Cities as heat engines: How urban design is making India's heatwaves deadlier for the poor

Nine out of India's ten hottest cities globally are located within the country's borders, with the majority concentrated in Uttar Pradesh and the surrounding plains, according to climate and energy expert Soumya Dutta, who has spent three decades studying climate change, environment, rivers and energy issues while working with communities across the world. In a recent detailed discussion, Dutta sought to explain why Indian and South Asian cities are becoming unbearably hot and who bears the brunt of this crisis.

Dutta pushed back against the popular belief that rising heat is simply a function of tree loss. "The relationship between trees and urban heat is not as simple as more trees meaning less heat, or fewer trees meaning more heat," he said, arguing that the real issue lies in how the human body experiences heat. He explained that discomfort and danger are not determined by air temperature alone, as reported by weather departments, but by a combination of air temperature and relative humidity. The human body cools itself primarily through the evaporation of sweat, a process that carries heat away from the skin and cools the blood before it circulates to protect the heart, brain and kidneys. When humidity is high, sweat cannot evaporate efficiently, and the body loses its primary cooling mechanism, even at temperatures as low as 35 to 38 degrees Celsius. Under such conditions, Dutta warned, people can fall seriously ill or die even without extreme air temperatures.

According to Dutta, Indian cities face this danger due to a combination of factors, not simply the loss of trees. Dense concrete and steel structures absorb solar heat during the day and release it slowly through the night, preventing cities from cooling down the way they once did. This is compounded by traffic and vehicle emissions; Dutta pointed out that Delhi alone has over 1.2 crore vehicles, which release both carbon dioxide and moisture while converting fuel energy into heat, effectively turning the city into what he called a "heat engine." A third factor is the disruption of natural wind flow due to unplanned construction, which traps heat and humidity over cities in what Dutta described as a "moisture dome" or "humidity island," a layer that also acts as a greenhouse gas trap. Together, he said, these three factors—urban heat retention, vehicular heat engines, and blocked ventilation—compound the crisis well beyond what climate change alone would cause.

Dutta emphasised that the impact of this crisis is deeply unequal. While middle-class residents with air conditioning face rising costs and inconvenience, an estimated 40 to 45 percent of the urban population, comprising construction workers, street vendors, rickshaw drivers, waste pickers and porters, work directly exposed to the sun, heat and humidity, and often cannot even recover overnight because night-time temperatures no longer drop enough for rest. He highlighted the rapid growth of gig and platform work in this context, citing food delivery workers who must complete deliveries in 12 to 15 minutes even when outdoor temperatures reach 42 degrees Celsius with 70 percent humidity, conditions he called "killing conditions," while riding several kilometres on scorching roads.

Dutta noted that continuous, cumulative heat exposure—rather than a single hot day—is what drives severe illness and even loss of consciousness among workers. He said many labourers report losing their capacity to work physically after the age of 40, whereas earlier they might have worked physically until 60. He referenced the IPCC's Sixth Assessment Report and India's own Ministry of Earth Sciences assessment from 2020, both of which identify heatwaves as the fastest-intensifying climate change impact, with the Ministry's report projecting a four to five-fold increase, or 400 to 500 percent, by the 2050s. Since over 90 percent of India's workforce is in the informal sector without protections or social security, Dutta questioned whether government and society are paying adequate attention to their plight, compared to environmental campaigns focused solely on tree preservation.

On solutions, Dutta called for community-level action combined with political mobilisation, arguing that the collective voice of millions of organised workers carries far more weight in policy circles than a handful of experts. At the community level, he suggested practical steps such as workplace shading, monitoring of heat conditions, improved ventilation, and low-cost interventions like reflective or "cool" roofing and earth roofing to reduce indoor heat, particularly since many workers live and cook in cramped, poorly ventilated housing.

Dutta acknowledged that heat action plans now exist in around 100 major Indian cities, a trend that began after Ahmedabad became the first city in South Asia to adopt one in 2013, following a 2010 heatwave that killed an estimated 1,300 to 1,400 people, with birds reportedly dropping dead from the sky. Since then, states such as Telangana and Rajasthan, the latter in 2024, have adopted statewide heat and human health action plans. However, Dutta criticised most existing plans as narrowly focused on water supply and urban greening, arguing that greening efforts often benefit privileged neighbourhoods while dense informal settlements like Mumbai's Dharavi have no space for trees at all and remain persistent heat islands regardless.

A major gap, Dutta said, lies in India's weather monitoring infrastructure. Weather stations, including major ones like Delhi's Safdarjung and Palam, are typically located in green, open areas such as near airports, and do not reflect actual conditions in dense working-class neighbourhoods. He also criticised the India Meteorological Department's heatwave declaration criteria as outdated, particularly for coastal regions, where existing norms fail to account for the deadly combination of high humidity and elevated temperatures. He noted that in coastal areas, a heat index can become dangerous even at 35 to 36 degrees Celsius with 90 percent humidity, yet current norms would not trigger a heatwave warning under such conditions. By the time official criteria are met, he warned, thousands of workers could already have fallen seriously ill or died.

Dutta urged that hyper-local, granular data collection using simple, affordable instruments be expanded across densely populated urban areas, alongside dialogue between employers, government bodies and worker communities. He called for the government to establish compensation and welfare policies for workers who lose wages due to extreme heat conditions, arguing that private insurance, driven by profit motives, will not fill this gap and that state intervention is essential given that these workers bear zero responsibility for causing climate change yet suffer its worst consequences.

Citing a Ministry of Earth Sciences report, Dutta warned that between 2035 and 2045, the Gangetic basin and peninsular India could see 35 to 40 days annually when outdoor physical work becomes impossible. With an estimated 50 crore Indians dependent on daily wage labour, he stressed that any policy response must include compensation mechanisms, since a loss of work is a direct loss of food, rent and basic survival for these families.

Dutta concluded by stressing that the crisis cannot be addressed through climate change mitigation alone. While rising sea surface temperatures since 2023, which have broken previous records by a large margin, have contributed to rising humidity, urban planning failures, including the blocking of natural wind corridors, are equally responsible for turning cities into heat traps. He called for integrated, holistic scientific assessments of heat exposure across different areas of cities, backed by adequate investment from both government and private stakeholders, warning that failure to act now would leave today's generation to answer difficult questions from children in the future about inaction in the face of a worsening crisis.