Warm, sunny weather can make walking, running and cycling particularly enjoyable. However, exercising in high temperatures places extra strain on the body and can lead to dehydration, heat exhaustion or life-threatening heatstroke.
This does not necessarily mean abandoning exercise throughout the summer. It means checking the conditions, adjusting your plans and recognising when it is safer to stop.
How heat affects the body during exercise
Your muscles produce heat whenever you exercise. In cooler conditions, the body releases much of this heat into the surrounding air. It also cools itself by producing sweat, which removes heat as it evaporates from the skin.
In hot weather, especially when it is humid, this cooling system becomes less effective. Your body sends more blood towards the skin to release heat while still supplying blood and oxygen to your working muscles.
As a result:
- your heart has to work harder;
- your heart rate may be higher than usual at the same pace;
- you sweat more and lose water and salts;
- familiar exercise can feel considerably harder;
- your pace, strength, coordination and concentration may decline;
- you may tire more quickly;
- your body temperature can continue to rise.
High humidity is particularly difficult because sweat does not evaporate as readily. You may be dripping with sweat without receiving its full cooling benefit.
Do not judge the risk from air temperature alone. Direct sun, humidity, a lack of wind, air pollution, heavy clothing and the intensity of your exercise all affect how much heat your body must handle.
Choose a cooler time
During hot weather, exercise early in the morning or later in the evening, when conditions are usually cooler.
Try to avoid strenuous outdoor activity during the hottest part of the day. The UK Health Security Agency also advises avoiding direct sunlight where possible between 11am and 3pm, when ultraviolet levels are generally highest.
Remember that the hottest air temperature may occur later in the afternoon. Check the hourly forecast rather than relying on a fixed time.
On very hot or humid days, consider:
- shortening your session;
- reducing your speed or intensity;
- choosing a shaded route;
- walking instead of running;
- moving your workout to a cooler indoor location;
- taking a rest day.
Changing a session because of the heat is sensible risk management, not a failure of commitment.
Check the conditions before leaving
Check:
- the hourly temperature and humidity;
- the Met Office forecast and weather warnings;
- any UKHSA Heat-Health Alerts;
- the UV forecast;
- local air-quality information;
- whether your intended route has enough shade and places to obtain water.
Air pollution can become worse during hot weather and may cause particular problems for people with asthma, lung disease or heart conditions.
Avoid long, exposed routes from which you cannot easily return. Tell someone where you are going and when you expect to be back, particularly if exercising alone.
Allow your body time to adjust
Someone who is fit is not automatically protected against heat illness. A fit person may actually generate more heat by exercising harder.
The body can gradually become better at coping with heat, but this does not happen immediately. During the first hot spell of the year—or after returning from a cooler climate—begin with shorter, easier sessions. Increase the duration and intensity gradually over several days.
Do not attempt your usual pace simply because it normally feels comfortable. Exercise according to effort rather than speed, and accept that your performance may be lower in the heat.
Begin properly hydrated
Start drinking regularly before you exercise rather than trying to compensate after you already feel unwell.
There is no single amount of water that is correct for everyone. Your needs depend on your size, exercise intensity, duration, temperature, humidity and how heavily you sweat.
Useful habits include:
- drinking regularly throughout the day;
- carrying enough water for the session, plus a reserve;
- taking regular drinks rather than waiting until the end;
- using water-refill points where available;
- checking that your urine is generally a pale straw colour.
Dark yellow, strong-smelling urine or urinating less often than usual can indicate dehydration. Completely clear urine throughout the day can sometimes mean that you are drinking more than necessary.
For most shorter or moderate sessions, water is normally an appropriate choice. If you are exercising hard, sweating heavily or continuing for several hours, an isotonic sports drink or an appropriate electrolyte drink may help replace some of the salts lost in sweat.
Sports drinks are not essential for every workout and can contain considerable sugar. Do not take salt tablets unless advised by a qualified healthcare professional.
It is also possible to drink too much water. Do not force down very large quantities in a short period, as this can dangerously dilute the sodium in your blood. Drink regularly and sensibly, taking account of your thirst, sweat loss and conditions.
If a doctor has told you to restrict your fluids, or you have a heart or kidney condition, ask a healthcare professional for individual advice.
Avoid alcohol before exercising
Alcohol can contribute to dehydration, interfere with judgement and make it more difficult to recognise the early signs of heat illness.
Be cautious with large amounts of caffeine too, particularly if you are not accustomed to it or it causes stomach upset, palpitations or more frequent urination.
Dress for the conditions
Wear lightweight, loose-fitting and breathable clothing. Light colours absorb less solar heat than dark colours.
Also consider:
- a lightweight, breathable hat;
- sunglasses offering suitable UV protection;
- sweat-resistant sunscreen with at least SPF 30 and four- or five-star UVA protection;
- anti-chafing protection where clothing or skin may rub;
- suitable shoes and socks, as heat and swelling can increase the risk of blisters.
Apply sunscreen generously before going outside and reapply it frequently, especially after sweating heavily or wiping yourself with a towel. Sunscreen reduces damage from ultraviolet radiation, but it does not prevent overheating.
Reduce your effort
Your normal pace is not a safe target in every condition.
Start slowly and include more frequent breaks in the shade. Use how you feel, your breathing and your ability to speak as guides. If you monitor your heart rate, expect it to be higher than usual and reduce your effort accordingly.
Do not use competition, a training plan or the desire to complete a particular distance as a reason to ignore warning signs.
Exercising with somebody else provides an additional safeguard. People developing heat illness may become confused and fail to recognise how unwell they are. Watch each other for unusual behaviour, poor coordination or a sudden decline in performance.
Who needs to take extra care?
Anyone can develop heat-related illness, but the risk may be greater for:
- babies, children and older people;
- pregnant women;
- people who are not accustomed to the heat;
- people with heart, lung or kidney disease;
- people with diabetes;
- people who are overweight;
- anyone who has recently been ill with a fever, vomiting or diarrhoea;
- people with a previous history of heat illness;
- anyone taking medicines that affect sweating, fluid balance, blood pressure or temperature regulation.
Some medicines can increase vulnerability to heat, but do not stop prescribed medication without medical advice. Ask a pharmacist, GP or NHS 111 if you are uncertain.
Children should not undertake strenuous physical activity on very hot days.
Recognising dehydration
Early signs can include:
- thirst;
- a dry mouth, lips or tongue;
- headache;
- tiredness;
- dizziness or light-headedness;
- dark yellow, strong-smelling urine;
- urinating less frequently.
Stop or reduce your activity, move somewhere cooler and drink fluids. Continuing to push yourself can allow dehydration and overheating to progress.
Recognising heat exhaustion
Heat exhaustion happens when the body is struggling with heat and the loss of water and salt. Symptoms may include:
- unusual tiredness or weakness;
- dizziness or feeling faint;
- headache;
- nausea or vomiting;
- heavy sweating;
- pale or clammy skin;
- intense thirst;
- cramps in the arms, legs or stomach;
- a high temperature;
- irritability;
- a fast pulse or breathing rate.
Skin-colour changes can be more difficult to see on brown or black skin, so do not rely on pallor or redness alone.
Heat exhaustion needs prompt action because it can progress to heatstroke.
What to do for heat exhaustion
Stop the exercise immediately.
- Move the person into shade or a cool indoor place.
- Remove unnecessary clothing, including a jacket, hat, socks or excess equipment.
- Give cool water, an isotonic sports drink or an oral rehydration drink if the person is fully awake and able to swallow.
- Spray or sponge the skin with cool water and fan it.
- Place cold packs wrapped in cloth around the neck or under the armpits.
- Stay with the person and monitor them.
They should begin to cool down and feel better within 30 minutes.
Contact NHS 111 if you are struggling to manage the symptoms, if you are concerned or if the person is getting worse.
Do not resume the exercise that day simply because the person begins to feel better.
Heatstroke is a medical emergency
Heatstroke occurs when the body can no longer control its temperature. It can cause damage to the brain and other organs and can be fatal without rapid treatment.
Warning signs include:
- confusion, disorientation or unusual behaviour;
- poor coordination or slurred speech;
- a very high temperature;
- hot skin, which may be dry or may still be sweating heavily;
- rapid breathing or shortness of breath;
- a fast heartbeat;
- a seizure;
- collapse or loss of consciousness;
- still being unwell after 30 minutes of cooling and drinking fluids.
Do not wait for every symptom to appear. In someone exercising, confusion, collapse or altered behaviour in hot conditions must be treated extremely seriously.
Call 999 immediately if heatstroke is suspected.
While waiting for the ambulance:
- move the person to a cooler or shaded place;
- remove unnecessary outer clothing;
- cool them continuously with cool water, wet sheets, sponging, fanning and wrapped cold packs;
- stay with them;
- follow the call handler’s instructions;
- place them in the recovery position if they become unconscious but are breathing;
- begin CPR if they are not breathing normally and follow the 999 operator’s guidance.
Do not give fluids to anyone who is confused, having a seizure, unconscious or unable to swallow safely. Do not give aspirin or paracetamol for suspected heatstroke.
A simple hot-weather exercise checklist
Before you leave, ask yourself:
- Have I checked the hourly weather, humidity, UV and air quality?
- Could I exercise earlier, later or indoors?
- Am I properly hydrated?
- Do I have enough water?
- Can I reduce the distance or intensity?
- Is there shade along the route?
- Can I return quickly if I feel unwell?
- Does someone know where I am?
- Am I carrying a charged phone?
- Do I know the signs of heat exhaustion and heatstroke?
If the conditions do not feel safe, change the plan. Missing or modifying one workout will not harm your fitness. Trying to complete it in dangerous heat might harm far more.
The most important message
In hot weather, slow down, shorten the session, drink regularly and exercise during the cooler parts of the day.
If you feel weak, dizzy, faint, sick, confused or unusually exhausted, stop immediately and move somewhere cool.
Heat exhaustion requires cooling and fluids. Suspected heatstroke requires 999 and immediate cooling.
This article provides general information for exercising in hot weather in the UK. People with medical conditions, those taking medication and anyone advised to restrict fluids should seek individual advice from a healthcare professional.
This is based on the current UKHSA hot-weather guidance, NHS guidance on heat exhaustion and heatstroke and NHS dehydration advice. One important detail for exercise is that a person with exertional heatstroke may still be sweating—so confusion, loss of coordination or collapse should never be dismissed merely because their skin is not dry.
