As a heat wave engulfs the United States, teams competing in the World Cup knockout phase will face an added headache: staying cool.The tournament was already anticipated to be the hottest World Cup since 1994, when it was last held in the United States. With high humidity and the threat of thunderstorms on top of temperatures expected to be near or over 38 degrees Celsius (100 degrees Fahrenheit) across most of the country, simply being able to cope with the conditions could be a crucial advantage.Combatting the heat goes far beyond in-match hydration breaks. At the first match of the tournament against Mexico in Mexico City, the South African squad lined up for the national anthem wearing what looked like silver, baggy jackets.Adding extra layers in soaring temperatures may seem counter-intuitive. But these jackets are part of Adidas’s ‘Climacool’ equipment, specifically designed to reduce players’ skin and core body temperature. As well as the jackets, there are cooling vests and overshoes.“Body temperature management and heat management is one of the biggest performance pillars we have in innovation,” Margherita Raccuglia, the director of athlete performance at Adidas, told The Athletic.Raccuglia said Adidas’s innovation group started to focus on athlete cooling in 2012, and introduced its first cooling vest for athletes before the Olympics in Rio four years later. Research was conducted with university partners, as well as making use of Adidas’s in-house innovation lab — which has a climatic chamber to simulate conditions — for professional and recreational athletes to understand how to better reduce body temperature and improve performance.Spain’s Lamine Yamal wearing an Adidas cooling vest during training (Ángel Martínez/RFEF)The Climacool jackets were first seen in Formula 1 at the start of 2025, with Mercedes drivers George Russell and Kimi Antonelli pictured in the jackets — labelled ‘space jackets’ within the F1 paddock — in Bahrain to cope with the heat in the Middle East. The pair wore them over cooling vests.Adidas were able to work closely with Russell and Antonelli to tailor the Climacool kit to their needs and sizes. Although similar cooling vests are common across the grid, wearing the jackets on top prevented them from being exposed to the warm temperatures typical on the starting grid next to cars being fired up. It ensured drivers stayed as cool as possible before jumping into their cars.The feedback from the Mercedes drivers and their physios to Adidas was positive, prompting them to remain in use through the rest of the season and into 2026. But it helped Adidas as it started to consider how it could use its cooling technology at the World Cup. Feedback was taken from last year’s Club World Cup, which was held during high temperatures in the U.S. last summer.“We decided to redesign the cooling vest and the jacket and to repurpose that technology for football,” said Raccuglia, “and then to add an additional product that is not part of the climate cool system offering for F1, but it is for football, which is the cooling overshoe.”Adidas worked closely with clubs, including Manchester United, Arsenal and Juventus, in the lead-up to the World Cup to adjust the Climacool kit for footballers, who would be putting the vests in particular on and off more regularly, typically during training and half-time, compared to F1’s drivers who do not use the vests once in the car.“We increased the (body) coverage of the vest to have more cooling, which is important in football where they’re having these short breaks,” said Raccuglia.“Then they need to put the vest on and off quickly, so we added a zipper at the front. Then we have one size for the vest, but you need to address different body shapes and sizes so (there is) adjustability on the side to ensure that optimum contact is achieved, which is ultimately driving optimal cooling.”Adidas supplied the kit to 14 of the 48 competing nations, including Germany, Spain, Argentina and Mexico, providing them with the jackets, cooling vests and overshoes for the tournament. The items are kept in freezer boxes to ensure temperatures stay low before they are worn.“The vests we have help us recover the player in a more efficient way by reducing the body temperature,” Carlos Cruz, a physio with Spain, said in a video on the team’s YouTube channel. “We use it to try to ensure that fatigue is reduced and the subsequent recovery process is much faster.“We are using them after training to aid recovery, but we could use them beforehand, especially when we start warming up or between the warm-up and the start of the match. It helps reduce body temperature, and helps the player feel more recovered and in better condition to compete.”Argentina’s players wearing the vests and overboots during a training session (Adidas)Much as Mercedes’ drivers wear the jackets as long as possible before getting in the car, the World Cup versions were designed to be “anthem jackets,” according to Raccuglia, so the players could wear them throughout the pre-match ceremonies.Raccuglia explained the jackets were intended to create a “cooling microclimate” around the body. By making the jackets oversized, it gives room for air to circulate around, while the lining on the outside is air proof to ensure the cold air does not escape. The reflective surface stops the outside temperature from heating up those wearing the jacket. “It’s function driving design,” she added.According to Adidas, the upper body kit can reduce the core body temperature by up to 0.5ºC, and skin temperature by as much as 13ºC.The jacket and vest combined weighs around one kilogram, but is designed to have close contact with the body. “For training purposes, the cooling benefit is overruling the additional cost of having to wear something heavy,” Raccuglia said.The addition of the overshoe, which can quickly be put on and off over the boots or barefoot, came from Adidas’s findings.“We gathered insights that foot temperature was a critical one,” Raccuglia said. “Increasing foot temperature could create feet swelling (and) not a very good fit in the boots, which can impact negatively comfort and performance.” The overshoe uses gel technology that Adidas said can cool the feet by 2ºC within minutes.“They help with recovery,” said Cruz. “The idea is to put them on or leave them on during the break in the changing rooms, so that when the players arrive or when temperatures are high, they can put them on and cool down and lower the body temperature through their feet.”Although Adidas has not been actively asking for feedback through the tournament, Raccuglia said the visible usage of the jackets, vests and overshoes was “good confirmation” it was proving effective.She also said the Spanish team reached out at the end of June to provide “really positive feedback” about the system, while some players have also asked to keep it to use individually when they go back to their club teams.What started in F1 and made its way to football is now bound for other sports, with talks taking place within Adidas on how the Climacool technology could be used or adopted in disciplines such as tennis, volleyball, hockey or track and field. Raccuglia said there was “a very long list of Adidas athletes that are requesting the system.”And as heat waves become more common across the world and sports adjust to the increased temperatures, finding ways to better regulate body temperature and stay cool will only be more vital.“It’s relevant not only for the pros, but for us consumers and regular, more recreational athletes,” said Raccuglia. “So the topic is not going to go away.”