Is it too hot to run?
There is no universal cut-off. The National Weather Service publishes heat index categories describing what becomes possible with prolonged exposure or physical activity — at a heat index of 90°F that includes heat stroke. Stop at the first sign of heat illness.
Guidance from US National Weather Service
- Heat stroke enters the bands at
- 90°Fheat index
- Full sun adds up to
- +15°Fon the published value
Conditions
| Threshold | Band | What to do |
|---|---|---|
| below80°F | Go | No heat-related restriction. Drink normally. |
| 80–90°F | Caution | Fatigue is possible with prolonged exposure or strenuous activity. Reduce the intensity or the duration. |
| 90–103°F | High riskExtreme Caution | Heat cramps, heat exhaustion and heat stroke are all possible with prolonged exposure or strenuous activity. Cut the session back substantially. |
| 103–125°F | Do not runDanger | Heat cramps and heat exhaustion are likely, and heat stroke is possible, with prolonged exposure or strenuous activity. |
| 125°Fand above | Do not runExtreme Danger | Heat stroke is highly likely with continued exposure. Do not run outside. |
Heat index, as US National Weather Service defines the bands. Each is one step; the ladder is not to scale.
What to do
- Read the heat index, not the air temperature. Humidity is what stops sweat evaporating, and evaporation is how you cool.
- Add up to 15°F to the heat index if you will be in full sun — the published value assumes shade and light wind.
- Reduce how hard and how long you run as the bands rise. The categories are defined for “prolonged exposure and/or physical activity”, which is a precise description of a run.
- Build up to hot weather over one to two weeks of shorter, easier exposure rather than all at once. Acclimatisation is gradual, and the guidance for people working in heat is built around exactly that.
Stop, and get help
- You stop sweating, or your skin becomes hot and dry.
- Chills or goosebumps in the heat.
- Confusion, disorientation, slurred speech, or stumbling.
- Nausea, vomiting, or a headache that arrives during the run.
- Dizziness that does not settle when you slow to a walk.
Why
Heat is the most common environmental risk in running and the one most often judged by the wrong number. Air temperature alone does not tell you how hard it will be to stay cool — humidity does most of the work, because you cool by evaporating sweat and humid air slows that down.
The heat index combines the two into a single figure, and the National Weather Service publishes risk categories against it. Those categories are defined for prolonged exposure and/or physical activity — which is a precise description of a run, and the reason they apply to you directly rather than as an analogy.
| Air temperature | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|
| 100°F | 109 | 118 | 129 | 143 | 158 | 176 | 195 |
| 95°F | 99 | 105 | 113 | 123 | 134 | 147 | 161 |
| 90°F | 91 | 95 | 100 | 106 | 113 | 122 | 132 |
| 85°F | 84 | 86 | 89 | 93 | 97 | 102 | 108 |
| 80°F | 80 | 81 | 82 | 83 | 84 | 86 | 89 |
Two things the heat index does not include
- Sun. The index assumes shade. Full sun can raise the effective value by up to about 15°F, which is often enough to move you a whole band.
- You. Acclimatisation, age, medication, illness, body size and how hard you intend to run all move the real risk. The bands describe conditions, not people.
Which is why this page gives bands and actions rather than a temperature at which running becomes unsafe. Anyone offering you that number is offering more certainty than the evidence supports.
Heat stroke is a medical emergency. The signs that separate it are neurological.
Heat exhaustion and heat stroke
Heat exhaustion is the body struggling to cope: heavy sweating, weakness, nausea, dizziness, a fast weak pulse. It resolves with stopping, cooling and fluids.
Heat stroke is a medical emergency. The signs that separate it are neurological — confusion, disorientation, slurred speech, stumbling — sometimes with hot dry skin because sweating has stopped. If you see those in yourself or another runner, stop, get into shade, start cooling, and call emergency services.
The distinction matters because the instinct in the first case is to push on and finish, and in the second case that instinct is dangerous.
Check today
For particle pollution EPA counts people with heart disease, people with lung disease including asthma and COPD, older adults, children and teenagers, pregnant people, minority populations and outdoor workers. Ticking the box applies EPA’s sensitive-group guidance rather than its general guidance. It is not medical advice.
Not today — move it indoors or to another time
- Do not runDanger110°F heat index in full sun
Heat cramps and heat exhaustion are likely, and heat stroke is possible, with prolonged exposure or strenuous activity.
US National Weather Service
Our suggestion Move the run indoors or to another day.
- GoGoodAQI 45
A great day to be active outside.
US EPA / AirNow
Heat and air quality today. Storms and daylight are the next signals to join this check. Enter conditions from your weather app or AirNow.
Related questions
What temperature is too hot to run?
No single number applies to everyone, and the Weather Service does not publish one. What it publishes are heat index categories — Caution (80–90°F), Extreme Caution (90–103°F), Danger (103–124°F) and Extreme Danger (125°F and above) — each describing what becomes possible with prolonged exposure and/or physical activity. Running is physical activity, so those categories apply directly, and they begin at a heat index of 80°F rather than at any particular air temperature.
Why does humidity matter so much?
You cool by evaporating sweat. High humidity slows evaporation, so the same air temperature carries much more heat stress — which is exactly what the heat index is calculating.
How much slower should I run in heat?
We do not publish a number, because no public authority supports one and the honest answer varies with the individual, acclimatisation and conditions. Run by effort instead: hold the effort you would normally hold and accept whatever pace that produces.
Does acclimatisation help?
Yes, and it takes time rather than willpower. Expect a week or two of easier running when hot weather first arrives before the same effort stops feeling disproportionate.
Sources
- What is the heat index? — the categories and the full-sun adjustment quoted here — US National Weather Service
- Heat Index — US National Weather Service
- Heat Forecast Tools, including HeatRisk — US National Weather Service
- Heat stress and heat-related illness — US Centers for Disease Control and Prevention