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.
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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.
