Running in 30°C Heat Barely Raises Your Heart Rate

Why Scientists and Race Organizers Use WGBT

WBGT is the standard method researchers and sports governing bodies use to quantify heat stress (or thermal load) on the human body during physical activity in direct sunlight. 

It combines four factors that affect how hot you feel and how much strain your body is under: air temperature, humidity, wind speed, and radiant heat from the sun (including cloud cover and reflections off surfaces).

The formula is:

WBGT = 0.7 × wet-bulb + 0.2 × globe temperature + 0.1 × dry-bulb

• The wet-bulb part captures evaporative cooling and is heavily influenced by humidity.
• The black globe measures radiant heat load from the sun and sky.
• The dry-bulb is the ordinary air temperature.

I estimated WBGT for every outdoor run session using data from the Open-Meteo API. Relative humidity and dew point feed the wet-bulb calculation (via Stull’s approximation), shortwave radiation and wind speed help proxy the globe temperature, and I kept a simple rain flag as a control variable. 

It’s not a perfect on-site measurement with a physical tripod, but it’s a practical estimate that captures far more of the real thermal stress than air temperature alone.

You may have noticed actual WBGT devices on the sidelines of football matches and athletics tracks, the tripod with the distinctive black globe. At the 2026 World Cup, those readings are getting attention. In football, FIFA’s current rules are fairly conservative: they’ve made 3-minute hydration breaks mandatory in every half of every match, regardless of conditions. 

Only when WBGT hits around 32°C do organizers seriously consider extra precautions or postponing and they don’t shorten matches the way some endurance sports do.

For comparison, the World of Triathlon uses a five-color flag system based on WBGT that is more granular and generally more conservative than football: from Green (< 25.7°C WBGT; low risk) to Black (> 32.2°C WBGT; extreme).

At Red and especially Black flag levels, they can shorten distances (for example, cutting the run leg) or even cancel the event. They also monitor conditions in real time during races and can make decisions mid-event if WBGT climbs, which would be communicated via flags during the bike leg.

What Does a WBGT of 32°C Feel Like?

You can hit WBGT 32°C with air temperatures in the mid-to-high 30s Celsius if humidity is moderate to high and the sun is strong. In drier conditions, you might need air temperatures closer to 40 °C, along with intense solar radiation, to reach the same WBGT. The black globe component is what makes it jump when the sun is beating down and drop when clouds roll in.

This 32°C threshold is actually quite controversial. FIFA uses it as the point where organizers should seriously “consider” extra measures or postponement. But the players’ union (FIFPRO) and many sports scientists argue that meaningful risk starts much earlier, they recommend cooling breaks from 26°C WBGT and postponement considerations from 28°C. 

There’s a clear gap between the official governing-body rules and what much of the research community considers safer.

Where the Heat Rules and the Science Disagree

Because high external heat stress is a big problem in elite sport, and I think this week’s data analysis demonstrates why.

I combined two large GPS-matched cohorts: 104,883 runs from six European countries (Jan 2023–Jan 2025) and 25,118 runs from verified marathon trainees (Jan 2025–Jan 2026), every session paired to hourly Wet Bulb Globe Temperature at the athlete's location.

At the population level, again, heat looked harmless. Median heart rate barely moves between cool and hot starts. But the median distance falls by roughly a kilometer. Median pace is slightly slower. Crucially, three-quarters of runners are less likely to train on hot days at all.

The heart rate looks flat because, for those who do train, they change how they train. This is a composition effect, not evidence that heat does not matter. When I hold effort constant, same runner, same pace band, a second layer appears.

In plane words, most people change their behavior most of the time to mitigate the physiological impact of heat. But some don’t, and that is where the problem is.

When Heat Does Change Behavior

1. People Skip

When I classified each runner's heat coping style using daily participation and session-level WBGT slopes. The dominant response is not showing up:

• 42% skip hot days and slow when they do run
• 34% skip hot days, but do not systematically slow
• Only 15% are "weather invariant," neither skipping nor slowing detectably

Marathon trainees are more skip-heavy than I expected (76% skip-dominant).

2. Runs Get Shorter

This is the strongest and most consistent behavioral signal in my models. With a random intercept per runner, each +1°C WBGT is associated with just over 30m of less running distance.

3. Pace Adjusts Modestly

Pace shifts are real but smaller than the distance. Here, the two datasets showed slightly different responses. Runners from the European data slow slightly at the population level. Those from the marathon training dataset show a weaker within-person pace response because the athletes still logging hot-day sessions are a selected subset, the ones willing to run at all.

My takeaway: heat reliably changes behavior. Skip, shorten, sometimes slow. That is the first line of defense, and for most people, it works; session-average HR stays flat.

When Heat Doesn't Change Behavior

Then there are the runners who maintain training intent despite WBGT: they still log sessions, hold marathon-pace efforts, and do not ease off enough for behavior to absorb the thermal load.

To test whether cardiac strain shows up at constant pace, I fitted a personal HR vs WBGT slope within each pace band (≥4 sessions, ≥2.5°C WBGT spread within the band). Europe recreational runners stay near zero across training paces (median +0.01 bpm/°C in the 5–6 min/km corridor; n = 1,157). 

Marathon trainees show a clearer signal at the same effort: median +0.12 bpm/°C (p < 0.01, n = 206), roughly +0.8 bpm per 7°C WBGT, consistent with the population matched-effort estimate of +0.07 bpm/°C. Heat does not stress everyone. It stresses people who keep doing the same kind of run when others would skip or shorten.

The Spearman Trade-Off: Behavior and Physiology on the Same Scale

To quantify "slows in heat" vs "HR rises in heat" at the individual level, I estimated each runner's WBGT slopes for pace and HR, then ask: do runners who slow more also show a smaller HR rise?

I used Spearman's rank correlation (ρ), which measures whether two variables move in opposite rank order without assuming a linear relationship

Across 1,996 runners with sufficient hot/cool exposure:

This means that runners who rank high on "slows down in heat" tend to rank low on "HR rises in heat", and vice versa. A negative ρ near −0.5 is a moderate-to-strong inverse association. This is a mathematical way of telling us something we already know: if a user eases up in the heat, the heart doesn’t show extra strain. If they don’t ease up, that strain shows up. It’s behavioral thermoregulation working. 

What This Means for Running in Heat

None of this is new in principle. Athletes, coaches, race organizers and medical teams already know that heat changes performance and increases risk. But what this analysis adds is scale. Across more than 130,000 real-world runs, the dominant response to heat is not a dramatic rise in heart rate. It is behavior change.

People skip. They shorten. They sometimes slow down. Most of the time, that behavioral adjustment protects the physiology.

The risk sits in the gap between intent and environment. When runners keep the same intensity intent in higher WBGT conditions, especially around common marathon-training paces, the physiological signal starts to appear. The average effect is not huge, but it is consistent: less behavioral adjustment in heat is associated with a greater heart-rate rise.

That is why WBGT matters. Air temperature alone misses too much of the real thermal load. And heart rate alone can miss the behavioral adaptation that came before it. To understand heat properly, we need to look at both: what the environment is doing to the athlete, and what the athlete does in response.