A heat wave does not care which side of the Atlantic you train on. The same physiology applies in Texas, Toronto and Toulouse: once hot air, sun and humidity overwhelm your ability to shed heat, a workout stops being training and starts being a medical risk. This guide sets out the limits in both Fahrenheit and Celsius, because a forecast in °F and a thermometer in °C should lead to the same decision.
What heat does to your body
Your core temperature normally sits near 37 °C (98.6 °F). Exercise generates heat fast, and sweating is your main cooling system — but sweat only cools you when it evaporates, which humid air prevents. As core temperature climbs toward 40 °C (104 °F), organs begin to struggle; above that, heat stroke becomes a genuine emergency. Air temperature alone understates the danger: a humid 32 °C (90 °F) afternoon can stress the body more than a dry 38 °C (100 °F) one.
Know the warning signs
Heat exhaustion usually comes first: heavy sweating, nausea, headache, dizziness, cramps, a rapid pulse and feeling faint. The response is to stop, move to shade or air conditioning, cool the skin with wet cloths and sip fluids. Heat stroke is the escalation — confusion, slurred speech, hot skin that may be dry rather than sweaty, seizures or collapse, with core temperature above 40 °C (104 °F). That is a call-the-emergency-number situation (911 in the US and Canada, 999 in the UK, 112 across Europe) and the person needs rapid cooling, ideally cold water immersion, while help comes. Never ‘push through’ confusion or faintness in the heat; those are brain symptoms, not weakness.
Humidity and heat stress, in plain language
Sports bodies often use a measure called WBGT (wet bulb globe temperature), which combines heat, humidity, sun and wind into one number. You do not need the instrument to use the idea. As a rough guide drawn from sports-medicine practice: a WBGT below about 25 °C (77 °F) is manageable for most people; 28 °C (82 °F) calls for caution, shorter sessions and more breaks; and around 31–32 °C (88–90 °F) most organisations postpone or cancel intense outdoor exercise for all but acclimatised athletes. Many weather apps now show WBGT or a ‘feels like’ index — when that index passes 39 °C (103 °F), treat outdoor training as off the table.
Hydration, in litres and ounces
Start hydrated: drink about 500–600 ml (17–20 fl oz, roughly 2 to 2½ US cups) two to four hours before training. During exercise, most people lose 0.5–2 litres of sweat per hour (17–68 fl oz), and a practical drinking target is 0.4–0.8 litres per hour (14–27 fl oz, about 2–3½ cups), adjusted to your own sweat rate. Weighing yourself before and after a session teaches you that rate: every 1 kg (2.2 lb) lost is about 1 litre (34 fl oz) of fluid to replace, and drinking 1.25–1.5 litres (42–51 fl oz) per kilo lost over the following hours covers ongoing losses. For sessions over an hour, or very salty sweaters — white marks on your kit are the clue — add electrolytes or salty food rather than water alone.
When to stop, and how to adapt
Move sessions to early morning or evening when the forecast peaks above 35 °C (95 °F). Cut intensity before you cut the session entirely: swap intervals for easy Zone 2 work, take shade breaks every 15–20 minutes, and wear light, loose clothing. Give yourself 7–14 days of gradually increasing exposure to acclimatise at the start of a hot spell — the first heat wave of the year catches more people than the third. And treat certain days as rest days by rule rather than by feel: if you are ill, sleep-deprived, hungover or already dehydrated, your margin is gone before you start.
