Nine matches during the World Cup group stage were played under dangerous heat and humidity levels, according to an analysis by The Guardian.
The findings have prompted global players' union Fifpro to demand that thermal conditions guide future scheduling decisions.
The evaluation monitored environmental metrics across the 72-game opening phase, which ended last Saturday.
Data showed that nine fixtures reached severe heat thresholds, conditions Fifpro previously argued should trigger match delays or postponements.
An additional 13 matches experienced ambient temperatures exceeding those dangerous thresholds, but those games took place in stadiums with advanced internal air conditioning systems.
The assessment tracked the wet bulb globe temperature (WBGT), which estimated conditions at or above 28°C (82°F) during kickoff times.
Unlike standard temperature readings, WBGT incorporates humidity, wind speed, and direct sunlight to measure thermodynamic stress on the human body.
Miami's Hard Rock Stadium hosted the two hottest matches among venues without air conditioning.
During a 2-2 draw between Uruguay and Cape Verde on June 21, the estimated WBGT surpassed 33°C, breaking the tournament record of 32.9°C set six days earlier during Uruguay's match against Saudi Arabia.
"The lesson for everyone in the industry is that with a warming planet, heat conditions will play a bigger part in tournament and league scheduling decisions in the future," a Fifpro spokesperson said.
United Nations officials also expressed deep concern about the intersection of global warming and athletic competitions.
"More extreme heat is not random, it's climate change, caused by more than a century of burning fossil fuels like coal, oil and gas," said Simon Stiell, executive secretary of the UN climate body.
"It affects the things we love, like football."
FIFA guidelines state that a WBGT reading above 32°C (90°F) should prompt mandatory cooling breaks and potential match suspensions.
To combat the severe North American summer, organizers implemented three-minute hydration breaks for all tournament fixtures.
Medical experts warned that extreme heat remains the deadliest form of severe weather globally. Joshua L.
DeVincenzo from Columbia University highlighted that fans face immense risks while navigating unshaded parking lots, transit platforms, and stadium entrances.
"Taking into account their entire journey to the stadiums, arriving early, leaving late, and how much time is spent in heat-conducting spaces like parking lots, cars, trains and platforms, and unshaded areas pose significant risks to the fan experience," he said.
"In addition to this, fans pose a wider range of vulnerabilities to the heat, where the elderly, children, and those with other cardiovascular or metabolic conditions should be extra prepared."
Labor advocates noted that manual workers at the venues face severe health hazards when heavy labor is performed above a 25°C WBGT without regulated rest periods.
Currently, only three tournament venues—located in Dallas, Houston, and Atlanta—are fully climate-controlled.
FIFA defended its safety frameworks, noting that matches were intentionally scheduled for late afternoon and evening slots.
The governing body utilized a tiered heat mitigation model, stationed on-site meteorologists, and provided cooling resources such as ice, misting fans, and electrolyte drinks.
"As FIFA and the 2026 World Cup have taken meaningful proactive steps in introducing these safeguards, there remains a critical need for more research on the lived experiences of fans, players, and workers under these new weather conditions," DeVincenzo added.
Extreme weather impacts have extended beyond high temperatures.
On June 22, severe rainstorms forced a two-hour delay for the match between France and Iraq, marking the first precipitation-related postponement at a World Cup since 1974.
"Sport is the world's greatest unifying force, and football is the world's biggest sport," Stiell said.
"If everyone who loves football speaks up to protect it from climate impacts like extreme heat, we can kick the dependence on planet-heating fossil fuels and move much faster to clean energy."
