When the World Runs a Fever

Understanding the escalating health crisis of excessive heat in a warming world and what we must do about it.

Photo by Javier Miranda on Unsplash

KEY STATISTICS AT A GLANCE

546,000 heat-related deaths per year (avg)

+23% rise in heat mortality since the 1990s

640B hours of labour hours lost to heat in 2024

+85% rise in heat deaths in over-65s (2000–2021)

+1.5°C global temp rise above 1850–1900 baseline (2024)

45x more likely heatwaves in Bangladesh

THE CRISIS

The Planet Is Overheating and Bodies Are Paying the Price

Every year in my practice, I see a familiar pattern emerge as temperatures climb: elderly patients presenting with confusion and rapid heart rates, construction workers arriving in emergency with heat exhaustion, and mothers worried about feverish infants in homes without cooling. What I am witnessing at the bedside is not an isolated phenomenon; it is the clinical face of a planetary emergency.

Climate change is no longer a distant, abstract threat. It is happening now, its effects measurable in emergency department admission rates, in excess mortality figures, and in the lived experience of hundreds of millions of people around the world. Of all the consequences of a warming climate, excessive heat is perhaps the most immediate, the most democratic in its reach, and the most medically consequential.

“The climate crisis is a health crisis. Every fraction of a degree of warming costs lives and livelihoods.” — Dr. Jeremy Farrar, WHO Assistant Director-General

The numbers are sobering. 2024 was the hottest year ever recorded on Earth, with global average temperatures exceeding 1.5°C above the pre-industrial baseline, a threshold once considered a distant red line. The past decade (2015–2024) stands as the ten hottest years in recorded history. Heat waves that once struck once in a century are now projected to arrive every few years. The world, quite simply, is running a fever, and that fever is killing people.

PHYSIOLOGY UNDER SIEGE

How Extreme Heat Attacks the Human Body

Heat stress occurs when the body’s thermoregulatory mechanisms are overwhelmed. Under normal conditions, sweating dissipates excess heat through evaporative cooling. But when ambient temperatures are extremely high — particularly when combined with high humidity — sweat evaporates slowly, this mechanism fails, and core body temperature begins to rise dangerously.

CLINICAL NOTE

Heat stroke, defined as a core temperature above 40°C with central nervous system dysfunction, is a medical emergency carrying a mortality rate of 10–50% even with treatment. Exertional heat stroke, seen in athletes, labourers, and military personnel, can progress to multi-organ failure within hours if cooling is delayed. Time to cooling is the most critical determinant of outcome.

Beyond the dramatic presentation of heat stroke, excessive heat exerts insidious damage across almost every organ system. The cardiovascular system bears a particularly heavy burden: heat forces the heart to work harder to circulate blood to the skin for cooling, increasing cardiac output, and placing dangerous strain on patients with pre-existing coronary artery disease or heart failure. The kidneys suffer from chronic dehydration; repeated or untreated heat exposure can cause irreversible renal injury and contribute to the global rise in chronic kidney disease, particularly among agricultural workers.

HOW EXTREME HEAT ATTACKS THE BODY

Cardiovascular System

Increased cardiac workload, elevated risk of heart attack, ischemic stroke, and arrhythmia. Heat is a major trigger for acute cardiac events.

Respiratory System

Heat increases ground-level ozone, worsening asthma, COPD, and respiratory infections. Wildfire smoke compounds the damage.

Neurological & Mental

Heat impairs cognition; extreme temperatures are linked to increased ED visits for mental health crises, agitation, and aggression.

Kidneys

Chronic dehydration promotes kidney stones, acute kidney injury, and contributes to progressive chronic kidney disease.

Maternal & Infant Health

Extreme heat in pregnancy increases the risk of preterm birth, low birth weight, stillbirth, and adverse neurodevelopmental outcomes.

Medications & Chronic Dx

Heat destabilises insulin and blood pressure medications. It worsens glycaemic control in diabetics and destabilises hypertension.

The mental health dimension of heat is underappreciated. Research consistently shows that periods of extreme heat are associated with significant increases in emergency department presentations for psychiatric conditions — including acute psychosis, anxiety, suicidal behaviour, and substance misuse. The mechanisms are multiple: disrupted sleep from hot nights, physiological stress responses, and the psychological toll of displacement and loss.

WHO IS MOST AT RISK

Vulnerability Is Not Random — It Is Structured

One of the most important lessons from my public health training is that risk is rarely evenly distributed. Heat is no exception. While extreme temperatures endanger everyone, the burden falls most heavily on those already disadvantaged by age, poverty, occupation, and housing.

HIGH-RISK POPULATIONS

▶ Adults over 65, especially those living alone and without air conditioning

▶ Infants and young children, whose thermoregulatory systems are immature

▶ Outdoor and agricultural workers with prolonged sun and heat exposure

▶ Pregnant women and their unborn children

▶ People with cardiovascular disease, diabetes, respiratory illness, or kidney disease

▶ Individuals on heat-sensitising medications (diuretics, antipsychotics, beta-blockers)

▶ People experiencing homelessness, lacking shelter and cooling

▶ Low-income communities in urban heat islands with poor housing and green space

▶ People with mental illness, who may lack awareness of heat danger or access to cooling

▶ Residents of refugee camps and informal settlements

The data on older adults is particularly alarming. Heat-related mortality among people over 65 has increased by approximately 85% between 2000 and 2004 and 2017 and 2021. As our populations age globally — and as temperatures continue to rise — this is a demographic time bomb that health systems must prepare for urgently.

SOUTH ASIA & BANGLADESH

A Region on the Frontline of the Heat Emergency

BANGLADESH & SOUTH ASIA FOCUS

A Region on the Frontline of the Heat Emergency

Bangladesh ranks ninth on the 2024 World Risk Index, despite contributing only 0.3% of global greenhouse gas emissions. In April 2024, temperatures reached 43.8°C — the most intense heatwave since records began in 1948 — forcing nationwide school closures affecting 33 million children. By 2030, nearly 90% of South Asia’s population will be at risk from extreme heat events.

43.8°C Peak temperature, Bangladesh 2024

45x Heatwaves are more likely due to climate change

148% Rise in heat deaths in over-65s (2017–2021)

19.9M Projected climate displaced by 2050

As a physician practising in Dhaka, I find these projections deeply concerning. South Asian cities are particularly susceptible to the urban heat island effect — dense construction, limited tree cover, and heat-absorbing surfaces conspire to make urban temperatures several degrees higher than surrounding rural areas. For the urban poor living in cramped, poorly ventilated housing without access to air conditioning, a heatwave is not a statistic: it is a life-threatening emergency.

Projections indicate that by 2100, up to 70% of Indian, Pakistani, and Bangladeshi populations could be exposed to extreme wet-bulb temperatures — conditions at which the human body simply cannot cool itself, regardless of fitness or health status. This is not a distant scenario. It demands action today.

PUBLIC HEALTH NOTE

The 2025 Lancet Countdown report found that the average person was exposed to 16 additional days of dangerous heat in 2024 that would not have occurred without human-induced climate change. For infants and older adults, this figure exceeded 20 heatwave days per person — a fourfold increase compared to twenty years ago.

THE ECONOMIC COST

The Price the World Is Already Paying

The health burden of extreme heat is inseparable from its economic consequences. In 2024 alone, heat exposure caused 640 billion potential labour hours to be lost globally — productivity losses equivalent to $1.09 trillion USD. The costs of heat-related deaths among older adults reached $261 billion in the same year. These are not projections; they are the measured costs of inaction already materialising in the present.

In agricultural economies like Bangladesh, the consequences are particularly acute. Farm workers are already among the most economically marginalised, facing the greatest physical exposure and the least access to occupational health protection. Heat reduces both their capacity to work and the crop yields they depend on, creating a vicious cycle of climate vulnerability and poverty.

WHAT WE CAN DO

Protection Is Possible — But Requires Collective Action

I am often asked whether there is reason for optimism when facing evidence of this magnitude. My answer, as a clinician and public health practitioner, is a qualified yes — but only if action at every level, from individual behaviour to international policy, is taken with genuine urgency.

PROTECTING YOURSELF & YOUR COMMUNITY FROM EXTREME HEAT

1. Hydrate proactively

Drink water regularly throughout the day. Do not wait until you feel thirsty — thirst is a late sign of dehydration in hot weather.

2. Avoid midday sun

Stay indoors or in shade between 11am and 4pm on hot days. Schedule outdoor work and exercise for early morning or evening.

3. Check on vulnerable neighbours

Older adults living alone, those with chronic illness, and families with young infants need regular check-ins during heat events.

4. Know the warning signs

Heat exhaustion: heavy sweating, weakness, cool, clammy skin. Heat stroke (>40°C + confusion): a medical emergency — call for help immediately.

5. Review medications with your doctor

Diuretics, antipsychotics, beta-blockers, and anticholinergics all impair heat tolerance. Ask your physician if an adjustment is needed in the summer.

6. Advocate for systemic change

Support urban greening, heat action plans, and policies that address the root causes of climate change. Individual resilience has limits.

At the policy level, the evidence base for effective interventions is growing. Heat action plans — which pre-designate cool public shelters, establish early warning systems, and coordinate emergency medical response — have been shown to substantially reduce heat-related mortality in cities that adopt them. Urban greening, cool roofing programmes, and protection of wetlands and tree canopy all help reduce the urban heat island effect.

“As physicians, we treat the consequences of the world’s choices. But we also have a professional and moral obligation to advocate for the conditions that prevent those consequences from arising in the first place.”

The most effective public health intervention for the heat crisis is rapid, deep decarbonisation of the global economy. Every fraction of a degree of warming averted translates directly into lives saved — lives of the elderly, the very young, the poor, and the sick who stand at the sharp end of this emergency.

The world is, in the most literal sense, running a fever. As healers, the evidence is clear about what we should do: cool it down.

REFERENCES & DATA SOURCES

1. World Health Organization. Heat and health: Fact Sheet. WHO, 2024. https://www.who.int/news-room/fact-sheets/detail/climate-change-heat-and-health

2. Romanello M et al. The 2025 Report of the Lancet Countdown on Health and Climate Change. Lancet, 2025.

3. Center for Climate and Energy Solutions. Extreme Heat and Climate Change. C2ES, 2025. https://www.c2es.org/content/heat-waves-and-climate-change/

4. National Academy of Medicine. Health Impacts of Extreme Heat. NAM, April 2025. https://nam.edu/product/extreme-heat-climate-health-hazard/

5. Global Climate Risks. Bangladesh on the Brink: A Heat-Driven Health Crisis. 2025. https://globalclimaterisks.org/insights/blog/bangladesh-on-the-brink/

6. World Bank. Bangladesh and South Asia Climate Resilience Report. November 2025.

7. Time Magazine / MIT. How South Asia Can Adapt to Extreme Heat. 2026.

8. World Weather Attribution. Climate Change Made the Deadly 2024 Asian Heatwaves More Frequent and Extreme. 2024.

9. HHS.gov. Extreme Heat — Climate Health Outlook. U.S. Department of Health & Human Services, 2024.

10. Wikipedia. 2024 Bangladesh heatwave. https://en.wikipedia.org/wiki/2024_Bangladesh_heatwave