On a hot day, the body behaves like a busy cooling system. Blood vessels near the skin widen, sweat appears and we seek shade, water or a slower pace. These responses usually keep our inner temperature within a narrow range. Extreme heat can overwhelm them, especially when humidity, exertion or illness make cooling harder.
The danger is not defined by one number on a weather forecast. Air temperature, sun, wind, humidity, activity and the temperature of the rooms where we recover all matter. The World Health Organization warns that hot days and nights can create cumulative strain.¹
How we shed heat
The body constantly makes heat through metabolism and muscle activity. It can release some heat to cooler surroundings, and it can send warm blood towards the skin. When the air is hot, evaporation of sweat becomes especially important: liquid water takes heat from the skin as it turns to vapour.
Sweating alone is not proof that cooling is working. Humid air slows evaporation, so sweat may drip away while carrying off less heat than expected. WHO guidance on indoor temperatures highlights this link between humidity and heat risk.² A breeze can help evaporation; direct sun or hot surfaces add to the load.
Meanwhile, sending more blood to the skin asks the heart to work harder. Sweat costs water and salts. During exercise or physical work, muscles generate additional heat just when the environment may be limiting its escape. A person's ability to cope depends on acclimatisation, fitness, health, medicines and access to cooler places.
When the system strains
Heat exhaustion can bring heavy sweating, weakness, headache, dizziness, nausea, cramps or a fast pulse. It is a warning that the body is struggling, often after fluid and salt losses or sustained heat exposure. The NHS advises moving to a cool place, removing unnecessary clothing, drinking cool fluids and cooling the skin; symptoms that do not improve promptly need further help.³
Heatstroke is different in severity. The body's temperature regulation has failed, and the brain may be affected. Confusion, altered behaviour, collapse or loss of consciousness are danger signs. Skin can be dry or damp, so waiting for a person to stop sweating is unsafe. Heatstroke is a medical emergency requiring immediate help and rapid cooling.⁴
This distinction matters because the word “heatstroke” is sometimes used casually for feeling awful in the sun. True heatstroke can injure the brain and other organs. Conversely, heat exhaustion should not be dismissed: a person who remains in the heat can deteriorate.
Why some people are more vulnerable
Older adults, young children and people with certain long-term conditions may have less physiological reserve or face practical barriers to keeping cool. Some medicines can affect sweating, circulation, thirst or fluid balance. The NHS recommends checking on children, older people and people with conditions such as diabetes or heart disease during hot weather.³
Risk is also social. A person may live in an overheated flat, lack a shaded route to work or have a job that cannot simply pause. A fan or bottle of water is not a complete answer if the surrounding air and building stay hot for days. WHO stresses that heat impacts depend on health systems, housing and the conditions in which people live and work.¹
Night-time heat deserves attention. If a home does not cool after sunset, the body has less chance to recover from the day's strain. Repeated hot nights can make a heatwave more dangerous even when the daytime maximum is unchanged.
What happens inside when cooling fails
As core temperature climbs, the heart and circulation struggle to supply both working organs and the skin that is trying to lose heat. Dehydration reduces the available fluid. Cells and tissues can become injured by sustained high temperature and the accompanying inflammatory response. The brain is particularly important to watch because altered thinking may make a person unable to recognise danger or take sensible steps to cool down. In severe cases, the kidneys, liver and other organs can be affected. This is why a person with suspected heatstroke needs urgent medical care even if they seem to recover briefly after being moved into shade.
The temperature is only part of the story
Heat risk rises with exertion because muscle work produces heat from within. It rises in humid weather because evaporation is less effective. It can also rise quickly in a closed car or a poorly ventilated room where the air becomes much hotter than outdoors. Workplace heat assessments therefore consider humidity, radiant heat and air movement as well as the thermometer reading.⁵
People can adapt somewhat to heat exposure over time, but acclimatisation has limits and can be lost. It does not make anyone immune to heatstroke. The sensible response is to match effort to conditions, create opportunities to cool down and recognise when symptoms signal that cooling is no longer enough.
A practical response begins before anyone is ill: drink regularly, seek cooler spaces, reduce demanding activity when possible and check on people who may be isolated. If heatstroke is suspected, call emergency services and begin cooling the person while help is on its way. These actions address the immediate problem: excess heat within a body that can no longer shed it fast enough.
Extreme heat turns an ordinary balancing act into an urgent physiological problem. The body can work hard to protect itself, but it needs an environment that lets heat escape. When behaviour or awareness changes, that balance may already have broken: prompt emergency care and cooling matter far more than proving how high the temperature reached.