Rapidly traveling between climate zones, all with different average temperatures, humidities, and oxygen levels, will place additional stress on players, staff, and spectators.
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During the 2026 FIFA World Cup, soccer teams will play a rapid sequence of games that will take them to 16 cities across Canada, Mexico, and the United States. A team of climate scientists and environmental physiologists evaluated the environmental stress the teams will experience during the tournament, finding that players will be at very high risk of extreme heat stress during matches played at 10 of the 16 stadiums. Matches at high-elevation stadiums will also put players at risk because of the lower oxygen content in the air.
“We hope our results will enable optimization of match schedules at individual venues, taking into account the health risks associated with extreme heat stress, but also the physiological reactions to heat potentially affecting the performance of players on the pitch,” said Katarzyna Lindner-Cendrowska, a geoecologist and climatologist at the Polish Academy of Sciences in Warsaw, and lead researcher on the study, which was published in Scientific Reports.
Stadium Swaps Add to Heat Stress
In places where the risks of heat-related illnesses are already high, research has shown that people engaging in intense physical activity such as professional sports are at an even greater risk.
Hot and humid conditions cause an athlete’s body to produce more heat than it can dissipate and enhance the risks of extreme dehydration via sweating. Low levels of atmospheric oxygen make it harder to breathe and oxygenate blood, further lowering a body’s ability to dissipate heat. These physical responses slow cognitive function and reaction times, inhibit the speed and precision of movements, and reduce concentration. Prolonged activity in these environments can also lead to long-term health conditions and, in extreme cases, death.
These risks are well known by sporting groups. The Fédération Internationale de Football Association (FIFA) considers environmental risks to players, coaches, and spectators ahead of events, typically by measuring the wet-bulb globe temperature (WBGT). If WBGT exceeds 32 C (89.6 F), cooling breaks are mandatory during both halves of a FIFA match. Recent research found that just six of the 16 host cities exceed this threshold during an average year, and each less than 5 percent of the time. (Four more cities exceed this threshold in a hot year.)
But despite its popularity among FIFA and other sporting organizations, WBGT “is considered an imperfect measure of heat load on athletes, as it is prone to underestimating the heat stress level,” Lindner-Cendrowska said. Some stadiums don’t have a way to measure WBGT on site, and the index itself doesn’t consider the additional thermal load if there is high humidity or poor airflow, which makes it harder to cool down, she added.
Lindner-Cendrowska and her colleagues calculated a more precise measure of athletes’ heat stress risk by modifying the universal thermal climate index (UTCI) to include biometric data on core body heat, water loss, and oxygen levels of soccer players while playing. The calculation is similar to the “feels like” temperature on a weather app but for physical exertion.
“UTCI is a measure of the human physiological response to the thermal environment and is considered a better index, as it provides an estimation of how the body feels under a given environmental condition specified by the air temperature, wind, humidity, radiation, clothing, and level of physical effort,” explained George Nassis, an environmental physiologist who focuses on cardiovascular and thermoregulatory systems at the University of Kalba in Sharjah, United Arab Emirates.
Nassis, who was not involved with the study, said the researchers’ choice to evaluate heat stress risk using a modified UTCI was “most appropriate.”
At 10 of the 16 stadiums, UTCI levels for active athletes could exceed 46 C (114.8 F), the threshold for extreme heat stress.
The researchers evaluated hourly average heat stress risk at each 2026 World Cup stadium during the 11 June to 19 July tournament. At 10 of the 16 stadiums, UTCI levels for active athletes could exceed 46 C (114.8 F), the threshold for extreme heat stress.
Midday matches in Houston and Arlington, TX; and Monterrey, Mexico, put players at the highest risk of heat stress from heat and humidity, with UTCI levels exceeding 50 C (122 F), but morning and evening matches at these locations were nearly as risky. In other stadiums, midday matches provided the most risk of extreme heat stress, but shifting matches to other times of day alleviated the risk.
In addition, matches played in Guadalajara and Tlalpan, Mexico, which are at elevations of 1,566 and 2,240 meters (5,138 and 7,349 feet) above sea level, respectively, could put athletes at risk due to lower levels of oxygen in the air. Though levels don’t vary greatly during the day, oxygen levels are the lowest just after midday in both stadiums.
“This altitude will also impose stress to the body of the coaching and supporting staff as well as to the visitors traveling to these locations from low-altitude locations,” Nassis said.
Teams moving from a low heat stress environment to a high heat stress environment will need to quickly adapt.
What’s more, the logistics of the 2026 World Cup present an additional complication: The 104 matches will take place in nine different Köppen-Geiger climate zones ranging from humid continental regions to subtropical deserts. The researchers called this an “unprecedented” diversity in biothermal conditions.
Teams moving from a low heat stress environment to a high heat stress environment will need to quickly adapt, Nassis said.
“This is a big challenge given that proper heat acclimatization … needs some days to take place,” Nassis added. “As a result, some players of these teams may be vulnerable to excessive heat stress that may compromise their health and performance.”
Helping Players Stay Cool
Most of the risk can be avoided by strategically scheduling matches at cooler and less humid times of day, the researchers concluded. When hot times can’t be avoided, stadiums could use air conditioning in strategic locations at the hottest times to help athletes cool down. The three stadiums that have retractable roofs could cover the fields.
Teams, too, can alter their training regimens to better prepare for the anticipated environmental stress.
“The top two priorities are acclimatization and hydration,” Nassis said. Teams could train outdoors or in artificial indoor environments to acclimatize players to anticipated heat conditions. They can also be more vigilant about monitoring hydration and teaching different ways to effectively cool down when overheated. Teams can implement different cooling strategies during halftime and during any WBGT-mandated cooling breaks, which are only three minutes long.
The modifications to match scheduling and training regimens would benefit soccer leagues not just for the 2026 World Cup but for future sporting events, too. The risk of heat stress is increasing around the world because of climate change, which is lengthening and intensifying heat waves and altering precipitation patterns.
“We hope that not only FIFA, but also other international sports federations and organizers of major sporting events will find our results inspiring enough to implement precautionary planning, which is essential for ensuring that sporting events are safe and satisfying experiences for everyone — athletes, spectators, and technical staff,” Lindner-Cendrowska said.
This story by Kimberly M. S. Cartier was originally published by Eos Magazine and is part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.
