Firefighters are at greater risk of having fatal heart attacks because prolonged exposure to heat can make their blood clot, a pioneering study has found.
Physical analysis of 19 firefighters in Scotland also found that tackling blazes put a strain on their hearts and worsened the functioning of their blood vessels.
Previous work showed that firefighters have the highest risk of heart attack of all the emergency services.
A heart attack is the leading cause of death for on-duty firefighters and they tend to suffer cardiac arrests at a younger age than the general population.
In the US, about 45 per cent of on-duty deaths each year among firefighters are because of heart issues, and researchers at the British Heart Foundation (BHF) and Edinburgh University believe the situation in the UK is comparable, although they did not know the cause.
They went twice, at least one week apart, and either performed a mock rescue from a two-storey building for 20 minutes or did light duties, in the case of the control group, for 20 minutes.
The firefighters wore heart monitors that continuously assessed their heart rate and its electrical activity.
Blood samples were also taken before and after, including measurement of a protein called troponin that is released from the heart muscle when it is damaged.
Those taking part in the rescue had core body temperatures that rose by 1C and stayed that way for three or four hours.
There was also some weight loss among this group, while their blood vessels also failed to relax in response to medication.
Their blood became "stickier" and was more than 66 per cent more likely to form potentially harmful clots than the blood of people in the control group.
Dr Mike Knapton, associate medical director at the BHF, said: "Firefighters routinely risk their lives to save members of the public. The least we can do is make sure we are protecting their hearts during the course of their duties."