The study may explain why heart disease is the leading cause of death among on-duty firefighters, the researchers from the University of Edinburgh said.
Firefighter Simon McNally, who was physically fit, had a heart attack while at work, at the age of 36.
"The doctor said if I'd gone home instead of coming to hospital I probably wouldn't have woken up."
He had been working as an instructor in Essex for three years where he set fires inside a shipping container three or four times a day, and was exposed to temperatures of 600-1,000C.