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. But amid sweltering North American conditions at this year’s tournament, the organization has introduced three-minute hydration breaks for every match.

Extreme heat is the deadliest form of extreme weather, causing more deaths every year than hurricanes, floods and wildfires combined. Sweltering conditions can put players at risk.

Spectators can also be affected, said Joshua L DeVincenzo, an assistant director at Columbia University’s National Center for Disaster Preparedness.

“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 and unions have warned that workers can also be put at risk by extreme heat during the matches. The Occupational Safety and Health Administration says that above WBGT readings of 25C, heavy lifting and other manual labor without rest breaks poses a severe risk of heat-related illness.

Air conditioning may be the most effective way to lower WBGT in stadiums, but only three World Cup venues this year, in Dallas, Houston and Atlanta, are fully cooled. Even those attending air-conditioned stadiums can be exposed to dangerous heat while entering and exiting the venues.

Fan festival viewing events in those cities can also pose risks. On opening day, more than 100 people at the Houston fan festival gathering required medical treatment, and four were hospitalized.

The Guardian asked Fifa if there is a WBGT that would trigger definite game delays or suspensions but did not receive a response.

The organization has defended its heat protections, noting it scheduled many matches of this year’s matches for late afternoon and evening to avoid the hottest summer hours and saying it was using a “tiered heat mitigation model” to protect players, referees, supporters, volunteers and staff.

Fifa has said it is stationing meteorologists at match venues to help prepare for extreme weather, it has said, adding that tournament planning includes “close coordination” with host city organizers as well as stadium authorities and national agencies, and that players, alongside mandatory hydration breaks, are being provided access to water, electrolyte drinks, ice, cold towels, fans, mist and shade.

“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, data that will be essential for informing the next generation of heat safety protocols at major sporting events worldwide,” said DeVincenzo.

A heatwave is expected to envelop swaths of the central and eastern United States this week, with some World Cup cities potentially set to see their hottest conditions of the year so far. Host cities including Boston, Philadelphia and Kansas City, Missouri, all of which have open-air stadiums, could see above-average levels of heat.

Philadelphia will be under extreme heat watch, declared by the National Weather Service, beginning Wednesday afternoon and into Saturday evening. Fifa’s fan festival gathering has shifted hours in an attempt to avoid the hottest weather.

An extreme heat warning is also in effect for Kansas City, and in New York City, an extreme heat watch will be in effect from Wednesday afternoon through Saturday evening.

The conditions could revive conversations about heat safety that cropped up during the brutally hot Club World Cup matches in the US last year.

“You can clearly see an effort to align the competition schedule planning and venue selection with the concerns around player health, but also player performance,” the Fifpro spokesperson said. “This is a clear outcome, which we welcome, and a lesson learned from the Club World Cup.”

But some games may still pose a threat, said the spokesperson, noting that “expanded competition formats as well as infrastructure limitations make it very difficult to eliminate risks completely.”

By the tournament’s end, one in four matches are expected to be played under WBGT conditions at or above 28C, according to estimates from World Weather Attribution. That could include the final, two quarter-finals and the third-place play-off.

Scorching heat is not the only form of extreme weather taking a toll on the World Cup this year. On 22 June, France’s match against Iraq was postponed by two hours due to rainstorms in the first weather-related delay to a World Cup match since 1974.

“Sport is the world’s greatest unifying force, and football is the world’s biggest sport,” said Stiell. “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. “