Is your home the real heat hazard? Study throws up 'concrete' clues

India’s heat governance framework has a blind spot the size of a concrete ceiling. When authorities declare a heatwave, they are responding to outdoor temperatures recorded at meteorological stations through instruments mounted in the open air, often in the shade, measuring conditions that tell only part of the story of heat exposure in a rapidly warming country.

A new study, released on May 13 by Climate Trends, a New Delhi-based research organisation, documents what that framework systematically misses: the chronic, round-the-clock heat burden borne by low- and middle-income urban residents inside their own homes.

The findings, based on high-resolution sensors installed across 50 residential units in Chennai between October 2025 and April 2026, are striking in part because the period of assessment was not peak summer.

The study period was designed to capture a baseline. And even at baseline, the numbers are alarming. Indoor temperatures across the monitored households frequently exceeded 32 degrees Celsius. Nighttime temperatures, between 8 pm and 6 am, rarely dropped below 31 degrees Celsius even during the cooler months.

The large majority of households recorded between 3,000 and 5,000 hours above the threshold of 32 degrees Celsius, and the worst-affected units logged between 5,700 and 5,800 hours above that threshold, the equivalent of eight continuous months of heat stress.

Perhaps the most counter-intuitive finding concerns timing. Indoor spaces did not peak at midday. They peaked between 8 pm and 9 pm, touching close to 34.7 degrees Celsius, as reinforced concrete structures released the heat absorbed through the day. Add humidity, consistently above 75 per cent through nighttime, suppressing the body’s ability to cool itself through evaporation, and the result is conditions that make restorative sleep physiologically difficult for extended periods.

The thermal conditions inside homes were shaped by building materials, predominantly reinforced cement concrete in roofing and walls, whose heat-retaining properties are largely uniform across income groups. But every high-income household in the sample had an air-conditioner (AC) while every low-income household had only a ceiling fan.

The cooling gap, as the researchers describe, is binary. Mechanical cooling offers relief for those who can afford it. But even that relief is partial. RCC (reinforced cement concrete) structures retain heat through the full 24-hour cycle, and when the AC is switched off, the heat reasserts itself. This suggests that addressing India’s indoor heat problem through energy subsidies or ACs, without addressing the thermal performance of buildings, is, at best, an incomplete solution.

India currently has over 300 Heat Action Plans, with 100 more in the works. Not one of them mandates monitoring of indoor temperature. All are triggered by outdoor meteorological thresholds. The study argues that this design flaw means heat governance frameworks will systematically undercount the exposure burden, underestimate associated health risks and misdirect adaptation spending.

The policy recommendations of the study are subsidised passive cooling retrofits, including cool roofs and reflective coatings for low- and middle-income households; building code revisions mandating cross-ventilation; promotion of materials like compressed stabilised earth blocks and autoclaved aerated concrete that absorb and retain less heat; and mandatory indoor monitoring as a component of urban Heat Action Plans.

A companion study published this week in the scientific journal Nature Communications, analysing data from nearly 9,000 large urban areas globally, reinforces the equity dimension. Urban tree cover was found to mitigate between 41 and 49 per cent of the potential urban heat island effect. However, its cooling benefits accrue most heavily to high-income countries and suburban areas. In the densely settled low-income urban zones where heat-risk is the highest, canopy cover is thinnest and cooling returns lowest.

Taken together, the two studies point to the same conclusion. The architecture of India’s heat response was built around an outdoor problem measured in the daytime. The people most at risk live the heat problem indoors, in the dark, in concrete rooms that do not forget the sun.

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