The Hottest World Cup in History: An Inside Look at the CORE Data from a FIFA Referee

To mitigate the extreme heat of the North American summer, FIFA referee Ilgiz Tantashev, along with fitness & performance coach Vadim Cherepanov, partnered with CORE, makers of the CORE 2 Thermal Sensor, to run heat adaptation protocols in preparation of the tournament as well as live, in-game monitoring of thermal vitals to avoid critical levels of heat stress.

“Previously, thermal safety monitoring was limited to environmental factors only, such as ambient temperature and humidity,” explains Ross McGraw, CORE’s CCO. “With this partnership, CORE have given FIFA referees the advantage of individual monitoring, allowing them to see what’s actually happening inside their bodies.”

FIFA referee Ilgiz Tantashev conducted an 8-week heat adaptation protocol which included several indoor, overdressed sessions on the treadmill and bike trainer. The goal of each session was to increase the heat strain to a level where the body starts to induce heat adaptations, such as increased blood plasma and lowering of heart rate at a given workload. 

Ilgiz used the CORE sensor attached to his heart rate monitor strap to track his thermal vitals and ensure the correct and safe amount of heat exposure, and using the CORE app, he tracked his adaptation across the lead-up to the tournament, eventually earning himself an 81% Heat Adaptation Score.

With his CORE sensor paired to his Garmin watch, Ilgiz live-monitored his thermal vitals, such as core temp, skin temp, CORE Heat Zones, and time in zones throughout each match, halftime, and hydration break. Access to this data in real-time allowed Ilgiz to see the real-time impact of cooling strategies such as water dousing, hydration, and cooling towels, and adjust accordingly.

All three matches showed similar patterns in Ilgiz’s Heat Strain Index (HSI), a measurement that combines core temp and skin temp into a value from 0-10 representing how hard the body is working to cool itself. Across the matches, HSI rose steadily throughout the first half, peaking at halftime. HSI came down significantly during the halves while Igiz was hydrating at rest, and then climbed steadily again throughout the second half. 

Interestingly, we see the HSI peak not at the end of the game, but at the end of the first half, likely due to two factors; the rate of play being faster in the first half and the ambient temperatures decreasing throughout the matches with the sun setting in some cases.

Despite the intense ambient temperatures and humidity during these matches, Ilgiz managed to avoid CORE’s Heat Zone 4 entirely, indicating his heat training effectively induced the proper adaptations to sustain the extreme conditions safely.

In two of the matches, we see the Heat Strain Index peak above 6/10 with max core temp of 39.2°C/102.5°F, indicating Ilgiz likely experienced a noticeable decline in both physical and cognitive performance. As HSI rises, the body diverts blood away from the muscles and brain towards the skin to prioritize cooling over performance, making it incredibly difficult to concentrate and make tough decisions. 

After officiating the France vs. Paraguay match where temperatures reached 38°C/100°F, Ilgiz faced heavy criticism for poor judgment and lopsided enforcement of discipline. It is possible that his high level of heat strain contributed to lapses in split-second judgment.

A September 2024 study by Plakias et al. titled The Impact of Heat Exposure on the Health and Performance of Soccer Players explored the topic, “...high heat exposure can affect the central nervous system. Brain functions such as visual perception, attention, anticipation, and memory are essential for decision-making and necessary for executing correct tactical actions…dehydration in soccer players increased the rate of incorrect choices in making the most appropriate pass.”

Ilgiz’s thermal data shows a clear impact from the hydration breaks that were newly introduced at this year’s World Cup. The two hotter matches show a brief dip or leveling off of heat strain before it continues to rise after the breaks. These dips seem to be quite small within the larger context of the intense heat strain throughout the match, but without a control data set, it is unknown how much higher heat strain would have gone without them. 

The cooler match shows a much more significant impact from the hydration breaks. After the first hydration break, heat strain stayed below the pre-break peak for approximately 20 minutes. It's possible that the break gave the referee enough cooling to keep HSI at manageable levels for longer.

Comparing total running distance with the heat strain index across all matches, we find that workload alone does not predict thermal strain. For example, in the Scotland vs. Morocco game, Ilgiz ran the most, but because this match featured the coolest environmental conditions, his heat strain index remained the lowest. Across these 3 matches, ambient temperature had a greater impact on heat strain than workload. 

This is not always the case across other sports. In the data we saw from World Tour cyclists at the Tour de France for example, ambient temperature was not as closely correlated to heat strain. Factors such as workload, wind convection, cooling strategies, and more played a significant role in heat strain.

Further exacerbating the intense levels of heat strain are the new rate-of-play rules introduced to the game, including the eight-second goalkeeper rule, faster restarts and the clampdown on time-wasting (IFAB, 2025). 

An editorial in Experimental Psychology titled The hidden heat penalty of speeding up football explains, “Previous analyses of elite matches (Siegle & Lames, 2012) have shown the ball to be in play for a little under 60% of the game time. In the heat, under the old rules, the game slows further as stoppages can be lengthened by players, as seen in other sports (Morante et al., 2007; Périard et al., 2014), causing realised playing time to fall towards 50%. In contrast, applying the new laws prevents this, likely resulting in the ball being in play for around 70% of the match even in the heat (Ashworth, 2026). Across a 90-min match that can amount to 15–20 more minutes of running, roughly 25% more total work, and consequently more heat production.”

With global temperatures continuing to rise each year, thermal data will continue to play a more critical role in keeping athletes and referees safe and performing their best. Individualized, live-monitoring offers an inside look at how these environmental inputs are playing out within the body in a way that was previously not possible. 

CORE is committed to offering the best solutions for measuring heat acclimation, cooling protocols, and hydration breaks as the sport continues to evolve to meet the changing demands of the environment.