India’sescalating heat crisis is no longer confined to scorching afternoons and heatwave alerts outdoors. A new study has found that indoor temperatures in low- and middle-income urban homes remained above 32°C for up to 5,800 hours — equivalent to nearly eight months of continuous heat exposure — while nighttime temperatures stayed above 31°C even outside peak summer months, exposing millions to chronic thermal stress inside their homes.
The findings come as Indian cities witness increasingly warmer nights amid rapid urbanisation, concrete-heavy construction and worsening urban heat island effects, raising concerns that indoor heat exposure may emerge as one of the country’s biggest but least monitored climaterisks.
The report by Climate Trends, based on high-resolution monitoring across 50 residential units between October 2025 and April 2026, found that indoor spaces across dense urban neighbourhoods failed to cool effectively after sunset due to prolonged heat retention in reinforced cement concrete structures.
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The study assumes significance at a time when India is recording more frequent extreme heat events, with temperatures in parts of the country touching nearly 52.9°C in 2024. While policymakers and heat action plans largely focus on daytime heatwaves, the report warns that rising nighttime temperatures are quietly eroding the body’s ability to recover from daytime heat stress.
Concrete Structures
According to the findings, most indoor temperature readings remained concentrated between 32°C and 36°C, with several instances extending beyond 35°C, indicating sustained and widespread indoor overheating. A large majority of households recorded between 3,000 and 5,000 hours above the 32°C threshold, translating into nearly four to seven months of persistent exposure.
The study found that indoor temperatures remained elevated throughout the night, with only marginal cooling between midnight and early morning hours. Nighttime temperatures hovered around 33.8-34°C and peaked close to 34.7°C during evening hours between 8 pm and 9 pm, reflecting delayed heat release from concrete structures.
“Indoor environments in the studied units do not provide meaningful respite from daytime heat,” the report said, adding that the issue was not driven by short-term heatwave spikes but by chronic heat retention linked to building materials, poor ventilation and dense urban layouts.
The report also highlighted a widening cooling divide across income groups. While all surveyed high-income households had access to air-conditioners, low-income families depended entirely on ceiling fans without access to active cooling systems.
Humidity levels remained consistently between 76% and 77%, significantly worsening discomfort and reducing the body’s ability to cool itself through evaporation. Residents reported frequent sleep disruption, fatigue and persistent nighttime discomfort, with nearly 45% of indoor conditions classified as “Hot” and another 20% categorised as “Very Hot”.
The most severe heat stress was recorded in dense pucca structures located in commercial areas. Shared housing units reported mean indoor temperatures of 42.7°C and maximum temperatures reaching 49.6°C, while some residential units saw indoor spikes as high as 53.4°C.
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Cooling Divide
The report warned that relying solely on outdoor temperature data significantly underestimates the scale of urban heat vulnerability, arguing that indoor nighttime heat stress is emerging as a serious public health threat in Indian cities.
To tackle the crisis, the study recommended large-scale deployment of cool roofs, reflective coatings, improved cross-ventilation systems and urban greening measures. It said cool roof technologies alone could reduce indoor temperatures by 2-4°C. The report also called for mandatory indoor heat monitoring under urban heat action plans and wider adoption of climate-responsive construction materials such as AAC blocks and compressed stabilised earth blocks.
The study concluded that India’s heat adaptation policies must move beyond daytime heatwave management and urgently prioritise nighttime cooling and indoor thermal resilience as urban homes increasingly turn into permanent heat traps.
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