At 13:00h on a summer day a runner sets out for an outdoor activity they have been looking forward to. The forecast predicts high temperatures, but they feel prepared: they are fit, experienced, and have carried enough water. Yet extreme heat does not always distinguish between beginners and experienced athletes. As we face increasingly frequent episodes of extreme heat, with temperatures reaching critical levels and authorities warning about associated health and fire risks, a simple question becomes increasingly relevant: when is it no longer safe to exercise outdoors?
A recent study published in the Journal of Science and Medicine in Sport examined a modified extreme heat policy designed to protect people participating in community sports. The researchers explored how heat risk can be assessed and translated into practical guidance for athletes, coaches, and the general population.
The problem is that heat affects the human body in several ways at the same time. During exercise, muscles produce heat, while the body relies mainly on sweating and blood circulation to maintain a safe internal temperature. When environmental conditions become too demanding, these cooling mechanisms become less effective, increasing the risk of heat exhaustion and, in severe cases, heat stroke.
Importantly, being physically fit does not make someone immune. Experienced runners, hikers, and other athletes may tolerate heat better because they are often more adapted, but extreme conditions can challenge anyone. The risk is especially relevant for people who are not acclimatized to high temperatures, who suddenly increase their activity during holidays, or who underestimate the combination of physical effort and heat exposure.
The researchers highlight the importance of moving beyond individual judgement alone and using evidence-based systems that consider environmental conditions. The modified Sports Medicine Australia policy includes an online heat risk assessment tool that combines environmental factors, such as temperature and humidity, with the characteristics of the activity and the participants. Depending on the level of risk, the tool provides guidance on whether an activity can continue as planned, should be modified, or should be postponed or cancelled. This approach aims to turn complex scientific information into practical decisions for athletes, coaches, and communities.
So, what should people do during a heatwave?
First, avoid treating every summer day as a normal training day. Early morning or evening sessions may reduce exposure, but they do not eliminate risk during extreme heat. Second, increase exposure gradually: the body needs time to adapt to exercising in hot conditions. Third, pay attention to warning signs such as dizziness, unusual fatigue, confusion, headache, or chills, and stop before symptoms become severe.
The message is not that people should avoid exercising in summer. Physical activity remains one of the best ways to improve health. The lesson is simpler: in extreme heat, preparation matters. Sometimes the smartest training decision is not pushing harder, but knowing when conditions demand a different plan.
Assistant professor at the University of Valencia.