NEW DELHI: A new study found that India’s urban centres are grappling with high indoor temperatures at night, which can be as dangerous as high outdoor temperatures during the day in summer, especially in low- and middle-income housing.
Indoor heat stress may significantly impact comfort levels and overall well-being, and calls for policymakers to consider indoor conditions in their heat management strategies.
Indoor temperatures frequently surpassed 32 degrees Celsius, with a few households enduring exposure levels equivalent to eight months of continuous heat—between 5,700 and 5,800 hours above the critical threshold.
A new study, Night-time Thermal Stress in Low and Middle Income Housing in India: Linking Indoor Temperatures and Relative Humidity with Perceptions of Comfort, was released today by Union Minister for New and Renewable Energy, Pralhad Joshi.
The study reveals the scale of indoor heat exposure faced by low-and middle-income urban residents in Chennai and makes a direct case for the integration of indoor heat monitoring into urban heat action plans nationwide.
The study calls for integrating indoor heat monitoring into national Heat Action Plans. It is based on monitoring of temperature and relative humidity in 50 residential units in urban Chennai neighbourhoods, focusing on conditions outside of peak summer to establish a baseline estimate of heat exposure between October 2025 and April 2026.
The findings are stark.
The majority of homes recorded between 3,000 and 5,000 hours of indoor heat exposure, highlighting a pervasive challenge faced by residents.
Alarmingly, night-time temperatures rarely dipped below 31 degrees Celsius, even in cooler months, making sleep and rest a daunting challenge for many families.
The study revealed that indoor spaces reached their highest temperatures not at midday, as one might expect, but between 8 PM and 9 PM.
This phenomenon is attributed to the heat-retaining properties of reinforced concrete structures, which release stored heat long after the sun has set.
Compounding the discomfort, relative humidity remained persistently high, above 75 per cent during the night, inhibiting the body’s ability to cool itself through evaporation.
Residents reported feeling "Hot" or "Very Hot" during daytime hours, with 45 per cent of indoor conditions classified as "Hot" and 20 per cent as "Very Hot."
Nighttime feedback from residents underscored the effects of relentless heat, with widespread reports of disrupted sleep and persistent fatigue.
The study's results underline a critical need to reassess heat governance in India, which has traditionally focused on outdoor temperature thresholds and daytime heatwave declarations.
“India firmly believes that green energy is the solution to both the climate crisis and the global energy crisis,” said Joshi.
“The power system of the future must be capable of integrating diverse renewable energy sources while maintaining stability and affordability. This is where the battery storage systems, pumped hydro-storage and long-duration storage technologies become critically important,” Joshi added.