Miami's extreme heat and humidity could push World Cup players to their physical limits

World Cup players heading to Miami face a physiological challenge that rivals anything their opponents can throw at them: a combination of heat and humidity so severe that experts warn it could trigger major heat illness and fundamentally alter the way football is played.

Dr. Jason Kostrna, an assistant professor and graduate kinesiology program leader at Florida International University, says teams from cooler climates are especially vulnerable if they have not had adequate time to acclimatise. The core problem, he explains, is that Miami’s thick humidity prevents sweat from evaporating — the body’s primary cooling mechanism.

“Unfortunately here in Miami, it’s very humid and so when you go and exercise, you start dripping sweat,” Kostrna told Reuters. “That means that you are not evaporating it. It’s just rolling off you and you are not getting that heat loss.”

As the body struggles to shed heat, the heart compensates by pumping blood to the skin at an accelerating rate. “Then when it can’t, it starts to really try and increase that heart rate,” Kostrna said. That cardiovascular strain, layered on top of the pressure of international competition, is expected to have visible consequences on the pitch.

“You’ll start to see the pace of play maybe drop a little, you’ll start seeing that decision-making becomes a little more conservative. Whether that’s consciously or unconsciously, depends on the athlete,” he added.

The numbers behind the warning are stark. When accounting for the Wet Bulb Globe Temperature (WBGT) — a composite measure that factors in humidity, sunlight, wind speed, and ambient air temperature — conditions in Miami could reach extreme levels. “Our wet bulb globe temperature for some of these games could be close to 120 degrees Fahrenheit (48.8°C) when you factor in radiative heat, humidity, the ambient temperature and throw in a little bit of the plastic or concrete jungle effect,” Kostrna said. “If we do not actively work to take care of everybody, that is enough to cause major heat illness.”

FIFA has introduced three-minute hydration breaks for all matches at the tournament, but Kostrna is sceptical of their effectiveness in isolation. “If you’re behind on hydration, this is not magically going to fix it,” he stated, describing the breaks as a “band-aid solution” for players who have not maintained adequate fluid intake in the days before a game. He acknowledged the breaks offer some benefit — helping to lower heart rate and deliver fluids mid-match — but stressed that pre-match preparation is the more critical factor.

Miami-Dade County is implementing a range of heat mitigation measures for supporters at Miami Stadium, including cooling stations, water trailers, hydration zones, and misting areas. Comparable protections for players on the field remain more limited, placing the burden of preparation squarely on national teams and their medical staff in the weeks before they arrive.