Nine matches in the World Cup group stage were played amid potentially dangerous heat and humidity, a Guardian analysis shows, as the global players’ union Fifpro warned that heat would have to “play a bigger part” in the sport’s future scheduling decisions. The findings come as probably record-breaking heat and humidity will hit the midwest and eastern US this week and could make conditions even more challenging for players and fans at some games. The Guardian’s latest analysis of conditions looked at the World Cup’s 72-game group stage, which ended on Saturday last week, and found an estimated nine matches were played in conditions of severe heat that Fifpro has previously said should trigger the delay or postponement of games.

An additional 13 may have been played in cities with temperatures beyond that level of heat, but where stadiums were cooled by air conditioning. These matches were played amid wet bulb globe temperatures (WBGT) that may have been at or over 28C (82F), according to data for the place and time of the games – conditions Fifpro has argued should trigger the delay or postponement of games. The two hottest games in a non-air-conditioned venue were both played at Miami’s Hard Rock Stadium.

During Uruguay’s 2-2 draw with Cape Verde on 21 June, the wet-bulb globe temperature may have reached or exceeded 33C, surpassing the previous tournament high of an estimated 32.9C recorded six days earlier, when Uruguay drew 1-1 with 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 spokesperson from the Fifpro union said in a comment when asked about the Guardian analysis findings. The UN’s top climate official also warned about how the climate crisis was affecting the sport.

“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,” Simon Stiell, executive secretary for the United Nations climate body, said in an emailed comment. “It affects the things we love, like football.” Unlike air temperature alone, WBGT accounts for humidity, direct sunlight and wind to provide a more accurate measure of heat stress and the body’s ability to cool itself through sweating. At high enough levels of heat and humidity, sweat can no longer evaporate efficiently, preventing the body from cooling itself and leading to rapid overheating that can cause heat illness or even death.

WBGT measurements require specialized equipment on site, but the Guardian has compiled approximations based on air temperature and humidity data for the time and place of each match, with data from open-source weather service Open-Meteo. These estimates were calculated using a formula devised by the Australian Bureau of Meteorology and used by the Canadian military and other institutions. The 2026 World Cup is projected to be the hottest such tournament to be held since the quadrennial competition began in 1930.

At WBGT readings above 32C (90F), Fifa says cooling breaks should be introduced and match delays or suspensions should be considered.