England fans in Texas for next month's FIFA World Cup opener against Croatia are among those most at risk of health complications caused by extreme heat, academics have warned.
The AT&T Stadium in Arlington is air-conditioned, meaning players and spectators within the venue will be protected from the high temperatures.
However, the World Weather Attribution (WWA) group of academics are especially concerned about the impact of climate on spectators on their way to matches, queuing to get in and attending outdoor fan festivals.
WWA has found there is a one in three chance of the Wet Bulb Globe Temperature (WBGT) exceeding 28 degrees Celsius (82 degrees Fahrenheit) in Dallas for the England match on June 17, among others.
WBGT is an index that considers how effectively the body can cool itself, with world players' union FIFPRO saying matches should be postponed where the WBGT exceeds 28 degrees.
Altogether, WWA expects nine of the 104 matches to be played above 26 degrees, where FIFPRO advises that safety measures be implemented, with around five expected to occur where the WBGT exceeds 28 degrees.
While players are elite athletes and can adapt how they exert themselves, Dr Chris Millington, from Imperial College London, believes the greatest risk is posed to spectators.
He said: "They're more medically diverse, maybe older or very young. They may have cardiovascular disease, kidney or metabolic disease. They may be taking medications that affect heat tolerance and they may be unacclimatised visitors.
"They may also be exposed for much longer than the match itself, walking to the stadium, queuing in direct sun, attending fan zones, sitting in exposed seats and travelling home on crowded transport.
"People who have got medical comorbidities, such as heart disease, will go into that situation. They'll probably not hydrate very well the night before. They might not sleep very well the night before, and then they might have some beer during the day. And gradually they'll get hotter and hotter and hotter. They probably won't put wear a hat.
"They may wear their sports shirt, or, as they sometimes do, they might be not wearing a top, and exposing themselves to those risks as well, and then what happens is, someone who has got that blocked coronary suddenly finds they're dehydrated and they're pushing all their blood flow to their skin, and suddenly not enough blood flow is going to their heart, and they suffer a heart attack that they wouldn't have suffered were they not in that situation."
FIFA has not publicly given an indication of the WBGT threshold which would trigger postponement. It has, however, introduced three-minute hydration breaks midway through each half at every match at this summer's finals as a player welfare measure.
Dr Millington suggested that does not go far enough to mitigate the heat impact on players, and said a longer half-time break should be implemented. However, he said efforts to reduce risks for players would increase the risk to spectators, by extending the whole match-going experience.#
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Dr Friederike Otto, Professor of Climate Science at Imperial College London, said that in future FIFA should give consideration to changing when it schedules World Cups.
"It would definitely be advisable to have these either earlier in the year or later in the year to have the ability to have a football party and not something that is a massive health risk," Dr Otto said.
A FIFA spokesperson said a tiered heat mitigation model would be in place at this summer's finals, where temperatures exceed defined thresholds.
In those circumstances, "spectators will be permitted to bring one factory-sealed water bottle, and venues will activate additional cooling capacity, including shaded areas, misting systems, cooling buses and expanded water distribution," the spokesperson said.
FIFA has also developed a medical set piece protocol for exertional heat illness for players that includes the use of bespoke cooling bags by trained match doctors that will allow immediate treatment at any location.