Work in progress
General information based on current NHS and UKHSA guidance. Not individualised medical advice. If you are worried about someone, call 111, or 999 if there are signs of heatstroke.
A side project, put together from published guidance and still being worked on. No clinician has checked it. Every card links its sources, so read those and talk to your GP or pharmacist. Never change a prescribed medicine because of anything here.
Drink fluids regularly through the day
Good evidenceSip water, diluted squash or lower-fat milk through the day rather than waiting until you feel thirsty.
Thirst lags behind fluid loss, further in older people, and losing 2 per cent of body weight as fluid already raises heart rate and core temperature.
Do not force down large volumes of plain water
Good evidenceDrink to thirst plus a bit more in heat, but not litre after litre of plain water in a short time.
Drinking more than you lose dilutes blood sodium, and the early signs look like heat exhaustion, so it is easy to make worse by drinking more.
Add salt or electrolytes only when losses are heavy
Some evidenceNormal meals replace everyday salt losses; after two hours of heavy sweating, or with cramps, use an oral rehydration or isotonic drink.
Sweat contains sodium as well as water, so replacing only water after heavy sweating dilutes what sodium is left.
Eat your fluids as well as drinking them
Some evidenceHave cold, water-rich food through the day: salad, cucumber, melon, berries, yoghurt, ice lollies or frozen fruit. On a fluid limit, count the water in food towards it.
Cold food tops up fluids and takes heat out of you as it warms, and it still works when appetite is poor or more water is off-putting.
Use a fan when the air is below about 35C
Good evidenceRun a fan on yourself when indoor air is below roughly 35C, aimed at your skin. Drink more, since the airflow speeds up fluid loss.
Moving air speeds sweat evaporation and, while air is cooler than skin, carries heat away too, reliably lowering heat strain and heart rate.
Above about 35C, wet your skin or turn the fan off
Some evidenceAbove about 35C a fan alone can add heat, so either wet your skin and clothes as well or switch it off and cool another way.
Once air is hotter than skin the airflow adds heat, and only wetting the skin restores enough evaporation for it to help again.
Wet or spray your skin
Good evidenceSpray or sponge your face, arms and legs with cool water, or wear a damp t-shirt, and repeat as it dries.
Water evaporating off skin removes a lot of heat without you needing to sweat, which makes it the cheapest cooling you have.
Take a cool shower rather than a cold one
Some evidenceFor everyday cooling at home, shower or bathe in cool water rather than the coldest setting.
Very cold water clamps down skin blood vessels and can trigger shivering, which slows heat loss from the core.
Cold water immersion for suspected heatstroke
Good evidenceFor someone collapsing or confused in the heat, call 999, then cool them in cold water while you wait.
Whole-body immersion cools several times faster than ice packs or fanning, and in heatstroke the damage tracks time spent too hot.
Hand and forearm immersion
Some evidenceDuring a break, put both hands and forearms into a bowl of cool water, around 10 to 20C, for 10 to 20 minutes.
Hands and forearms carry dense surface blood vessels, so immersing both in cold water lowers core temperature measurably.
Ice slushies and cold drinks
Some evidenceHave a cold drink or an ice slushy before and during heat exposure or exercise.
Melting ice inside you absorbs more heat per mouthful than a cold drink, lowering core temperature modestly.
Neck cooling: feels good, does little
Limited evidenceA cold cloth or collar on the neck is fine for comfort, but do not treat it as cooling someone down.
The neck is full of temperature receptors, so it feels much cooler, but trials show no change in core temperature.
Cooling towels and vests
Limited evidenceDamp towels and evaporative vests are for comfort. For real cooling use an ice or phase-change vest, and expect a limited window.
Evaporative vests only work while damp and fail in humid air, and a small ice mass runs out well before immersion would.
Close curtains and blinds on sunlit windows
Good evidenceDuring the day, shut curtains, blinds or shutters on any window the sun is hitting. External shading works better than internal.
Sunlight through glass is the largest single source of heat gain in most homes, and blocking it early keeps the whole day cooler.
Open up when outside is cooler than inside
Good evidenceKeep windows shut while it is hotter outside than in, then open them safely once outdoor air is cooler.
Overnight ventilation flushes stored heat out of the building fabric, so the next day starts cooler.
Go somewhere cooler than your home
Some evidenceIf your home is hot and you cannot cool it, spend part of the day somewhere cool: a library, community centre, place of worship, shopping centre or a shaded park with a breeze.
A flat that has soaked up heat over several days can be hotter than the shade outdoors, and a few hours somewhere cooler gives your body a break.
Turn off what is heating your home from inside
Some evidenceSwitch off lights and equipment you are not using, check the heating is off, and cook early or late rather than in the afternoon.
Everything electrical in a room ends up as heat in that room, and an hour of oven use can undo a morning of closed curtains.
Sleep in the coolest room, even if it is not your bedroom
Some evidenceIf your bedroom stays hot, sleep in a cooler room for a few nights and swap the duvet for a single sheet.
Upstairs rooms hold heat longest, and broken sleep in the heat removes the overnight recovery your body needs for the next hot day.
If you have air conditioning, set it to 27C and add a fan
Good evidenceSet the thermostat to about 27C rather than the lowest setting, run a fan in the same room, and keep the door and curtains closed.
27C with moving air feels about as cool as 23C with still air and costs far less to run, so you are more likely to keep it on.
Stay out of the sun between 11am and 3pm
Good evidenceKeep out of direct sun in the middle of the day, and move errands, exercise and outdoor work to early morning or evening.
UV and radiant heat both peak at midday, adding a large load on top of air temperature, and sunburn itself impairs sweating.
Wear loose, light, breathable clothing
Some evidenceChoose lightweight, loose, light-coloured clothes that breathe, and cover up with long sleeves or a UPF layer for long spells in the sun.
Loose clothing lets sweat evaporate and light colours reflect radiant heat, while covering up beats bare skin in strong sun.
Let your body adapt over days
Good evidenceBuild up heat exposure and outdoor effort gradually over one to two weeks rather than going hard on the first hot day.
Repeated exposure increases blood volume and starts sweating earlier, with most of the benefit within a week.
Move exercise, shorten it, or drop the intensity
Good evidenceTrain early or late, cut the duration, take more breaks, and cool actively in them rather than chasing your usual numbers.
Working muscle generates most of the heat load, so intensity is the single biggest thing you control.
Go easy on alcohol
Good evidenceCut back on alcohol in hot weather, and do not use it to rehydrate.
Alcohol suppresses the hormone that conserves water and blunts judgement about how hot you are, so early warning signs get missed.
Put a thermometer in the rooms you use
Some evidenceKeep a cheap room thermometer where you sit and where you sleep, aiming to keep the bedroom below about 26C.
People judge indoor temperature poorly, and worse when illness or medication blunts the signal, so a number tells you when to act.
Keep medicines below 25C
Good evidenceCheck your packaging, and move anything that says below 25C out of sunlit rooms, car glove boxes and window sills.
Heat degrades many medicines, so they may not work as expected. The label states the limit for that product.
Use sunscreen generously and reapply
Good evidenceApply plenty of at least SPF 30 with 4 or 5 star UVA protection, and reapply often, especially after sweating or swimming.
Sunburn damages skin and reduces its ability to sweat, leaving you less able to cope with the next hot day.
Wide-brimmed hat and wraparound sunglasses
Some evidenceWear a wide-brimmed hat that shades the face, ears and neck, and wraparound sunglasses.
A brim cuts radiant heat on the head and covers the spots people miss with sunscreen, and wraparound lenses block side-on UV.
Check on neighbours, relatives and friends at risk
Good evidenceDuring hot spells, look in on or ring people who live alone, are over 65, or would struggle to cool themselves.
Most heat deaths happen at home to people who did not call for help, and early confusion removes the ability to ask.
Plan journeys around transport heat
Some evidenceCarry water on public transport, travel outside the rush hour, avoid deep underground lines on very hot days, and have a plan if stuck.
Carriages and stations get far hotter than the street with no airflow and no way to leave, and crowding adds more heat.
Check the colour of your pee
Some evidenceAim for pale straw colour. Dark yellow means drink more, and not needing to go for many hours is also a sign.
It is the simplest signal of fluid status and needs no equipment, though some vitamins and foods change the colour.
Weigh yourself to track fluid loss
Some evidenceWeigh yourself at the same time each morning, or before and after long exercise. A drop over about 2 per cent in a day is fluid.
Short-term weight change in heat is almost entirely water, and gaining weight during long exercise warns of hyponatraemia.
Know when it is 999 and when it is 111
Good evidenceCall 999 for any sign of heatstroke, or if someone is no better after 30 minutes of cooling. Call 111 if you are struggling or unsure.
Heat exhaustion usually settles with cooling and fluids, while heatstroke is an emergency where outcomes depend on cooling fast.
Caffeine is not the problem people think
Common mythTea and coffee count towards your fluids. Keep to about 3 to 5 cups a day, or switch to decaf.
At everyday doses caffeine has only a small diuretic effect and does not dehydrate you on balance.
What to do
What to do
hyponatraemia (low blood sodium from drinking too much)
acute kidney injury (AKI)
diabetic ketoacidosis (DKA)
lithium toxicity
lactic acidosis (metformin)
low blood glucose (hypoglycaemia) in the heat