With the country facing a severe heatwave, and extreme heat alerts across several regions issued by the Indian Meteorological Department (IMD), a new study released on May 13 by Climate Trends has documented, with high-resolution sensor data, the scale of indoor heat exposure faced by low- and middle-income urban residents in Chennai, India.
The report, based on its findings, claims that the country’s heat governance framework, in particular its heat action plans, which have been built almost entirely around outdoor temperature thresholds and daytime heat wave declarations, should also consider indoor nighttime heat as a key indicator.
India currently has over 300 such plans, with a hundred more in development. “None mandates indoor temperature monitoring,” the Climate Trends study reminds us.
On May 13, Pralhad Joshi, Union minister for new and renewable energy, addressing the India Heat Summit 2026 virtually, also acknowledged the role of heat stress. “Rising temperatures and extreme weather events are realities affecting our cities, villages, economies, and our daily lives as citizens. Heat stress has emerged as one of the defining challenges of our times,” he said.
Peak heat hour in the evening
The Climate Trends study, titled ‘Nighttime Thermal Stress in Low and Middle Income Housing in India: Linking Indoor Temperatures and Relative Humidity with Perceptions of Comfort,’ argues for the direct integration of indoor heat monitoring into urban heat action plans in India.
The study monitored temperature and relative humidity inside 50 residential units in Chennai’s urban neighborhoods between October 2025 and April 2026.
According to the Climate Trends research, temperatures inside homes in the monitored households often exceeded 32°C. The worst-affected households recorded between 5,700 and 5,800 hours of exposure above this threshold, equivalent to eight months of continuous heat, from October 2025 to April 2026.
The biggest share of households recorded between 3,000 and 5,000 hours, which means four to seven months of persistent exposure.
Little sleep, chronic fatigue
Nighttime temperatures, between 8 pm and 6 am, hardly dipped below 31°C even during the cooler months of the study period. Temperature inside homes reached its peak not at midday but between 8 pm and 9 pm, the period during which reinforced concrete structures released the heat stored through the day and remained above 34°C well into the night.
Relative humidity remained steadily above 75% through the night, adding to the discomfort by restricting the body’s ability to cool itself through evaporation.
Residents’ experience of the heat confirmed what the sensors recorded. During the day, 45% of indoor conditions were classified as “hot” and 20% as “very hot.”
Responses about nighttime heat indicated widespread disrupted sleep and chronic fatigue.
Housing material is key
The nature of housing affects the experience of heat. “Reinforced cement concrete (RCC) used in roofing and walls prolong indoor heat exposure,” says the Climate Trends release.
“The study brings into focus how nighttime heat retention must also receive attention. It is concerning to note the level of baseline heat exposure and heat stress for the residents, and what’s most pertinent is that even at night, their indoor temperatures rarely dropped below 31°C,” said Aarti Khosla, founder and director, Climate Trends.
“Chennai’s high outdoor temperatures and relative humidity anyway pose difficult conditions. That its residents must also face chronic heat exposure indoors and have it affect their sleep and recovery periods is a matter that seeks urgent interventions,” she added.
More than economic status, housing material was responsible for exposing residents to heat, says the study. Across the 50 households, nearly all had concrete roofs with similar thermal mass properties.
Income impacts relief
“Income determined whether residents had any means to cope. Every high-income household in the sample had an air conditioner. Every low-income household had only a ceiling fan,” the release added. RCC buildings retain heat throughout the 24-hour cycle. With the AC turned off, the heat makes itself felt again.
“Most people spend the majority of their time indoors, where they face the dual threats of heat stress and poor air quality. These exposures carry uncertain physiological trade‑offs and add psychological as well as economic strain on individuals and households,” said Naveen Puttaswamy, associate professor, Faculty of Public Health, SRIHER, Chennai, and co-author of the study.
The study recommends including cool roofs and reflective coatings for low- and middle-income households; cross-ventilation as a design requirement; promotion of climate-responsive construction materials, including compressed stabilized earth blocks and autoclaved aerated concrete; and mandatory indoor heat monitoring as a component of urban heat action plans.