Heat kills even in cities that know it is coming

In Nagpur, where summer temperatures routinely exceed 40°C, a 2020 study published in the International Journal of Occupational and Environmental Medicine tested an assumption that public health workers in hot cities often rely on: that a population acclimatised to extreme heat carries lower mortality risk from it. The Nagpur data said otherwise. The researchers documented that excess mortality in the city increased measurably with each additional degree above the seasonal norm, even in a population that has lived with severe heat for generations. Acclimatisation buffers discomfort. It does not eliminate the mortality signal.

India is now in the peak weeks of its pre-monsoon heat season. The India Meteorological Department defines a heat wave when maximum temperatures reach at least 40°C in the plains and the departure from the date’s normal is 4.5°C or higher. By that standard, Rajasthan, Madhya Pradesh, Odisha, and parts of Telangana have already been inside heat wave conditions this season, as they are most years between March and June. What mortality data from Indian cities has begun to answer is how much those weeks actually cost in lives, and which populations bear the most of that cost.

What city-level data shows

The longest Indian dataset comes from Ahmedabad. A study published in Environmental Research in 2021 reconstructed heat-attributable mortality in the city from 1987 to 2017. Across three decades, the authors found a statistically significant increase in mortality risk during high-ambient-temperature periods, with the relationship holding across different sub-periods within that span. The 2010 heat wave brought that abstract trend into focus: the city’s health authorities eventually estimated around 1,344 deaths, a figure revised upward after accounting for excess mortality in the weeks following the peak.

Hyderabad extends the picture. Researchers examining all-cause mortality in the city from 2006 to 2015, publishing in African Health Sciences in 2021, found a consistent association between high summer temperatures and increased deaths, with the strongest signal among elderly residents and people with chronic conditions. The Nagpur study, tracking excess mortality by temperature increment, showed that this mortality-temperature relationship does not disappear in cities long-adapted to extreme heat. Adaptation helps at the margins. It does not shift the underlying curve.

What these cities share, and what matters for interpreting the numbers, is that vulnerability within a city is not uniform. Urban heat islands, the proportion of outdoor workers, housing construction quality, and access to shade and water all modify how the same ambient temperature affects different people in the same district. A migrant construction worker sleeping in a tin-roofed shelter carries a fundamentally different heat exposure from a government employee two streets away.

Why the cardiovascular signal matters more now

Most people connect heat-related illness to heatstroke, the most visible end of a broad spectrum. The mortality story runs further than that. A review published in Future Cardiology in October 2023 examined how sustained high ambient temperatures affect the cardiovascular system. The authors found that the body’s heat-dissipation response, redirecting blood flow to the skin for cooling, increases cardiac output demands substantially. In people with coronary artery disease or heart failure, this circulatory shift can precipitate events that cooler temperatures would not.

This mechanism carries growing weight in India because of a shift in the country’s disease profile. Hypertension and type 2 diabetes, both of which impair vascular function and reduce heat tolerance, have become far more prevalent over the past two decades. The population that now sits at the intersection of heat exposure and cardiovascular vulnerability is substantially larger than it was when the 2010 Ahmedabad data was gathered. The mortality literature from Indian cities has not yet reflected that compounding.

What the Ahmedabad response showed

Ahmedabad introduced South Asia’s first formal Heat Action Plan in 2013. The plan brought together early warning alerts tied to IMD temperature forecasts, mobilisation of community health workers toward high-risk neighbourhoods, activation of cooling spaces in public buildings, and public communication campaigns timed to heat wave onset. Knowlton et al., publishing in the International Journal of Environmental Research and Public Health in 2014, documented the development and early implementation of the programme and found it reduced heat-related mortality in the years following launch relative to the 2010 baseline. The reductions were uneven. Neighbourhoods with denser informal housing showed consistently smaller gains, which points to the limits of awareness campaigns when the underlying exposure conditions remain unchanged.

Odisha and Telangana have since formalised their own heat action protocols. A 2022 qualitative study of community heat adaptation in Iranian urban settings, published in the Journal of Education and Health Promotion, found that communities with stronger informal social networks managed heat emergencies more effectively than more atomised ones, suggesting that neighbourhood-level trust and communication may matter as much as official warning systems. Whether that dynamic holds in Indian cities at scale has not been examined directly.

Where structural protections are still missing

Heat action plans address awareness and acute response. They do not address the structural conditions that put outdoor workers at disproportionate risk before any emergency is declared. Rescheduling outdoor work away from peak midday hours, ensuring access to shade and water at worksites, and building in structured rest breaks during peak heat hours cost less than building cooling centres, and yet most Indian states have not formally incorporated heat illness into occupational health frameworks.

Urban planning decisions made in the next several years will shape heat vulnerability for a generation. Tree cover density, the albedo of road and roof surfaces, the location of informal settlements relative to urban heat islands, and the thermal design of low-income housing all determine how hot the lived environment actually becomes during a heat event. Some Indian municipal corporations have begun integrating heat risk into master plans. Implementation remains uneven.

I keep returning to the Nagpur result. The acclimatisation story is appealing because it implies that populations adapt and risk diminishes over time. The data does not support that conclusion. What Ahmedabad’s experience shows is that building institutional heat response capacity, covering early warning systems, community outreach, cooling infrastructure, and formal occupational protections, does reduce mortality when that capacity is actually maintained and reaches the people most exposed. Whether those systems are growing fast enough to outpace the rising temperature trend is the question the next decade of data will have to answer.

This article is for educational purposes only and does not constitute medical advice, diagnosis, or treatment recommendations. Consult a qualified healthcare provider for any health concerns. See our Medical Disclaimer.

Sources

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