"Working in hot temperatures increases the risk of suffering a heart attack," BBC News reports.
It has been known for some time that the leading cause of death amongst serving firefighters is heart attacks and not fire-related injuries as some people might assume. Researchers wanted to establish why this is the case.
The new study featured 19 healthy firefighters who either took part in a "fire suppression simulation" (putting out an actual fire in a simulation facility) or light, non-emergency activities.
The researchers looked at blood samples and measurements of core body temperature after these activities. They found that the simulation of putting out a fire increased the "stickiness" of the firefighters' blood – making it more likely to clot – in part due to them being dehydrated. It also resulted in the blood vessel walls being less elastic, and in signs of some minor injury to the heart muscle due to lack of oxygen.
All of these factors, as well as the added stress of dealing with an emergency, may combine to increase heart attack risk. The findings are important, but further larger studies are needed to confirm the results and establish useful precautions that can be taken to reduce the risk.
In the meantime, current advice is that firefighters stay well hydrated and take time to cool down after putting out a fire.
Similar advice applies to anyone exposed to a high temperature environment, especially if they are taking part in a strenuous activity, such as endurance running.
The study was carried out by researchers from the University of Edinburgh and was funded by the British Heart Foundation and the Colt Foundation. A number of authors also received grants from the British Heart Foundation, the Wellcome Trust and the Fire Brigade Union.
The study was published in the peer-reviewed medical journal Circulation. The UK media's coverage of the study was accurate.
This was a randomised controlled crossover trial in healthy firefighters which assessed the effects of a standard training exercise to put out a fire on cardiovascular health.
In the US, around 45% of firefighter on-duty deaths are due to cardiovascular events such as heart attack. It is believed the risk increases when they are required to put out fires as they experience increased physical exertion, high temperatures, and exposure to air pollution.
The researchers wanted to look at whether they could identify any biological effects of putting out a fire that might cause an increase in risk.
This type of trial is the best way of testing whether a factor or situation – in this case fighting a fire – definitely has an effect. In this case it might have been difficult (and possibly dangerous) to assess the firefighters while putting out a real fire, so they were assessed during a training exercise.
The study included 19 healthy firefighters. They all took part in a fire simulation exercise and also a set of light, non-emergency activities on two different days.
The researchers took various measurements, such as their heart rate, core body temperature and blood tests during and after these exercises to see how the firefighters' cardiovascular systems responded.
The firefighters were selected at random from the Scottish Fire and Rescue Service to be invited to participate. To join the trial they could not be:
or have any of the following:
The firefighters had to attend for each activity after 48 hours off duty to reduce the risk that their recent regular work activities might affect the results. They were also required to not drink any alcohol for 24 hours before each test day and not have food, tobacco, and caffeinated drinks for at least four hours before each test.
The firefighters all took part in both a standard simulated firefighting exercise (exposure) and light duties similar to those undertaken during a shift free of an emergency (control), on different days.
The simulated firefighting exercise was carried out in a special training facility and lasted on average about 20 minutes (median). The firefighters entered the facility as part of a four person team, went up the stairs carrying a water filled hose, finding and putting out a fire on the first floor, and identifying and rescuing a 80kg dummy "casualty".
A number of measurements were taken before, during and after the activities. Blood was taken at four different time points:
The blood samples were tested for various different markers that indicate level of cardiovascular risk. For example, they tested how prone to clotting the blood was (as blood clots can lead to heart attack or stroke), whether the proteins in the blood indicated that the heart muscle was experiencing damage, and how elastic the blood vessel walls were.
The firefighters were fitted with portable heart (ECG) and blood pressure monitors at least half an hour before each activity and for the following 24 hours. They also swallowed an ingestible temperature monitor the evening before which measured core body temperature before, during and for six hours after each activity.
Sweat loss was determined using body mass before and after each activity and the firefighters were also asked to rank their perceived level of exertion immediately after each activity.
The trial featured 19 healthy non-smoking firefighters (16 men and three women) who were 41 years old on average. Only 17 of them completed both activities.
The average core body temperature of the firefighters was 37.4C at the start of the study, increasing to 38.4C at its peak. There was an increase in core body temperature (1.0C) and reduction in weight (by 0.46 kg) among the firefighters following the fire simulation training, due to dehydration.
When compared to the control non-emergency activity, one to two hours after the fire simulation activity the firefighters' blood samples showed an increased tendency to form clots (was more "sticky") in tests in the laboratory.
Immediately after the fire simulation exercise the firefighters' blood samples also showed increases in other factors such as haemoglobin, volume of red blood cells, platelets (fragments of cells found in the blood that are involved in clotting), and white blood cells, in comparison with the control activity.
Blood flow through the firefighters' forearms increased after the fire simulation exercise, but the blood vessels were less responsive to certain drugs which widen the blood vessels.
A marker protein (called cardiac troponin)which indicates heart muscle injury due to the muscle not receiving enough oxygen showed small increases in the hour following the fire simulation compared with after the control activity. The levels of this protein were still within normal ranges, suggesting that the extent of lack of oxygen in the heart muscle was relatively small.
The firefighters themselves did not experience any symptoms of cardiovascular injury during the study.
The researchers conclude that exposure to extreme heat and physical exertion during firefighting increases tendency for clot formation, impairs blood vessel function, and results in reduction in oxygen in the heart muscle and heart muscle injury in healthy firefighters.
They say: "Our findings provide pathogenic mechanisms to explain the association between fire suppression activity and [heart attacks] in firefighters."
This randomised crossover trial aimed to assess whether putting out a fire has an effect on the biological signs of cardiovascular health of firefighters.
By simulating a fire fighting scenario the researchers found that exposure to these conditions increased tendency of the blood to clot, reduced the stretchiness of the blood vessel walls, and caused a slight increase in a marker of heart muscle damage.
This trial is thought to be the first assessing this link. Whilst a randomised controlled trial is the best way of investigating this link, there are some limitations to consider.
This is a small study, and whilst the findings do suggest ways in which firefighting might affect cardiovascular risk, further larger studies would be required to confirm the results and also establish any precautions that can be taken to reduce the risk.
Current advice to firefighters is to make sure that they stay well hydrated. Dr Mike Knapton, The British Heart Foundation's associate medical director, said: "It's essential that firefighters are aware of this risk and take simple steps such as taking time to cool down and rehydrate after tackling a blaze. It's also important for them to be aware of the early warning signs of a heart attack so that, if the worst should happen, they can receive medical attention as soon as possible."
While the research involves firefighters, the results do highlight the fact that even people who assume that they are in perfect health can suddenly develop a heart attack.
Read more about the early warning signs and symptoms of a heart attack.