Extreme Heat Is More Dangerous Than You Think: How High Temperatures Affect Your Body

A Global Health Threat That Has Quietly Overtaken Other Climate Hazards

The 2025 Lancet Countdown report, the ninth annual assessment produced by 128 researchers across 71 institutions, found that heat-related deaths reached an average of 546,000 per year between 2012 and 2021, a 63 percent increase since the 1990s. The most vulnerable groups, infants under one year and adults over 65, experienced a record average of 20 heatwave days in the most recent reporting year alone. The economic toll is just as stark. Lost labor productivity due to heat reached an estimated 1.09 trillion US dollars in potential income losses in 2024, with low and middle income countries bearing the heaviest share relative to the size of their economies.

The World Health Organization's European Region, the fastest warming of the six WHO regions, recorded more than 175,000 heat-related deaths a year, accounting for roughly 36 percent of the estimated 489,000 annual heat deaths recorded globally between 2000 and 2019. WHO officials have been blunt about why. Heat stress exacerbates cardiovascular, respiratory, cerebrovascular, mental health, and diabetes-related conditions, and it places particular strain on elderly people living alone and on pregnant women. These are not abstract global statistics. They describe a physiological process that plays out identically in a hospital in Madrid, in Manila, or in a rice field in Central Luzon.

Europe Is Living This Right Now

The scale of this threat is not theoretical or confined to past reporting periods. As of this writing, Europe is in the middle of its second major heatwave of 2026. The first struck in late May, an unusually early and historic heat dome that pushed temperatures 10 to 15 degrees Celsius above normal and broke all-time spring temperature records across Spain, the United Kingdom, Germany, France, and Ireland. Portugal recorded a national May record of 40.3 degrees Celsius in the town of Mora on May 27, while London recorded its hottest May day on record at Kew Gardens twice in consecutive days, reaching 35.1 degrees Celsius. That first heatwave alone caused at least 11 deaths in the United Kingdom and seven heat-related deaths in France.

A second, more severe heatwave began in mid-June, just before the summer solstice. France's national weather service, Météo-France, confirmed that June 23, 2026 was the country's hottest day since record-keeping began in 1947, with temperatures reaching 44.3 degrees Celsius in parts of the south. By June 26, at least 327 heat-related deaths had been registered across Europe since the heatwave intensified the previous Sunday. Red, the highest-level heat alert, has been declared simultaneously in the United Kingdom, Germany, France, Spain, Switzerland, and Luxembourg. Spain's national weather agency, AEMET, reported highs above 45 degrees Celsius in the south of the country, with nearly the entire nation placed under some form of heat alert. In Austria, GeoSphere Austria recorded temperatures of at least 35 degrees Celsius at 18 separate weather stations on a single day, with Vienna forecast to reach 39 degrees Celsius.

The disruption has not been limited to direct casualties. Hundreds of schools across the United Kingdom closed early or shortened their schedules. Rail operators in Belgium and Sweden canceled trains to reduce the risk of heat-induced track warping, after a 600 meter freight train derailed near Bollebygd, Sweden, on June 25 due to track distortion caused directly by the heat. Public health officials in France are investigating the deaths of two young children found unconscious in a family car in Carpentras, where temperatures exceeded 39 degrees Celsius, as heat-related. This is precisely the pattern the Lancet Countdown's global data describes: a fast-warming region, a population and infrastructure not built for sustained extreme heat, and a death toll concentrated among children and the elderly.

What Actually Happens Inside the Body When It Gets Too Hot

The human body normally maintains a tightly regulated core temperature near 37 degrees Celsius. To stay there, it relies on two primary cooling mechanisms: vasodilation, which widens blood vessels near the skin so heat can radiate outward, and sweating, which cools the body as moisture evaporates from the skin surface. Under prolonged or extreme heat exposure, these mechanisms become overwhelmed, and the strain shifts from the skin to the body's most critical organ systems.

The cardiovascular system absorbs the first major burden. As blood vessels near the skin dilate to dump heat, the heart has to pump faster and harder to keep blood pressure stable. This is compounded by fluid loss through sweating, which reduces blood volume and can cause blood pressure to fall even as the heart works harder. The American Physiological Society's 2025 review of heat-related health impacts identifies this combination, elevated cardiac output followed by volume depletion, as a primary driver of adverse cardiovascular events during heat exposure, including arrhythmias and acute myocardial infarction. Heat is directly toxic to heart muscle tissue at sustained high temperatures, capable of triggering inflammation and the release of enzymes associated with cardiac injury.

The kidneys are hit by what specialists describe as a triple threat: dehydration, falling blood pressure, and direct heat-related cellular stress. As fluid losses mount, less blood reaches the kidneys, impairing their ability to filter waste and regulate electrolytes. Research published through the National Center for Biotechnology Information has documented that exertional heatstroke can produce acute kidney injury in an estimated 30 to 40 percent of cases in cohort studies of military personnel and athletes, with innate immune responses, including neutrophils and macrophages, now understood to play a direct role in heat-induced kidney damage even at sub-heatstroke temperatures.

The brain is especially sensitive to even small increases in core temperature. As blood is redirected toward the skin to aid cooling, the brain receives comparatively less oxygen and fewer nutrients, slowing neural communication and impairing memory, concentration, and decision-making. If core temperature continues to climb past approximately 40 degrees Celsius, the threshold that defines clinical heatstroke, brain cells can begin to swell, and central nervous system dysfunction sets in. This is what makes heatstroke a true medical emergency rather than simply a severe case of feeling unwell: confusion, slurred speech, seizures, and loss of consciousness can develop rapidly once this threshold is crossed.

Heat strain extends further still. Muscle tissue can break down under extreme thermal stress, a process called rhabdomyolysis, releasing proteins into the bloodstream that further damage the kidneys. Blood itself can develop coagulation abnormalities. The liver faces metabolic overload as it works to process the cascade of cellular damage occurring elsewhere in the body. None of these systems fail in isolation. Once core temperature breaches the critical threshold, organ damage can occur in parallel, which is why untreated heatstroke carries a real risk of death even in people who were otherwise healthy before the exposure began.

Why the Philippines Is Especially Exposed

The Philippines has experienced some of its most extreme recorded heat index values in 2026. PAGASA's danger category, covering heat index readings between 42 and 51 degrees Celsius, has been recorded across dozens of localities this year, with readings as high as 45 degrees Celsius reported in areas including Sangley Point in Cavite and San Ildefonso in Bulacan during peak summer months. The agency's extreme danger classification, 52 degrees Celsius and above, carries a warning of potential imminent heatstroke with prolonged sun exposure. To help the public track this risk in real time, DOST-PAGASA launched its Interactive Heat Index Mapping and Prediction platform, known as iHeatMap, in March 2026, using data from the European Centre for Medium-Range Weather Forecasts to generate localized, high resolution heat index forecasts across the country.

The Department of Health has documented the human cost through its event-based Surveillance and Response System. In the comparable period across recent dry seasons, the agency has recorded cases ranging into the hundreds nationally, with deaths reported as either directly attributable to heatstroke or heat-influenced, meaning underlying conditions such as cardiovascular disease worsened by extreme ambient heat to the point of becoming fatal. A significant share of reported heat-related illness cases in past years has concentrated among students aged 12 to 21, a pattern that prompted the DOH to recommend alternative delivery modes for classes during periods of dangerous heat index.

• PAGASA's danger category (42 to 51°C) carries documented risk of heat cramps, heat exhaustion, and heat stroke with prolonged exposure
• The extreme caution category (33 to 41°C), which regularly covers Metro Manila during summer months, can still produce heat cramps and exhaustion with continued outdoor activity
• Outdoor and informal sector workers, who cannot limit midday sun exposure, face disproportionately higher cumulative heat strain
• The 2025 Lancet Countdown identifies children, pregnant women, and older adults living alone as the populations facing the steepest rise in heat-related health risk

In recognition of this burden, PhilHealth has confirmed that its inpatient benefit packages cover heat-related conditions directly. As of April 2026, heat stroke, heat exhaustion, heat collapse, heat cramps, and sunstroke are covered up to 12,675 pesos, while heat fatigue and other heat and light related effects are covered up to 18,135 pesos, and moderate to severe dehydration is covered up to 7,800 pesos. Emergency care for suspected heat illness, heat-related or otherwise, is also available through PhilHealth's Outpatient Emergency Care Benefit package at any accredited Level 1 to 3 hospital.

Recognizing the Warning Signs Before They Become an Emergency

The Department of Health has consistently emphasized that heat-related illness exists on a spectrum, and that recognizing early symptoms can prevent progression to a medical emergency. Dizziness, excessive sweating, fatigue, headache, and nausea are common early indicators. The DOH advises that anyone experiencing these symptoms should rest immediately in a shaded or air-conditioned area, apply cold compresses, hydrate with water rather than caffeinated or sugary drinks, and seek medical attention if symptoms do not improve quickly. Heat stroke, marked by confusion, very high body temperature, hot and dry or profusely sweating skin, rapid pulse, and possible loss of consciousness, is a medical emergency requiring immediate transport to a health facility.

Prevention remains the most effective intervention available. Public health guidance, consistent across the DOH, PAGASA, and WHO, centers on a small number of repeated principles: drink water before thirst sets in rather than after, avoid the period between 10 a.m. and 4 p.m. for outdoor activity when possible, wear light and loose clothing, use shade, umbrellas, or hats when sun exposure cannot be avoided, and check regularly on elderly relatives or neighbors who may be living alone, since this group faces the steepest documented rise in heat-related mortality risk worldwide.