How hot is too hot to run? Limits in °C and °F, with sources

How hot is too hot to run?

It's too hot to run hard when the heat stress, not the thermometer, reaches High. The American College of Sports Medicine (ACSM) puts the race limit at a WBGT of 28 °C (82 °F). Above that, it recommends postponing or cancelling.

WBGT, the wet bulb globe temperature, blends humidity, sunshine, wind and air temperature. That's why one 25 °C (77 °F) day is easy and another is dangerous. Our hub on running in heat and humidity shows how much each degree slows you.

For a training run, use the four levels as a guide:

• Low, below 18 °C (65 °F) WBGT: run as planned.
• Moderate, 18–⁠23 °C (65–⁠73 °F): slow down; heat will build during the run.
• High, 23–⁠28 °C (73–⁠82 °F): any runner may suffer heat illness. Run easy, shorter or earlier.
• Extreme, above 28 °C (82 °F): the ACSM's level for postponing races. Don't run hard outdoors.

Official race flags and medical advice override this article. It isn't medical advice.

Why isn't there one temperature limit?

Because humidity and sun can count for more than the air. WBGT is 70% wet bulb, 20% black globe and 10% air temperature, so a thermometer reading tells you little on its own.

We found the air temperature at which each level begins, for three dew points, in full sun and with none. You can check your own run's numbers in Map your run.

Read across a row and the sun's effect is plain. At a dew point of 15 °C (59 °F), High arrives at 30.1 °C (86 °F) before sunrise but at 20.4 °C (69 °F) in midday sun. Our dew point running chart explains the humidity column.

What do the official limits say?

The ACSM's 1996 position stand on distance running set four WBGT bands, and they're still the basis for race flags. Later research has argued for lower limits at some races.

Hadley's rule adds air temperature and dew point in °F. For example, 30 °C (86 °F) with a dew point of 20 °C (68 °F) sums to 154, which his chart says means slowing by 4.5–⁠6%. It's a coach's rule of thumb, not a study. Our guide to heat-adjusted pace calculators compares it with others.

How hot is too hot to run a marathon?

For a mass marathon, the ACSM's 28 °C (82 °F) WBGT is the outer limit, and some races set stricter ones. What matters is the heat when you'll be running, not at the start.

Roberts found that problems at the Twin Cities Marathon rose quickly once the start WBGT passed 13 °C (55 °F). A marathon also lasts long enough for the sun to climb. In our reanalysis of the London Marathon on 22 April 2018, the air was already 18.9 °C (66 °F) at the start, and a 4-hour runner felt up to 26.2 °C (79 °F) at their pace.

Our guides to marathon heat flags and the ideal marathon temperature cover race day in detail.

Who should stop sooner?

Anyone not used to the heat, unwell or dehydrated should ease off at a lower level. The ACSM's 2007 position stand names runners who aren't acclimatised, are on certain medications, are dehydrated or were recently ill as more prone to collapse.

The 2015 consensus recommendations led by Sébastien Racinais add that the first hot days of the year are riskier. That applies to a spring race in Boston or London as much as to an early summer run in Sydney or Johannesburg. If you've had a fever this week, the heat isn't the day to test it.

Ultra runners face the longest exposure of all; our guide to running an ultramarathon in the heat covers it.

How do you run safely when it's hot?

Move the run, slow it down and plan the drinks. Then watch your body, not your watch.

• Go when the WBGT is lowest. That's often early, but dawn humidity can be high. See the best time to run in hot weather.
• Run by effort. Accept a slower pace from the first kilometre.
• Pick shade and water. Loops past a tap beat a long exposed out-and-back.
• Know when to stop. The ACSM's 2021 consensus statement calls exertional heatstroke a true medical emergency. Fast recognition, stopping and rapid cooling are essential.