One in four 2026 World Cup games could take place in very hot conditions as climate change has increased the risk of extreme heat in North America since the US hosted the 1994 tournament, researchers warned Thursday.

Concerns about heat already prompted Fifa to mandate a cooling break during each half of the World Cup matches that will take place between June 11 and July 19 in 16 stadiums across the United States, Mexico and Canada.

“Players and fans face a much higher risk of gruelling heat and humidity at the 2026 World Cup compared to the 1994 tournament on the same continent,” World Weather Attribution (WWA), a network of climate scientists, said in a statement.

WWA’s scientific analysis estimates that 26 of the 104 matches could occur in conditions reaching at least 26C (79 Fahrenheit) Wet Bulb Globe Temperature (WBGT) – an index that measures how effectively the human body can cool itself under heat, humidity, sun exposure and wind.

At 26C WBGT, the international footballers’ union Fifpro recommends cooling breaks as heat strain becomes a real risk for players.

Of those 26 matches, 17 will be played in stadiums with cooling systems, reducing risks for players and fans.