As heatwaves grow longer, hotter and more frequent, extreme temperatures are no longer just a weather story — they are a public health issue. From subtle dehydration to life-threatening heatstroke, heat places extraordinary stress on the human body, often in ways people underestimate or misunderstand.
Doctors and researchers warn that heat affects far more than comfort. It alters how the heart, brain, kidneys and immune system function — and for some groups, the risks escalate quickly.
How the Body Tries to Stay Cool
The human body is designed to operate within a narrow temperature range of around 36.5–37.5°C. When external temperatures rise, the body activates its cooling system.
Blood vessels near the skin widen, allowing heat to escape. Sweat glands release fluid that evaporates, drawing heat away from the body. Heart rate increases to move blood more efficiently to the skin.
These mechanisms work well in moderate heat, but they are limited. High humidity reduces sweat evaporation, dehydration limits sweat production, and prolonged heat exposure overwhelms the body’s ability to cool itself.
Dehydration: The First and Most Common Impact
One of the earliest effects of heat is dehydration. As sweating increases, the body loses water and essential electrolytes such as sodium and potassium.
Even mild dehydration can cause fatigue, headaches, dizziness and reduced concentration. In hot conditions, dehydration can develop quickly — sometimes before thirst is felt.
Left unaddressed, dehydration thickens the blood, strains the heart and impairs kidney function. It also reduces the body’s ability to regulate temperature, creating a dangerous feedback loop.
The Heart Works Harder in the Heat
Heat places significant stress on the cardiovascular system. As blood vessels widen to release heat, blood pressure can drop, forcing the heart to beat faster to maintain circulation.
For healthy individuals, this increased workload is usually manageable. But for people with heart disease, high blood pressure or respiratory conditions, extreme heat can trigger chest pain, irregular heart rhythms or cardiac events.
Studies consistently show spikes in hospital admissions for heart-related conditions during heatwaves, particularly among older adults.
Heat Exhaustion: When Cooling Systems Falter
When the body struggles to keep up with rising temperatures, heat exhaustion can occur. Symptoms include heavy sweating, weakness, nausea, vomiting, muscle cramps and fainting.
At this stage, the body is still trying to regulate temperature, but it is falling behind. Immediate cooling, rest and hydration are essential to prevent progression to more severe heat illness.
Ignoring heat exhaustion can have serious consequences, especially during prolonged exposure or physical exertion.
Heatstroke: A Medical Emergency
Heatstroke is the most severe heat-related illness and a medical emergency. It occurs when the body’s core temperature rises above 40°C and cooling mechanisms fail completely.
Symptoms include confusion, seizures, loss of consciousness and, in some cases, cessation of sweating despite extreme heat. Organs begin to malfunction, and without rapid treatment, heatstroke can be fatal.
Unlike heat exhaustion, heatstroke requires urgent medical intervention. Cooling the body quickly — often with ice packs or cold water immersion — can save lives.
The Brain and Cognitive Function
Heat affects the brain in subtle but significant ways. High temperatures impair concentration, memory, reaction time and decision-making.
Researchers have found that heat exposure increases irritability and aggression while reducing impulse control. This may help explain rises in accidents, workplace errors and even violence during extreme heat events.
For students, outdoor workers and drivers, these cognitive effects can increase the risk of injury and mistakes.
Kidneys Under Pressure
The kidneys play a critical role in fluid balance, and heat places them under strain. Dehydration reduces blood flow to the kidneys, increasing the risk of acute kidney injury.
Repeated heat exposure, particularly among manual labourers, has been linked to chronic kidney disease in some regions of the world. Health experts warn that rising temperatures may exacerbate kidney-related health burdens globally.
Who Is Most at Risk
While heat affects everyone, certain groups face heightened danger:
• Older adults, whose temperature regulation is less efficient
• Infants and young children, who overheat quickly
• People with chronic illnesses, including heart, lung and kidney conditions
• Outdoor workers and athletes, exposed for long periods
• People without access to cooling, such as air conditioning
Social factors — including housing quality, income and isolation — also influence heat vulnerability.
Heat, Sleep and Recovery
Hot nights can disrupt sleep, preventing the body from recovering from daytime heat stress. Poor sleep compounds fatigue, weakens immune function and increases vulnerability to illness.
When nights remain warm, the body never fully cools, increasing cumulative stress and raising health risks during prolonged heatwaves.
Heat in a Warming World
Climate scientists warn that extreme heat events are becoming more intense and more frequent. Public health officials increasingly view heat as a silent killer — often underestimated because its effects are gradual rather than dramatic.
Cities are adapting with heat action plans, cooling centres and early warning systems. But individual awareness remains critical.
Protecting Yourself in High Heat
Health experts recommend simple but effective measures:
• Drink water regularly, even before feeling thirsty
• Avoid strenuous activity during peak heat
• Wear light, breathable clothing
• Seek shade or air-conditioned spaces
• Check on vulnerable neighbours and family members
Listening to the body’s warning signs can prevent serious harm.
More Than Discomfort
Heat is not just about sweating or discomfort. It affects nearly every system in the body, from the heart and brain to the kidneys and immune defences.
As temperatures rise, understanding what heat really does to the body becomes essential — not only for personal wellbeing, but for public health planning in an increasingly hot world.
7 years in the field, from local radio to digital newsrooms. Loves chasing the stories that matter to everyday Aussies – whether it’s climate, cost of living or the next big thing in tech.
