Extreme heat is increasingly recognised as a major health threat, yet many primary care settings in Africa lack practical, synthesised guidance on its clinical and community-level impacts.^1,2 The purpose of this article is to provide a clear and concise overview of the health effects of extreme heat that are most relevant to primary health care, and to outline the key mechanisms, vulnerabilities and clinical considerations that practitioners should be aware of.

Extreme heat is emerging as a major cause of global morbidity and mortality as temperatures rise and heatwaves become more frequent, longer and more intense.¹ Studies of temperature-related mortality show that non-optimal temperatures contribute significantly to global deaths, with heat-related mortality increasing fastest in regions that already experience high baseline temperatures.¹ Many African countries face additional vulnerability because of widespread outdoor labour, informal housing, limited access to cooling and constrained water and energy supply systems. These environmental and socio-economic conditions heighten population exposure and reduce the capacity to adapt to extreme heat.^1,2

Climate-related stressors already influence service delivery, patient presentations and health worker well-being across the primary care platform. A recent review examining climate change and primary health care identified heat as a key hazard affecting workload, medication safety and continuity of care in African settings.² Primary care facilities therefore play a central role in recognising, managing and preventing heat-related illness.²

A global meta-review by Thiel et al. further highlights the breadth of conditions associated with heat exposure, including electrolyte disturbances, cardiovascular and respiratory disease, renal injury, mental health effects and increased infectious disease incidence.³ These findings show that heat interacts with climatic factors such as humidity and with social factors such as age, disability and socio-economic status.^2,3 This underscores the need for primary care practitioners to understand both the physiological effects of heat and the social vulnerabilities that shape risk.

Human thermoregulation maintains core temperature through conduction, convection, radiation and evaporation. When environmental temperatures rise, the hypothalamus increases skin blood flow to enhance heat loss. Under significant heat stress, skin blood flow may reach 6 L to 8 L per minute.⁴ Sweating provides the dominant cooling mechanism, but high humidity greatly reduces evaporation. Once humidity exceeds roughly 75%, sweat cannot evaporate effectively, causing heat to accumulate.

When environmental heat exceeds the body’s ability to dissipate it, core temperature rises, and thermoregulatory mechanisms become overwhelmed, leading to uncompensated hyperthermia. There are five major physiological processes triggered by heat exposure that can progress to organ dysfunction: ischaemia, heat cytotoxicity, systemic inflammation, disseminated intravascular coagulation and rhabdomyolysis.⁵ These interacting mechanisms reduce blood flow to vital organs, damage cell membranes, increase permeability to toxins and pathogens and promote widespread inflammatory and coagulation disturbances. The combined effects can impair the heart, kidneys, brain, liver, pancreas, lungs and gastrointestinal tract, creating a cascade of multi-organ stress that may rapidly progress to failure if cooling and resuscitation are delayed. Underlying these pathways are cellular derangements such as mitochondrial dysfunction, oxidative injury and endotoxin translocation from the gut, which further amplify tissue damage. In severe cases, these mechanisms reinforce one another, producing a self-propagating cycle of cellular injury and circulatory collapse characteristic of heat stroke.⁵

Heat stress also impairs fluid balance and cardiovascular stability through vasodilation, dehydration and plasma volume loss, which increase cardiac workload and reduce renal perfusion.⁴ These responses disproportionately affect older adults and individuals with chronic disease. These responses disproportionately affect older adults and individuals with chronic disease, as age-related declines in sweat gland function and thirst perception reduce physiological adaptability, while medications commonly used in chronic disease management, including diuretics, anticholinergic medications, beta blockers, antipsychotics and certain antidepressants, further impair thermoregulation and exacerbate fluid loss.^4,5

Mild heat-related illnesses include heat rash, heat oedema, heat cramps and heat syncope.⁴ Heat rash presents as small pruritic vesicles caused by sweat duct blockage and local inflammation.⁵ Heat oedema results from peripheral vasodilation and dependent fluid pooling, especially in the legs.^4,5 Heat cramps arise from painful spasms in large muscle groups because of salt loss during sweating, while heat syncope is a brief loss of consciousness with rapid recovery following peripheral vasodilation and dehydration.⁵ These conditions usually resolve with rest, cooling and rehydration.⁵

Heat exhaustion is a moderate heat-related illness characterised by fatigue, weakness, headache, dizziness, nausea and tachycardia.⁵ It typically follows recent exposure to high temperatures and is driven by dehydration and sodium depletion.^4,5 Core temperature may be normal or mildly elevated. Without intervention, heat exhaustion can progress to heat stroke.⁶

Heat stroke is the most severe heat-related illness.⁶ It occurs when core temperature rises above 40 °C with central nervous system dysfunction such as confusion, agitation, seizures or coma.⁶ Mortality is approximately 10% and increases significantly with hypotension. There are two main forms. Exertional heat stroke affects younger, healthy individuals who generate large amounts of metabolic heat during strenuous activity.^5,6 Classic heat stroke occurs during extreme ambient heat and humidity and primarily affects older adults, young children and those with chronic illness, disability or limited access to cooling.⁶

Several factors increase vulnerability to heat illness.

These include dehydration, certain medications, poor heat acclimatisation, impaired mobility, chronic heart or lung disease and renal impairment.^4,5 Accurate core temperature measurement, ideally rectal, is essential because peripheral readings may underestimate severity. Urgent, aggressive cooling remains the cornerstone of management, particularly for heat stroke where delays significantly worsen outcomes.^4,5,6

In many African settings, the health effects of extreme heat do not occur immediately but unfold over several days.⁷ Mortality and morbidity often peak 6 days to 12 days after exposure, with older adults, young children and people with chronic disease most affected.⁷ This lag effect has important implications for follow-up and monitoring during and after heatwaves.

Most heat-related deaths arise from worsening of pre-existing conditions rather than heat stroke itself.⁵ High temperatures increase the risk of acute cardiovascular events, renal injury, respiratory exacerbations and metabolic decompensation.⁸ Heat exposure also affects mental well-being, contributing to irritability, reduced concentration and sleep disturbance.

Community research from Agincourt reported headaches, dizziness, sleep disturbance and reduced productivity during hot periods.¹³ Many households lacked effective cooling strategies and awareness of heat-related mortality. Extreme heat also amplifies existing health burdens at the community level, with residents noting worsening of chronic conditions such as hypertension, asthma and arthritis during very hot days.¹³ Communities frequently experience increased dehydration, heat-related fainting and disrupted sleep, which further compound stress, fatigue and reduced daily functioning.^7,13 Socio-economic conditions, such as housing quality, access to water, vegetation cover, population density and disability status, all influence vulnerability.¹⁴

Beyond individual patient assessment and acute management, primary health care has a central role in reducing preventable morbidity during periods of extreme heat. This includes supporting community preparedness and ensuring facility-level readiness for surges in patient demand. The following sections outline these broader responsibilities.

Extreme heat is an escalating health challenge across Africa. Its physiological effects, interactions with chronic illness and wider social consequences place significant demands on primary health care. Integrating heat awareness into routine practice, medication review, pregnancy care, community outreach and facility preparedness is essential for strengthening climate-resilient health systems and protecting vulnerable populations.