-year, and regional variations.
• Quantifies the increase in outdoor occupational heat stress and highlights caste-based inequalities in exposure.
• Demonstrates a shrinking diurnal UTCI range in India, contrasting with some air temperature-based findings and emphasizing the importance of comprehensive thermal comfort indices.

Funding

• Bill & Melinda Gates Foundation, INV-002992

Citation

@article{Shah2025Spatiotemporal,
  author = {Shah, Arpit and Sugathan, Anish and Malghan, Deepak and Kim, Rockli and Subramanian, S. V.},
  title = {Spatiotemporal changes in heat stress exposure in India, 1981-2023},
  journal = {Nature Communications},
  year = {2025},
  doi = {10.1038/s41467-025-64840-x},
  url = {https://doi.org/10.1038/s41467-025-64840-x}
}