Subscribe
A new study tracking indoor temperatures inside 50 low-and middle-income homes in Chennai found that temperatures rarely dropped below 31 degrees Celsius (°C) and often remaining above 34°C for hours after sunset, even in winter.
The research, conducted between October 2025 and April 2026 by Climate Trends, and drawn from seven months of high-resolution sensor data, points to a growing but under-recognised climate threat: chronic indoor heat exposure driven by climate change, urbanisation and heat-retaining construction materials.
Indoor temperatures frequently exceeded 32°C and climbed above 35°C during the warmer months. Relative humidity stayed stubbornly above 75 per cent throughout, a figure that matters for coastal cities like Chennai as humidity amplifies the effects of heat on the body by weakening its primary defence: sweating.
“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, and co-author of the study.
Most alarming was the persistence of night-time heat. Temperatures peaked around 8 and 9 PM, reaching nearly 34.7°C, a delayed surge caused by concrete walls and floors releasing the heat they had been storing since morning. Even in the early morning hours, homes saw little relief, with temperatures hovering around 33.8°C to 34°C.
“Overall, the diurnal profile demonstrates a compressed temperature range with limited fluctuation, indicating that indoor environments do not experience meaningful cooling at any point during the day. This sustained thermal exposure underscores the challenge of achieving thermal comfort—especially during night-time hours when recovery from heat stress is critical,” the report highlighted.
The structural reasons behind this are: construction materials, ventilation, and housing density. Low-and middle-income housing in Indian cities tends to be built from brick and reinforced concrete, materials with high thermal mass that absorb solar radiation efficiently and release it slowly.
Previous research from Ahmedabad and Hyderabad has previously recorded night-time indoor temperatures running 2-5°C hotter than outdoors for exactly this reason.
Across the 50 households studied, nearly all had concrete roofs with similar thermal mass properties. Additionally, 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.
Critically, the report found, that even air conditioning cannot mitigate the baseline heat exposure embedded in the structure itself. RCC buildings retain heat throughout the 24-hour cycle. With the AC turned off, the heat makes itself felt again.
Research published in The Lancet Planetary Health demonstrates strong associations between elevated nighttime temperatures and increased mortality during heat wave events. Sleep studies further indicate that indoor temperatures above 26-28°C significantly impair sleep quality, while temperatures exceeding 30°C contribute to chronic discomfort and cumulative physiological stress.
Aarti Khosla, Founder and Director, Climate Trends said, “The study brings into focus how night-time heat retention must also receive attention. It is concerning to note the level of baseline heat exposure and heat stress for the residents.”
The report was released during the India Heat Summit, 2026. Addressing the summit virtually, Pralhad Joshi, Union Minister for New and Renewable Energy, said, “Rising temperatures and extreme weather events are realities affecting our cities, villages, economies, and our daily lives of citizens. Heat stress has emerged as one of the defining challenges of our times.”