Over the past few years, summer temperatures around the world have reached levels that would have seemed extreme just a decade ago. Heat records are being broken across Europe, North America, and beyond, and for runners, that shift is impossible to ignore.
What used to be a few uncomfortable weeks in July has, for many, become months of navigating genuinely dangerous conditions.
So when does hot become too hot? And when you do decide to head out, how do you do it safely?
Those aren’t simple questions. The answer depends on more than just the thermometer — humidity, sun exposure, wind, your fitness level, and how heat-adapted your body is all play a role.
A 90-degree day in the American Southwest feels very different from 90 degrees in the humid Southeast, and both feel different again after two weeks of training in the heat versus stepping out after a cool spring.
This guide breaks it down: the temperature thresholds worth knowing, the real risks of running in the heat, and eight practical tips to help you train smarter when the conditions turn against you.
When Is It Too Hot To Run Outside?
There is no single temperature that is universally agreed upon as the cutoff for safe running, and anyone who tells you otherwise is oversimplifying.
The real answer depends on a combination of factors — air temperature, humidity, sun exposure, wind, your fitness level, and how well heat-adapted your body is.
That said, there are useful benchmarks. The Road Runners Club of America1Hot Weather Running Tips. (n.d.). Road Runners Club of America. https://www.rrca.org/education/for-runners/hot-weather-running/ advises avoiding outdoor running when temperatures exceed 98.6°F and humidity climbs above 70–80%. That’s a reasonable absolute ceiling, but it’s also quite a high bar — 98.6°F is body temperature, and most runners will be in serious trouble long before conditions reach that point.
A more practical and scientifically grounded measure is the Wet Bulb Globe Temperature, or WBGT. Unlike a standard thermometer, WBGT accounts for air temperature, humidity, solar radiation, and wind all at once — in other words, it measures how well your body can actually cool itself in a given environment, not just how hot the air is.
It’s the standard used by World Athletics for race cancellations, the U.S. military for outdoor exercise limits, and sports medicine organizations for mass participation events. Research widely cited in elite athletics suggests runners should not be training outdoors when WBGT exceeds 28°C (82°F).
Why does that matter? Because a moderately hot, humid day can push WBGT past that threshold without the thermometer reading anything alarming.
Humidity is the variable most runners underestimate. Sweating is your body’s primary cooling mechanism — but it only works if sweat can evaporate. In low humidity, sweating operates at roughly 50% efficiency. In very high humidity, that drops to around 16%. That’s why a 90°F day with high humidity can be more dangerous than 100°F in dry desert heat.
When in doubt, check both the temperature and the humidity before you head out — and if conditions feel oppressive even before you start running, trust that instinct.
The Risks of Running In the Heat
When you run, your core temperature rises. That’s normal — but only up to a point. Push it too high and your body’s cooling systems start to fail, opening the door to heat exhaustion and, in serious cases, heat stroke.
Your body has two primary ways of managing heat during exercise: sweating and redirecting blood flow toward the skin’s surface to release heat away from your core. Both are effective under the right conditions.
The problem is that heat and humidity together can overwhelm both mechanisms faster than most runners expect.
This is where the Heat Index becomes important. The Heat Index combines air temperature with relative humidity to give you the “real feel” — what conditions actually feel like to your body rather than what the thermometer reads.
Humidity has little effect below 40%, but above that threshold, its impact grows rapidly and non-linearly. The numbers can be startling: at 88°F with 40% humidity, the real feel stays at 88°F.
Push humidity to 70% at the same temperature and it feels like 100°F. At 85% humidity, that same 88°F day feels like 110°F.
As covered above, high humidity dramatically reduces sweat evaporation — and it’s that compounding effect that makes the Heat Index such a critical number for runners to check before heading out.
You can find the current reading for your area using the National Weather Service calculator.
8 Tips for Running In the Heat
#1: Give Your Body Time to Acclimatize
Heat acclimatization is real and it takes time. Most research2Tyler, C. J., Reeve, T., Hodges, G. J., & Cheung, S. S. (2016). The Effects of Heat Adaptation on Physiology, Perception and Exercise Performance in the Heat: A Meta-Analysis. Sports Medicine, 46(11), 1699–1724. puts the full adaptation period at around 10–14 days of consistent exposure, during which your body learns to sweat earlier, sweat more efficiently, and manage cardiovascular strain more effectively.
During that window, be patient — slow down, shorten your runs, and treat every hot session as a training stimulus in itself rather than a performance test.
#2: Time Your Run Around the Heat Index, Not Just the Temperature
Before heading out, check both the temperature and humidity for each hour of the day, not just the headline forecast.
Early morning often feels like the obvious choice — and the temperature is usually lower — but humidity tends to peak overnight and into the morning, which can make the Heat Index surprisingly high. Midday brings the highest air temperatures and most direct sun, but humidity is typically at its lowest.
Evening is often the sweet spot: temperatures drop as the sun sets, and humidity tends to follow. If you can run on a shaded trail, that changes the calculation further — the canopy can make a meaningful difference to real-feel conditions even on a humid morning.
#3: Hydrate Before, During, and After
Dehydration3McCubbin, A. J., Allanson, B. A., Caldwell Odgers, J. N., Cort, M. M., Costa, R. J. S., Cox, G. R., Crawshay, S. T., Desbrow, B., Freney, E. G., Gaskell, S. K., Hughes, D., Irwin, C., Jay, O., Lalor, B. J., Ross, M. L. R., Shaw, G., Périard, J. D., & Burke, L. M. (2020). Sports Dietitians Australia Position Statement: Nutrition for Exercise in Hot Environments. International Journal of Sport Nutrition and Exercise Metabolism, 30(1), 83–98. https://doi.org/10.1123/ijsnem.2019-0300 significantly raises the risk of heat illness and impairs performance long before you feel thirsty.
Aim for 4–8 fluid ounces of water or an electrolyte drink every 15–20 minutes during your run, adjusting for your sweat rate and the conditions.
In the heat, plain water isn’t always enough — sodium and electrolytes help your body retain fluid and replace what’s lost through sweat.
If you want to make your own, here’s a guide to homemade sports drinks.
#4: Run by Effort, Not Pace
Heat slows you down — that’s not a fitness problem, it’s physics.
Your cardiovascular system is working harder just to keep your core temperature in check, leaving less capacity for pace. Chasing your usual splits in 90-degree heat increases the risk of ignoring early warning signs from your body.
Instead, use perceived effort as your guide. A hard effort in the heat is still a hard effort, even if the pace looks nothing like it would in October.
#5: Seek Out Shade
Dark asphalt absorbs and radiates heat, turning exposed roads into a significant additional heat source beyond the air temperature.
Wherever possible, choose shaded trails or tree-lined paths. Natural surfaces like soil and wood chips also absorb less heat than tarmac, keeping the ground temperature lower underfoot. If your only option is an exposed road, try to run when the sun angle is lowest.
#6: Dress for the Conditions
Wear lightweight, light-colored, breathable fabrics and as little as you’re comfortable with. Dark colors absorb solar radiation; light colors reflect it.
A visor is a better choice than a full running cap in extreme heat — it shields your face from direct sun while allowing heat to escape from the top of your head. And don’t skip sunscreen. A broad-spectrum SPF 30 or higher protects your skin and reduces the additional heat load from sunburn.
#7: Use Water to Cool Down
Cooling your skin directly lowers your core temperature and reduces cardiovascular strain.
Soak a bandana in ice water and tie it around your neck before you head out. Carry an extra bottle to pour over your head mid-run, or plan your route around water fountains where you can douse yourself.
Even wetting your wrists and forearms — where blood vessels run close to the surface — can help bring your temperature down quickly.
#8: Know the Warning Signs — and Know When to Go Inside
Heat exhaustion and heat stroke are medical emergencies, and the window between feeling off and being in serious trouble can be shorter than most runners expect.
Warning signs include heavy sweating followed by a sudden stop in sweating, dizziness, nausea, confusion, and a rapid heart rate that won’t settle.
If you experience any of these, stop running, get into the shade or indoors, and cool down immediately. When conditions are genuinely dangerous — high WBGT, excessive humidity, or a heat advisory in effect — the treadmill is not a compromise. It’s the right call.
References
RunClub — Free
Join 300,000+ runners in RunClub
Free training plans for every distance, a daily running game, exclusive coaching content, and a real community of runners. Create your free account in 20 seconds — no card.
Already a member? Log in →
Join the conversation
Great article for summer, thank you!