In the 21st century, the battle against climate change has often focused on melting glaciers, rising sea levels, and erratic weather patterns. Yet, a more intimate and silent enemy lurks in our neighbourhoods, cities, and homes heat. No longer just a seasonal inconvenience, excessive heat has evolved into a public health emergency, claiming lives and disrupting communities with alarming regularity. The World Health Organization (WHO) now classifies heat as a critical environmental and occupational health hazard, and data confirm that this invisible threat is accelerating in both intensity and reach.
Recent figures from the WHO show that heat-related mortality among individuals aged over 65 surged by approximately 85% between 2000–2004 and 2017–2021. Across two decades, the world saw an average of 489,000 annual deaths attributable to heat, with Asia and Europe bearing 81% of this burden. The European continent alone recorded over 61,000 excess deaths in the summer of 2022 due to extreme heat, a deadly reminder that rising temperatures are not just about discomfort but deadly consequence.
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In 2024, researchers examined 29 extreme weather events that resulted in more than 3,700 deaths and displaced millions globally. Their analysis revealed that 26 of these incidents were directly influenced by human-induced climate change, with elevated ocean temperatures and warmer atmospheric conditions significantly contributing to the intensity of devastating storms and record-breaking rainfall.
Heat: A Slow and Silent Killer
Unlike sudden natural disasters, heat does not announce itself with dramatic visuals or noise. Instead, it infiltrates daily life, particularly affecting the elderly, infants, outdoor labourers, individuals with chronic illnesses, and those living in low-income or informal settlements. The most severe manifestation, heatstroke, is a life-threatening emergency, but the effects of heat extend to cardiovascular diseases, asthma, renal failure, and even mental health. The connection between extreme temperatures and accidents, both occupational and domestic, is also gaining increased attention among public health professionals.
What makes the situation more concerning is the cumulative nature of heat. Repeated exposure to high day and night-time temperatures wears down the human body. Prolonged heatwaves strain internal organs and exacerbate existing medical conditions. As per WHO, heatwaves are becoming longer, more frequent, and more intense, even in regions that previously did not experience such extremes.
When Cities Become Ovens
Urban design is failing humanity in the face of rising heat. Many cities, especially in the Global South, are constructed in ways that amplify, rather than mitigate, thermal stress. The “urban heat island” effect, where built-up areas trap heat, makes densely populated urban zones even more hazardous. Loss of green spaces, excessive use of metal roofing, and inadequate ventilation all contribute to overheating.
Informal settlements and poor housing are particularly vulnerable. According to the WHO, such neighbourhoods can be several degrees hotter than surrounding urban zones. In places like Delhi, Lagos, and Rio de Janeiro, families living under tin roofs report unbearable indoor temperatures that often surpass the external heat. This is not merely an inconvenience; it is a threat to life. Research indicates that people in substandard housing are significantly more likely to experience heat-related illnesses or fatalities.
Who Bears the Brunt?
Though the climate crisis is global, its medical consequences are not evenly distributed. Countries in tropical and subtropical zones are naturally hotter and more vulnerable. However, temperate regions are increasingly feeling the sting of climate-driven heatwaves, and often with more deadly consequences because populations, infrastructure, and healthcare systems are ill-prepared.
The 2010 heatwave in Russia, which lasted 44 days, led to over 56,000 deaths, while the 2003 European heatwave claimed nearly 70,000 lives. These numbers eclipse the death tolls of many more visible disasters. Vulnerability is not just geographical; it is also social and economic. The poor, the elderly, outdoor workers, women (particularly those in domestic settings), and people with pre-existing health conditions are disproportionately affected.
For example, in rural India, many women and children suffer heat exposure due to indoor cooking in poorly ventilated huts during extreme heat. In Nigeria, construction workers in cities like Abuja and Lagos report increasingly intense and prolonged exposure, leading to a rise in exertional heat stress.
Health Systems on the Edge
Healthcare systems are finding themselves at breaking point during heat events. Heatwaves often coincide with power outages, which can disrupt the very services needed to treat heat-related illnesses, air conditioning in hospitals, clean water supplies, and transportation for emergency services. In 2022, France and Spain saw a surge in hospital admissions for heat-related complications, straining emergency departments already impacted by the COVID-19 pandemic.
Furthermore, many healthcare professionals remain insufficiently trained or aware of the full extent of heat-related medical risks. The WHO has called for more comprehensive training, stating that health professionals must adjust both clinical guidance and health policies in light of escalating heat trends. Despite mounting evidence, public awareness of the medical dangers of heat remains dangerously low.
The Price of Inaction
The economic cost of heat-related illness is staggering. In developing economies, where a significant proportion of labour is manual and outdoors, rising temperatures directly translate to lost productivity and income. The International Labour Organization (ILO) estimates that by 2030, the equivalent of 80 million full-time jobs could be lost annually due to heat stress, primarily in agriculture and construction sectors.
In nations like Bangladesh, Kenya, and the Philippines, heat stress is not just a health issue, but a labour and food security crisis. Farmers report reduced work hours, spoiled crops, and chronic fatigue. When human bodies cannot adapt to external heat, the economic engine stalls—sometimes irreversibly.
A Call to Action
In recognition of the increasing threat, the WHO has issued official guidance for managing heat risk, including heat-health warning systems, early alert mechanisms, and urban planning interventions. The Global Framework for Climate Services (GFCS) and the United Nations Framework Convention on Climate Change (UNFCCC) also encourage the integration of climate information into health planning.
Under the Paris Agreement, signatory countries are expected to implement climate adaptation strategies, including public health responses to extreme heat. However, implementation remains uneven and underfunded, particularly in the Global South. The WHO stresses that heat-related illness and death are largely preventable—but prevention requires political will, cross-sectoral collaboration, and urgent investment.
Where Do We Go From Here?
A future where heat becomes a primary public health concern is not speculative, it is already here. The health risks associated with extreme heat are predictable, well-documented, and growing. Yet, the world has been slow to act, treating rising temperatures as a meteorological rather than a medical crisis.
Addressing this health emergency demands a multifaceted approach: climate-resilient urban planning, better-equipped health systems, public education, and international cooperation. Without these, the air we breathe and the heat we feel will continue to claim lives in silence, one heartbeat at a time.
