Exercising safely in heat, cold and outdoors
On hot days, stay out of the sun between 11am and 3pm, wear light, loose clothing and ease in over 10 to 14 days. Drink enough to replace what you lose, not as much as you can. If you feel unwell in the heat, stop, cool down and drink fluids.
At a glance
- Plan around the middle of the day: on hot days, avoid being outdoors between 11am and 3pm and wear light-coloured, loose clothing1.
- Ease in: the first hot days of the season catch people out, and it can take 10–14 days of repeated activity to adapt2,3.
- Drink to replace what you lose: good hydration helps prevent heat illness, but drinking too much can also cause harm4.
- Stop if you feel unwell: move somewhere cool or shaded and drink fluids5.
How do you stay safe exercising outdoors?
On hot days, the main steps are to avoid the hottest part of the day, dress for the heat, ease in gradually and stop if you feel unwell. Heat-related illness is largely preventable by getting used to the heat, drinking enough and avoiding physical activity in extreme temperatures5.
- Pick your time. Plan outdoor activity for before 11am or after 3pm, or choose a shadier, cooler spot. On very hot days, cut back or skip hard activity6,5.
- Dress for the heat. Wear light-coloured, loose clothing, and a hat and sunscreen if you're outside1,6.
- Ease in. People who aren't used to the heat, such as on the first hot days of early summer or in a sudden hot spell, are at higher risk of heat illness2. If the weather turns hot suddenly, shorten your sessions and go easier, then build up over 10–14 days of repeated activity3.
- Stop if you feel unwell. Stop, move somewhere cool or shaded and drink fluids5. If it turns into heat exhaustion, the first steps are to stop exercising, move away from heat sources, drink cool fluids and loosen or remove clothing7.
Take extra care if you are sick now, recovering from a recent infection, or have had diarrhoea, as these may raise the risk of heat injury2.
Does hydration help prevent injury?
Good hydration helps prevent heat illness. Whether it protects against sprains, strains or falls isn't clear yet.
In people who are active, losing more than 1% of body mass in fluid affects how the body controls its temperature, and the risk of heat illness rises further once the loss passes 3%4. Staying well hydrated is something you can control. Both too little fluid and too much fluid can harm health and safety during activity4.
What to do:
- Drink through the day. Start activity having drunk regularly, and have extra fluids after you've been sweating. Water is the best way to replace what you lose8.
- Check your urine colour. Most people should aim to drink enough for their pee to be a clear pale yellow8.
- Sip steadily. Replace the fluid you lose in sweat rather than drinking as much as you can4.
- Skip the big drink for cramps. If you get muscle cramps, don't try to fix them by drinking large amounts of water; replacement should be limited to what you've actually lost4.
When to check with a health professional
If you're starting exercise after a long break, it's always a good idea to see your doctor first.
If you live with a long-term condition or take regular medicines, ask your doctor how to manage exercise in hot or cold weather.
If dizziness, nausea or feeling unwell doesn't settle once you've stopped, cooled down and had a drink, or you feel worse, call your local emergency number. For a sore muscle or joint, ease off, and get advice if it persists.
For an injury or a health condition, see a health professional, such as your doctor or a physio.
Call your local emergency number for chest pain or pressure (or pain spreading to the arm, neck, jaw or back, especially with sweating or feeling sick), severe breathlessness, fainting or collapse, signs of a stroke, a serious injury, or someone who isn't safe.
Quick answers
How do I know if I'm dehydrated?
Early signs of not having enough fluid include thirst, dry lips and mouth, tiredness, flushed skin, weariness and cramps. With a bigger shortfall you may also get dizziness, headache, nausea, vomiting or chills. These signs tend to build as the shortfall grows, so noticing thirst early is useful.
How much should I drink when I'm exercising?
Start well hydrated and replace what you lose in sweat, rather than drinking as much as you can. As a daily guide, the NHS gives 6 to 8 cups or glasses of fluid a day, and you may need more if it's hot, or if you're active for long periods or recovering from illness.
Can drinking too much water be dangerous?
Yes. Drinking too much can cause hyponatraemia, which means dangerously low blood sodium, and it can be life-threatening. It matters most in prolonged exercise. To lower the risk, replace only the fluid you lose in sweat and don't gain fluid weight during activity.
Related guides
- GuideGood form and common mistakesGood form mainly lowers your chance of injury. The common mistakes are doing too much too soon, skipping the warm-up, holding your breath, swinging weights and not resting. Get the movement right with light weights first, and ask a qualified professional to watch you if you're new to strength training or unsure.
- GuideOvertraining and overuseSigns you may be overdoing it include workouts feeling harder than usual, soreness or tiredness that won't clear, poor sleep, low mood and losing the drive to exercise. No single test confirms it, so treat these as clues and take a break if several appear together.
- GuideAdapting exercise to your bodyYou can adapt exercise by doing strength work seated, choosing a different exercise for the same muscles, lowering the effort or using a smaller movement. Start at a comfortable level, build up gradually, and avoid movements that put pressure on a painful joint.
- GuideFirst aid for minor exercise injuriesFor a minor sprain or strain, protect and rest the area for the first 2 to 3 days. Use ice and a snug bandage, and raise it. Don't rest for long: a few days later, start moving again gently, and ease off if pain increases.
- GuideStretchingFor more flexibility, slow, gentle static stretches (holding a stretch) are the best choice. Before a walk or workout, keep holds short or use slow, controlled movements. Move smoothly, don't bounce, and stop at mild discomfort, never pain.
- GuideWarm-ups and cool-downsStart slowly for at least 6 minutes so your heart rate rises gradually, and warm up every time you exercise. A cool-down is a comfortable way to finish rather than a must-do for recovery; about 5 minutes of gentle stretching or slow walking is enough.
Sources
- NHS. Heat exhaustion and heatstroke.
- Racinais S et al. (2015). Consensus recommendations on training and competing in the heat. British Journal of Sports Medicine.
- American College of Sports Medicine. Exercising in hot and cold environments.
- McDermott BP et al. (2017). National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active. Journal of Athletic Training.
- Gauer RL et al. (2026). Heat-Related Illnesses. American Family Physician.
- Healthdirect Australia (2016). Hot weather risks and staying cool.
- Kenny GP et al. (2018). Heat exhaustion. Handbook of Clinical Neurology.
- NHS. Water, drinks and hydration.
General information to help you get active, not medical advice. If you have a health condition or symptoms, check with a health professional.
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