When the Heat Becomes the Hazard

A complete guide to heat-related illness: what happens to your body, how to recognise the signs, and what to do 

Looking for the quick version? Our short guide covers the essentials on a single page: https://blog.constellationtraining.co.uk/heat-illness-spot-it-stop-it/

On 25 May 2026, the UK broke its all-time record for the hottest day in May. Kew Gardens registered a provisional 35.1°C, a temperature once considered implausible before summer had properly begun. The following day, 26 May, the record was broken again.

These are not statistical curiosities. They are the new baseline. Research by the Met Office found that breaking a May temperature record of this magnitude is now around three times more likely than it would have been in a climate without human-caused greenhouse gas emissions. What was once a one-in-a-hundred-year event now occurs roughly once every thirty-three years. Extreme heat is becoming a recurring part of the UK climate.

During the summer of 2022, when temperatures topped 40°C for the first time in recorded British history, an estimated 4,507 deaths in England were linked to the heat. In summer 2025, the UK’s warmest on record, there were 1,504 heat-associated deaths even with proactive NHS and public health alerts in place. The lower figure is evidence that awareness and action save lives. It also suggests that many of the deaths which did occur may have been preventable with earlier recognition and intervention.

This guide explains the full spectrum of heat-related illness: from the early warning signs of dehydration to the life-threatening emergency of heatstroke. It covers vulnerable groups and the specific considerations for children and babies. If you are a parent, a carer, or simply someone who wants to be prepared when the temperature climbs, this is the information you need.

Why Heat Is More Dangerous Than We Think

There is a persistent tendency in Britain to celebrate hot weather, and understandably so. Sun is rare enough here that most of us welcome it without reservation. That cultural reflex is partly what makes heat dangerous.

Heat illness is not something that happens to other people in other countries. It does not require sitting in direct sunlight for hours at a time. The physiological mechanisms behind heat exhaustion and heatstroke can be set in motion by sustained exposure to warm indoor temperatures, by physical exertion, by poor ventilation, or by a combination of any of these factors.

The human body maintains its core temperature at approximately 37°C. When the surrounding environment becomes hotter than the body can comfortably manage, a cascade of compensatory responses begins: heart rate increases, blood is redirected to the skin, and sweating accelerates. These mechanisms are effective under moderate conditions, but they have limits. When they fail, the consequences escalate quickly.

According to the Office for National Statistics, once ambient temperatures exceed 25°C, temperature-related mortality increases by approximately 50 per cent. That threshold was crossed across large parts of England every day of the 2026 May heatwave.

Dehydration: The Precursor to Everything Else

Every heat-related illness is made worse by dehydration, and dehydration is the state most people enter long before they notice it. By the time thirst becomes apparent, the body is already operating at a deficit.

In hot weather, the body can lose fluid rapidly through sweat. An adult working or exercising outdoors can lose well over a litre per hour in high temperatures. Older adults have a reduced thirst response, meaning they do not register the need to drink until the deficit is significant. Children have a higher surface area to body weight ratio, meaning they lose a proportionally larger amount of fluid relative to their size.

Recognising Dehydration in Adults

What to Do

• Drink water consistently throughout the day, not just when thirsty.
• During exercise or heavy work in heat, consider a sports rehydration drink to replace lost salts as well as fluids.
• Avoid alcohol and excessive caffeine, both of which increase fluid loss.
• Pale, straw-coloured urine is the best simple indicator of adequate hydration.
• If someone shows signs of moderate dehydration, rest them in a cool area and give water or a rehydration solution. If they cannot keep fluids down or symptoms worsen, seek medical attention.

Heat Exhaustion: The Warning Stage

Heat exhaustion is the body’s distress signal. It occurs when the thermoregulatory system is under severe strain but has not yet failed completely. It is the stage at which intervention makes the most difference. Treated promptly, most people recover fully. Left unaddressed, it progresses to heatstroke, at which point the situation becomes life-threatening.

Signs and Symptoms

• Heavy sweating
• Pale, ashen, or clammy skin
• Headache
• Dizziness or light-headedness
• Nausea or vomiting
• Muscle cramps
• Weakness or fatigue
• Rapid, weak pulse
• Temperature may be raised but usually below 40°C

A person with heat exhaustion is typically still conscious and able to communicate. They will often feel unwell but may not connect their symptoms to the heat. This is one reason heat exhaustion is frequently not recognised in time.

What to Do: The Four-Step Response

Monitor the person continuously. The NHS advises that heat exhaustion should begin to improve within thirty minutes with these measures. If it does not, or if symptoms worsen at any point, call 999 or take them to A&E. Do not drive yourself.

Heatstroke: A Medical Emergency

Heatstroke is not simply severe heat exhaustion. It is a qualitatively different and life-threatening condition in which the body’s core temperature has exceeded 40°C and the brain’s thermostat has effectively failed. Without rapid treatment, heatstroke causes brain damage, organ failure, and death.

The transition from heat exhaustion to heatstroke can happen quickly. One critical warning sign is the disappearance of sweating. The skin becomes hot and dry. If you notice this change in someone who was previously sweating heavily, the situation has escalated.

Signs of Heatstroke

What to Do: Call 999 First

Call 999 immediately. Do not wait to see if the person improves. Do not drive them to hospital yourself unless specifically advised to by the 999 call handler.

 While waiting for the ambulance:

•  Move the person to the coolest available environment.
• Remove outer clothing.
• Wrap them loosely in a cold, wet sheet or clothing and pour cold water over it continuously.
• If no sheet is available, sponge the body with cold water and fan them aggressively.
• Apply ice or cold packs wrapped in cloth to the armpits, groin, neck, and scalp.
• If they are conscious and able to swallow, give sips of cool water.
• If they lose consciousness, place them in the recovery position and continue cooling.
• Do not give paracetamol or aspirin. These are ineffective for heat-related temperature elevation and may cause harm.

Heat Exhaustion vs Heatstroke: Quick-Reference Comparison

UV and Sunburn: The Parallel Risk

Throughout the heat illness sequence above, UV exposure runs as a parallel and compounding risk. Sunburn is often treated as a minor inconvenience. It is not.

Sunburn is a radiation injury. It damages DNA in skin cells, suppresses the skin’s local immune response, and impairs the body’s ability to regulate temperature at the skin surface. A sunburned person is more vulnerable to heat exhaustion because the damaged skin cannot manage heat exchange as efficiently.

 The UV index in the UK during a heatwave can reach 8 or higher, classified as very high. At that level, unprotected skin can begin to burn in under fifteen minutes. An overcast hot day still carries significant UV risk. Cloud does not block UV.

• Apply broad-spectrum SPF 30 or higher sunscreen thirty minutes before going outdoors and reapply every two hours or after swimming.
• Reapplication is not optional. Most people apply too little, and correctly applied sunscreen still degrades with time and perspiration.
• Protective clothing and shade are more effective than sunscreen alone for prolonged outdoor exposure.
• Areas that are frequently missed: face, scalp, backs of hands, tops of feet, backs of ears, and the back of the neck.
• UV passes through cloud cover. An overcast hot day is not a safe day.

Children and Babies

Children are not simply smaller adults when it comes to heat risk. Their physiology makes them genuinely more vulnerable, and several warning signs present differently to those you would observe in an adult.

Why Children Are at Greater Risk

• Children have a higher surface area to body mass ratio, meaning they absorb heat from the environment more rapidly relative to their size.
• Their thermoregulatory systems are less mature and less efficient.
• They rely on adults to recognise that they are overheating and to act.
• Young children and babies cannot communicate discomfort clearly, so early signs can go unnoticed.
• Children are often physically active during hot weather, accelerating fluid loss.

 An additional hazard specific to children is the risk of being left in a hot car. When the outside temperature is 34°C, the interior of a parked car can reach 52°C within twenty minutes. A child’s core temperature can reach dangerous levels with frightening speed. A child should never be left alone in a car in warm weather, even briefly, even in shade.

Recognising Heat Illness in Children

Early signs that a child is overheating

Sheffield Children’s NHS Foundation advises that the first sign a child is overheating is often irritability or a complaint of headache. If this happens, act immediately: move them to a cooler environment, remove clothing, and give fluids. Do not wait for signs to worsen.

• Grumpiness, irritability, or unusual fussiness
• Complaints of headache or dizziness
• Flushed or reddened face
• Heavy sweating
• Lethargy or reluctance to play
• Nausea

Signs of heat exhaustion in children and babies

The NHS notes that heat exhaustion symptoms are largely the same in children as in adults, with the addition that children may become notably irritable. In babies, additional signs include:

• Skin that is pale and clammy
• Crying more than usual or being difficult to settle
• A fontanelle (soft spot on the head) that appears sunken
• Fewer wet nappies than normal, indicating dehydration
• Rapid breathing

Signs of heatstroke in children

In addition to the adult signs, children with heatstroke may become sleepy or floppy. In babies and toddlers, signs include a dry mouth and tongue, difficulty settling, and muscle cramps that may cause them to lie very still or cry continuously.

Cooling a Child or Baby

• Move immediately to a cool, shaded, or air-conditioned space.
• Remove outer clothing and nappy for babies.
• Use a cool, damp flannel or sponge, focusing on armpits, neck, and groin.
• A cool (not cold) water spray is effective and less distressing than immersion.
• Do not apply ice packs or ice-cold water directly to a child’s skin.
• Breastfed babies should be offered feeds more frequently. Bottle-fed babies can be offered small amounts of cooled, boiled water in addition to feeds.
• Babies under six months do not need water offered separately but may be fed more often.
• Children over six months can be offered cool water regularly throughout the day.
• If using a cool bath, do not leave the child unattended, and ensure the water is not so cold it causes shivering. Shivering generates heat.

Breastfeeding mothers need particular attention during hot weather. Milk production depends on adequate maternal hydration, and the additional fluid demands of lactation mean that a mother who is not drinking enough will notice a reduction in supply before she feels significantly thirsty. In hot conditions, breastfeeding mothers should increase fluid intake beyond their usual daily amount, aim for pale straw-coloured urine as the simple indicator of adequate hydration, and not wait for thirst to prompt drinking. A dehydrated mother is also at elevated risk of heat exhaustion herself, which creates a secondary risk to the baby if she becomes too unwell to feed or care for them.

UV Protection for Children

The UV guidance for children follows the same principles as adults, with some age-specific adjustments: 

• Keep babies and young children out of direct sun, particularly between 11am and 3pm when UV is at its peak.
• Shade, a wide-brimmed sun hat, and UV-protective clothing are the primary defences.
• Sunscreen is not recommended as the first line of protection for babies under six months. Shade and clothing are preferred.
• For children over six months, use SPF 30 or higher on all exposed areas and reapply regularly.
• Dress children in loose, light-coloured, breathable fabrics.

Who Is at Greatest Risk

UKHSA data from 2024 showed that the highest heat-associated mortality rate was among people aged 85 and over, at 521 deaths per million people. But age is not the only factor.

Managing the Indoor Environment

It is a common misconception that staying indoors automatically means staying safe in a heatwave. Indoor temperatures in UK homes, particularly those with poor ventilation or south-facing windows, can remain dangerously high long after outdoor temperatures have peaked.

The NHS advises keeping curtains and blinds closed on south and west-facing windows during the hottest part of the day, and opening windows in the early morning and evening when outdoor air is cooler. Electrical equipment and lighting generate heat; turning them off when not in use makes a meaningful difference.

• Closing curtains and blinds during peak sun hours significantly reduces indoor temperature.
• Ground-floor rooms and north-facing rooms are generally cooler.
• Electric fans are effective at lower ambient temperatures but have limited benefit once air temperature exceeds skin temperature.
• A cool shower or bath lowers core body temperature directly.

Common Mistakes That Make Things Worse

The Broader Picture: This Is Not a One-Off

Dr Garyfallos Konstantinoudis of Imperial College London has described heatwaves as ‘silent killers’: the people who die in them typically have pre-existing conditions, and heat is rarely listed as a contributing cause of death on death certificates. The deaths are real, but they are systematically undercounted.

Since 2020, more than 10,000 people have died during UK heatwaves. Despite this, the 2025 data provides genuine grounds for optimism. The UKHSA estimated that proactive heat health alerts and NHS preparedness prevented roughly 1,500 deaths in summer 2025 compared to the modelled baseline. Awareness, preparation, and timely action work. That is the case this guide is making.

If Someone Is in Trouble

To summarise the action thresholds:

Knowing what to look for is the first step. The second is acting on it without hesitation. Every minute matters once heatstroke takes hold.

Constellation Training provides first aid training throughout the UK. Our courses are FAIB-accredited and cover emergency response to heat-related illness as part of wider first aid training. If you or your organisation want to be better prepared, you can find out more about our programmes on our website.

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References

^[1]  Met Office. ‘UK May and spring temperature record provisionally broken for second day in a row’. May 2026. Available at: www.metoffice.gov.uk

^[2]  Office for National Statistics. ‘Excess mortality: heat and cold deaths in England and Wales’. 2023. Available at: www.ons.gov.uk

^[3]  UK Health Security Agency. ‘New UKHSA data shows 1,504 heat-related deaths during summer of 2025’. 2026. Available at: www.gov.uk/government/news

^[4]  NHS. ‘Heat exhaustion and heatstroke’. Available at: www.nhs.uk/conditions/heat-exhaustion-heatstroke

^[5]  KidsHealth. ‘Heat cramps, heat exhaustion, and heatstroke’. Available at: www.kidshealth.org

^[6]  Sheffield Children’s NHS Foundation Trust, cited in British Red Cross. ‘Heatstroke first aid’. Available at: www.redcross.org.uk/first-aid/learn-first-aid/heatstroke

^[7]  NHS / Black Country ICB. ‘Local GP shares signs of heat exhaustion and heatstroke’. Available at: www.blackcountry.icb.nhs.uk

^[8]  Mother & Baby / Dr Chodhari. ‘Heatstroke in babies and toddlers’. 2026. Available at: www.motherandbaby.com

^[9]  UK Health Security Agency. Heat mortality data 2024. Available at: www.gov.uk

^[10] London School of Hygiene & Tropical Medicine / Imperial College London. ‘Nearly 600 heat-related deaths expected in UK heatwave’. June 2025. Available at: www.lshtm.ac.uk