Tour de France Has Run on Luck as Heat Stress Risk Builds Across French Race Locations

On a July afternoon in Nimes during 2020, the Wet Bulb Globe Temperature - the composite heat stress metric used by cycling's governing body - hit 30 degrees Celsius. The Tour de France was not racing through Nimes that day. In 2019, Paris reached 28.8 degrees WBGT. The race visited Paris on its final stage, but the worst day passed without a stage scheduled there. So far, the world's most prestigious cycling race has navigated its way through five decades of rising temperatures with what researchers from the French National Research Institute for Sustainable Development (IRD) describe as considerable fortune.

That fortune has a quantified basis. A study published in Scientific Reports analyzed climate data associated with more than 50 editions of the Tour de France, covering 12 frequently visited locations from 1974 to 2023. The research was led by IRD within the European project TipESM, in collaboration with the London School of Hygiene and Tropical Medicine and the Barcelona Institute for Global Health.

How heat stress in cycling is measured

The Union Cycliste Internationale uses the Wet Bulb Globe Temperature (WBGT) as its primary heat safety metric. WBGT combines air temperature, relative humidity, solar radiation, and wind speed into a single index designed to estimate the physiological strain imposed by environmental conditions. The UCI protocol designates a WBGT above 28 degrees Celsius as a high-risk threshold that triggers protective measures.

The researchers retrieved historical meteorological records for each of the 12 locations and calculated WBGT values for all July days from 1974 to 2023, then compared the values recorded specifically on Tour de France race dates against the distribution for all July days in each location. The gap between these two numbers - the maximum experienced on a race day versus the maximum that occurred in July overall - captures how close the race has come to encountering its worst possible conditions.

Where the risk is concentrated

The data show that heat risk is not uniformly distributed across the route. Toulouse, Pau, Bordeaux, Nimes, and Perpignan in the south consistently recorded the highest WBGT values. In Bordeaux, the worst recorded race-day value was 28.7 degrees in 1995, while the worst July value at that location was 30.1 degrees in 2019 - meaning the race missed a potentially dangerous day by several degrees. In Nimes, the worst race-day value was 27.9 degrees in 2019, while the location reached 30 degrees in 2020 when no stage was scheduled.

Paris and Lyon are an emerging concern. Paris crossed the 28-degree UCI threshold five times in July between 1974 and 2023, four of those since 2014. The race has avoided each of those days by scheduling its Paris finish before or after the peaks.

"In a way, we can say that it is an extremely fortunate race, but with record-breaking heatwaves becoming more frequent, it is only a matter of time before the Tour encounters extreme heat stress day that will test existing safety protocols," said Ivana Cvijanovic, researcher at IRD and first author of the study.

The mountain stages remain comparatively safe

Historically cool locations in the high Alps and Pyrenees have maintained low to moderate risk levels throughout the study period. The Col du Tourmalet reached a maximum race-day WBGT of 23 degrees in 2006, and Alpe d'Huez peaked at 20.1 degrees in 1992. Neither has recorded an extreme heat episode on a race day. The altitude and proximity to snow-fed air masses have provided a buffer that lowland stages lack.

The timing pattern within race days also has safety implications. Morning hours remain the safest period, while high heat stress can persist into the late afternoon. For stages finishing in the mid-to-late afternoon in southern France, this timing overlap is precisely the combination most likely to create problems.

Physiological data gap

One of the study's honest limitations is that WBGT-based risk thresholds were not specifically calibrated for elite endurance cyclists. General population heat stress models may not accurately capture the risk to athletes operating at sustained near-maximal exertion, who generate substantial metabolic heat on top of whatever the environment imposes. Elite athletes also differ from the general population in thermoregulatory capacity, acclimatization, and cardiovascular fitness.

"Science still has many unanswered questions about how the human body responds to heat, and even more so in the case of elite athletes, who face sustained physical exertion while also having physical conditioning and training levels well above those of the general population," said James Begg, researcher at Galson Sciences and co-author of the study. "To investigate sport-specific vulnerabilities, we would need access to anonymised physiological data that would allow us to go beyond heat indices alone."

No universal heat safety standard exists across international sports federations. FIFA, the UCI, and other organizations each define their own thresholds and response protocols, creating a fragmented landscape for an increasingly shared problem as warming continues.