Heat Exposure and Health Effects

Heat exposure and health effects covers the consequences of high ambient temperature for human health, from acute heat-related illness to the elevated mortality and hospitalisation seen during heat waves. As climate change raises mean temperatures and the frequency and intensity of extreme-heat events, this is among the most directly attributable links between a warming climate and population health.

Definition

Heat exposure and health effects refers to the adverse physiological and epidemiological consequences of elevated ambient temperature, including heat exhaustion and heat stroke, exacerbation of cardiovascular and renal disease, and increased mortality and hospital admissions during periods of high heat.

Scope

The topic addresses how the body responds to heat stress, the spectrum of heat-related illness, the relationship between ambient temperature and mortality, and the populations and conditions that raise vulnerability. It is reference-educational: it explains the epidemiology and physiology of heat effects rather than providing clinical management instructions.

Core questions

• How does ambient temperature relate to mortality and morbidity?
• What physiological responses and failures underlie heat-related illness?
• Which individuals and groups are most vulnerable to heat?
• How do heat waves differ from chronic warming in their health impact?

Key concepts

• Heat stress and thermoregulation
• Heat exhaustion and heat stroke
• Temperature-mortality relationship
• Minimum-mortality temperature
• Heat waves
• Vulnerable populations (elderly, chronically ill, outdoor workers)
• Urban heat island

Mechanisms

When ambient heat exceeds the body's capacity to dissipate it through sweating and increased skin blood flow, core temperature rises, producing a spectrum from heat exhaustion to life-threatening heat stroke. Heat also strains the cardiovascular and renal systems and can worsen pre-existing chronic disease, which is reflected in rising hospital admissions during hot periods. Epidemiological studies describe a characteristic U- or J-shaped relationship between temperature and mortality, with risk rising steeply above a population-specific minimum-mortality temperature; age, chronic illness, social isolation, and limited access to cooling raise individual susceptibility.

Clinical relevance

Recognising the conditions and populations at heightened risk during hot weather supports surveillance, early warning, and preparedness in clinical and public-health settings. This entry is descriptive: it explains how heat produces health harm and is not a substitute for clinical protocols on the diagnosis or treatment of heat-related illness.

Epidemiology

Large multi-country studies attribute a substantial share of deaths to non-optimal ambient temperature, with both heat and cold contributing; Gasparrini and colleagues quantified this temperature-attributable mortality across many populations. Studies in the elderly show that heat and heat waves raise hospital admissions, and the Lancet Countdown reports rising exposure of vulnerable populations to extreme heat as the climate warms.

History

Episodic heat disasters, such as severe heat waves in major cities, drew early attention to heat as a cause of excess death. Time-series and case-crossover methods later allowed researchers to quantify the temperature-mortality relationship systematically, and multi-country collaborations extended these estimates globally. As climate projections pointed to more frequent and intense heat extremes, heat exposure became a central concern of climate-and-health research.

Key figures

• Antonio Gasparrini
• Anthony J. McMichael

Seminal works

• gasparrini-2015
• gronlund-2014

Frequently asked questions

Why do deaths rise during heat waves?

High ambient heat strains thermoregulation and the cardiovascular and renal systems and can worsen existing chronic disease; studies show mortality rises steeply once temperature exceeds a population-specific minimum-mortality threshold, with the elderly and chronically ill at greatest risk.

Does only extreme heat matter, or does moderate warming also affect health?

Both. Severe heat waves cause acute spikes in death and illness, but multi-country studies find that a large share of temperature-attributable mortality occurs across moderately non-optimal temperatures rather than only at extremes.