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The morning before Keith Titmuss died suddenly after a training session with the Manly Sea Eagles he seemed "fine" and "normal," according to his teammates.
The rising NRL star was young, fit and healthy, while the temperature outside never reached above 25 degrees Celsius.
But after training finished on November 2020, the 20-year-old collapsed, had a seizure, and then died of a heart attack at a hospital in Sydney a few hours later.
A coronial inquest into Mr Titmuss's death has revealed he died due to exertional heat stroke (EHS).
Experts say this illness can affect young, healthy Australians and its incidence is "massively underestimated".
What is exertional heat stroke?
EHS occurs when a person's body is pushed too hard while exercising in warm or hot weather, raising their internal temperature to a dangerous level.
This can trigger an accelerated heart rate, hyperventilation, vomiting, seizures, fainting, and in some cases, death.
In the United States, EHS is the third leading cause of death in athletes during physical activity.
According to the Australian Bureau of Statistics, heat was a contributing factor in 579 hospitalisations and 22 deaths in 2022.
Samuel Chalmers, an expert in sports heat policy at the University of South Australia, said EHS was more common in Australia than the statistics suggested.
"When people die … historically, heat illness has not always been what's been recorded," Dr Chalmers said.
"Heat illness can trigger a number of other issues and organ failures, which may ultimately be the ones that have been coded as the cause of death, even though they may not have happened if the heat illness didn't occur."
Lauren Fortington, an expert in injury epidemiology and sports medicine at Edith Cowan University, said the statistics also underestimated the number of people hospitalised with EHS.
"In a hospital or emergency setting … it's difficult to capture and code them correctly," Dr Fortington said.
"Injury data is coded to an activity, but illness data is not.
"These heat cases fall between the lines.
"If a person who's been active attends [hospital] they might have dehydration or cramps or nausea … and be put aside as a virus or just overexertion."
How is it different to classic heat stroke?
EHS is triggered by exercise while classic heat stroke is primarily caused by hot weather, according to Ollie Jay from the University of Sydney's Thermal Ergonomics Laboratory.
"Classic heat stroke you often see in old people who have impaired thermal regulation, they're exposed to extreme heat or extreme humidity during heat waves and their body can't cope," Professor Jay said.
"Whereas an exertional heat stroke … the stimulus is the enormous amount of heat that's generated inside the body from physical exertion."
Professor Jay said EHS often occured in "physically active, fit, and healthy individuals".
While hot weather significantly increases the risk of EHS, temperatures don't need to be particularly high for it to occur.
The temperature was only between 21-25C outside the day Mr Titmuss died.
EHS is the most severe form of exertional heat illness.
Heat-related illness can often be treated by giving someone a cold drink, immersing them in iced water, and moving them to a cooler spot.
However, if this progresses to heat stroke, emergency medical care is needed.
"Once your body reaches above 40-41 degrees Celsius, the endothelial barrier of the gut becomes quite leaky and starts becoming permeable," Professor Jay said.
"The things that normally stay safely inside your gut like endotoxins, they start leaking out.
"They can enter the circulation, and then set off a cascade of effects … blood clotting all over the body, multi organ failure, and ultimately death."
Is enough being done to prevent deaths?
Dr Chalmers, Dr Fortington, and Professor Jay believe that most Australian sporting organisations are doing a pretty good job of adopting research-informed heat policies to prevent EHS in their athletes.
But it's not just elite sportspeople who are at risk.
A 2021 study headed by Dr Fortington found that the majority of EHS deaths in Australia occurred during outdoor recreation, not organised sport.
"It's males aged 15 to 45 years going out for a hike or motocross riding that are high risk," Dr Fortington said.
"They may not tell someone, they may underestimate the risk, or they may overestimate their ability. And people don't realise that heat can kill."
Dr Fortington believes some of these deaths could have been avoided if the community viewed EHS as a more serious threat.
"If it's a hot day and you've planned to go out trail riding, bushwalking, hiking, etc, you might like to revisit your plans," she said.
As climate change continues to drive up temperatures, Dr Chalmers says it is essential for all sporting organisations — both amateur and elite — to adopt robust heat policies.
"We need to get everyone on board because with global warming and hotter summers this is going to become an issue that gets more and more prominent," he said.
"There shouldn't be a focus on just 'When do we cancel'?"
"There are things like cold ice towels draped over players necks and thighs … using fans, more cold-water consumption, and cooling vests."