On Monday, 3 August, Fijian rugby player Saimoni Vunilagi set off from home to train with his team-mates.

After the session, he was taken to hospital. A few days later, the 26-year-old was dead.

In a statement announcing his death, Japanese second division club Kyuden Voltex said Vunilagi "displayed symptoms consistent with severe heatstroke" during the training session, adding "despite the best efforts of the medical team, he sadly passed away".

Japan, like the UK, Europe and many other parts of the world, suffered severe heatwaves this summer.

By noon on the day Vunilagi became ill, the temperature in Fukuoka - where Kyuden Voltex are based - had risen to 35C. The humidity was more than 75%.

In the weeks before, the daily high was between 32 and 37C. It had been a month since the daily high was below 30C.

No formal cause of death has been publicly released, but the link suggested by the club between training in heat and Vunilagi's death has caused alarm, given the increase in global temperatures and the rising frequency of extremely hot days.

So, how serious a threat is extreme heat to athletes, what measures could help and do perceptions within sport need to change?

There have been a variety of heat-related medical incidents during sports events in 2026.

Tennis players Jakub Mensik and Ekaterina Alexandrova collapsed during matches played at more than 30C, and eight runners had to be taken to hospital during a marathon in South Korea in April at which temperatures reached 29C.

Tour de France winner Tadej Pogacar called this year's heat "dangerous" during the event, while Matthew Brennan asked "what are we doing?" after the temperature topped 40C at the Vuelta a Espana, in which he won five stages.

This summer's World Cup also featured controversial hydration breaks at every match, which Fifa insisted were introduced to aid player welfare.

Many sports - including tennis, athletics and cycling - monitor potentially dangerous levels of extreme heat using the Wet Bulb Globe Temperature (WBGT), which produces a measure for heat stress levels by assessing air temperature, humidity, wind speed, sun angle and solar radiation.

But some experts believe athletes need greater protection, or the risk of further fatalities will increase.

"We have seen an increase in the number of heat illness events in rugby and many other sports around the world in recent years," says Dr Madeleine Orr - assistant professor of sport ecology at the University of Toronto.

"People assume we've seen such extreme heat before and think, 'it was fine, we pushed through, we can do it again'.

"That's not necessarily true - the heatwaves we're experiencing today are hotter than the ones in your childhood, because heatwaves are relative to the local normal, and 'normal' levels have risen."

Though the requirements of each sport mean the impact of high temperatures can differ, there are growing calls for blanket rules to be implemented across sports which offer more cast-iron protection when things become really extreme.

"Different leagues have different policies - there needs to be more of a standardised approach so that everyone just knows what to do," says Orr.

"It's similar to lightning - most people are familiar with the idea that when there's lightning near a sports event, we take a break for 30 minutes and start once it has gone away.

"We need a similar policy on heat and we need it to apply everywhere - it's easier for people to get behind if it's universal that once we get to a certain threshold, we know what happens."

One of the big challenges with heat-related illnesses in sport is convincing athletes and their coaches to do what is best for their health.

"I've got no doubt that a lot of coaches think that heat regulation is a hindrance," says Christian Day - general secretary of the Rugby Players' Association.

"And it's a tough one for the player, isn't it? Because you don't want to be the player who puts their hand up and says they're not feeling well, or that it's too hot to train on a certain day. Players need protecting from themselves sometimes.

"It's important for team staff to have a regulatory number that they know if it hits it, there's no discussion, you just stop playing.

"It's also about educating players as to what signs to look for to identify one of their team-mates who is struggling in training."

World Rugby updated its heat and air-quality guidelines in 2025, including use of a heat stress index - a measure from 0 to 250 used to determine how dangerous hot conditions are for players, with provisions for water breaks and extended half-time periods at low levels of danger, and potentially suspending or postponing matches at the higher end of the scale.

But players spend far more time training than they do playing matches.

"If this level of heat is going to become the norm, then we need central regulation and governance as to how we help clubs manage their players and how they should react to these kind of temperatures," says Day.

"Maybe there are cases where perhaps training needs to be limited in time, or doesn't take place at all that day outdoors.

"It's not a matter of trying to call people out or produce problems. It's trying to ensure the safest possible training environment."

Pre-season can be an especially dangerous time for athletes across sports.

On 5 June 2026, the Japan Rugby Football Union (JRFU) warned clubs heatstroke was an increasing concern during the summer, and told teams safety should be the overriding priority when planning summer training camps.

Heatstroke is a sudden, dangerous health condition which occurs when the body overheats and can no longer cool itself.

This can damage the organs, cause the intestines to release toxins and potentially be fatal within little more than an hour.

Symptoms include dizziness, nausea, headache, confusion and slurring.

"Heatstroke is 100% preventable, so every single death is one too many," says Orr.

"It's not the case that athletes can handle heat better because of their fitness.

"And the bigger the body the faster it heats up, so bigger bodies are more vulnerable as well. Moving that mass around takes a lot of energy.

"So while yes, professional athletes are extraordinarily fit, that doesn't save them when the issue of concern is cooling off the body."

Amateur athletes playing for the love of the game can be at risk too.

"Whatever the elite level does, we'll see it trickle down into grassroots," says Day.

"And that's why it's crucial that the elite level of every sport sets the right example because the kids, the players of tomorrow, will absolutely copy whatever they see their heroes doing."