Heat Illness: Spot It, Stop It
A practical guide to recognising dehydration, heat exhaustion, and heatstroke in adults and children, including warning signs, first aid, and when to call 999.
A quick guide for parents and anyone spending time outdoors in hot weather
On 25 and 26 May 2026, the UK broke its all-time May temperature record twice in two days. Kew Gardens reached a provisional 35.1°C, a figure that would have been considered implausible before summer had properly begun. It will not be the last time.
Heat illness does not require lying in direct sun. It can develop in a poorly ventilated room, during a school sports day, or while gardening on a warm afternoon. Most people who develop heat exhaustion or heatstroke do not realise how unwell they are becoming until symptoms are already significant.
This guide gives you what you need to recognise the signs early and act quickly.
Three Conditions, One Spectrum
Heat illness runs along a single spectrum from manageable to life-threatening. Understanding where someone is on that spectrum determines what you do next.
Dehydration is the earliest stage and the most common. Left unaddressed, it can lead to the others.
Heat exhaustion is the body struggling to maintain its core temperature. It is serious, but it responds well to prompt first aid.
Heatstroke is a medical emergency. The body’s thermostat has failed. Core temperature is dangerously high, typically above 40°C. Without immediate cooling and emergency medical care, it causes brain damage and death.
Spot It Fast: Recognising the Signs
| DehydrationTreat promptly | Heat ExhaustionIf not improving within 30 minutes: call 999 | HeatstrokeCall 999 immediately |
|---|---|---|
| Thirst | Heavy sweating | Skin hot and dry, with no sweating |
| Dark or infrequent urine | Pale, clammy skin | Flushed, hot skin |
| Headache or dizziness | Headache and dizziness | Confusion or agitation |
| Fatigue | Rapid, weak pulse | Rapid, strong pulse |
| Dry mouth | Muscle cramps | Seizure or loss of consciousness |
| Reduced concentration | Nausea or vomiting | Temperature 40°C or above |
Hot, dry skin after heavy sweating = heatstroke. Call 999
This is the most important warning sign to recognise. If sweating stops and skin becomes hot and dry, the situation has become a medical emergency.
What to Do
Speed matters at every stage. The sooner you act, the better the outcome. Use the table below as your action guide.
Act Now: What to Do and When to Call 999
| Condition | What to do |
|---|---|
| DEHYDRATION |
|
| HEAT EXHAUSTION |
|
| HEATSTROKE |
|
Children and Babies: What Is Different
Children are at greater risk than adults in hot weather. Their thermoregulatory systems are less mature, they lose fluid more quickly relative to their body size, and they rely entirely on the adults around them to notice that something is wrong.
Early warning signs in children
- Unusual irritability or grumpiness
- Complaint of headache
- Flushed face
- Reluctance to play or unusual tiredness
- Nausea
Act on these immediately. Do not wait for more obvious signs. Move the child to a cool space, remove clothing, and give fluids.
Additional signs in babies
- Crying more than usual or difficult to settle
- Sunken fontanelle (the soft spot on the head)
- Fewer wet nappies than normal
- Rapid breathing
- In heatstroke: floppy, unresponsive, or having a seizure
Call 999 immediately if a child or baby:
Becomes confused, floppy, or unresponsive
Has hot, dry skin and has stopped sweating
Has a seizure
Cannot be roused
Cooling a child
- Cool, damp flannel or spray to armpits, neck, and groin.
- Do not use ice or ice-cold water directly on skin.
- Do not allow shivering to develop: it generates heat.
- For babies under six months, milk feeds should remain the main source of hydration. Feed more frequently during hot weather.
Breastfeeding mothers should also increase fluid intake in hot weather. Milk production depends on maternal hydration, and supply can be affected before thirst develops. Pale, straw-coloured urine is the simplest indicator that fluid intake is adequate. A dehydrated mother is also at greater risk of heat exhaustion herself.
Prevention
- Drink water consistently throughout the day, before you feel thirsty.
- Pale, straw-coloured urine means you are adequately hydrated. Dark urine means you are not.
- Apply SPF 30 or higher thirty minutes before going outside and reapply every two hours.
- Keep children out of direct sun between 11am and 3pm.
- People develop heat illness indoors as well as outside. Close curtains on south-facing windows during the hottest part of the day.
- Avoid alcohol and excessive caffeine during a heatwave.
This guide covers the essentials. If you want the full clinical picture, including a detailed section on UV risk, vulnerable groups, the indoor environment, and the broader context of heat mortality in the UK, the complete guide is available here: https://blog.constellationtraining.co.uk/heat-related-illness-guide/
Knowing what to look for and acting quickly are the two things that make the most difference. Heat exhaustion is manageable. Heatstroke does not have to happen.
If you want to be more confident in a genuine emergency, including heat illness, Constellation Training runs open first aid courses. Our FAIB-accredited courses teach the practical skills needed to recognise emergencies early and respond confidently. Details are on our website.
References
[1] Met Office. ‘UK May and spring temperature record provisionally broken for second day in a row’. May 2026.
[2] NHS. ‘Heat exhaustion and heatstroke’. Available at: www.nhs.uk/conditions/heat-exhaustion-heatstroke
[3] British Red Cross. ‘Heatstroke and heat exhaustion first aid for children and babies’. Available at: www.redcross.org.uk
[4] Sheffield Children’s NHS Foundation Trust, cited in British Red Cross. ‘Heatstroke first aid’.
[5] UK Health Security Agency. Heat mortality monitoring data. Available at: www.gov.uk
