Keeping it cool under heat stress – key recommendations for the prevention and treatment of heat related illness

Published:  01 August, 2013

The Country Fire Authority (CFA) in Victoria, Australia, conducted extensive body monitoring trials, using the latest technology in order to assess the physical and mental effects of heat stress on firefighters in action. Based on the results the CFA was able to make several essential key considerations for the treatment and prevention of this life threathening condition, writes Peter Langridge, health and wellbeing officer for the CFA.

Heatstress is one of the top three leading causes of injury during bush fire suppression and an increasing cause of injury during structural firefighting. While personal protective clothing (PPC) provides considerable protection, current structural PPC is heavy, multilayered and contains a moisture barrier that traps air, creating a microclimate between the skin and clothing. Consequently, during physical exertion and exposure to a hot environment, firefighters lose their ability to dissipate body heat through sweating − the natural means of thermoregulation. These factors increase the vulnerability of firefighters to thermal strain.

The human body will tolerate a core temperature increase of only 3°C before sweating fails to cool the body and heat stroke occurs. As the surroundings become hotter, all methods of heat elimination− radiation, conduction and convection− become ineffective in protecting the firefighter against heat from these mechanisms.

For the safety of firefighters, it is important to keep cardiovascular and thermoregulatory strain below critical levels while working in PPC. Under high ambient temperature, the body may experience exhaustion, mental confusion, disorientation, loss of consciousness, heart attack and, in extreme cases, death. A number of rehabilitation procedures exist within the fire service to decrease incidence of heat stress such as crew rotation, fluid replacement, education about signs of heat illness and passive cooling (for example, opening of PPC or sitting in a shaded area). Although these current practices help to relieve heat stress, the National Fire Protection Association Rehabilitation Standard (NFPA 1584) states that active cooling methods should be included in standard operating procedures.

Active cooling is the process of using external methods or devices (eg hand and forearm immersion, misting fans, ice vest) to reduce elevated core temperature. Currently, there are no rehabilitation standards in Australian fire services.

Previous studies have proven that active cooling, particularly hand and lower arm immersion, may be the most effective method to reduce core temperature, because the hands and forearms act as an effective heat exchange point due to the significant increase in vasodilation and peripheral blood flow associated with an increased core temperature. Recently it has been shown that combining two active cooling methods (ice vests with hand /forearm immersion) provides no additional benefit compared to hand/forearm immersion alone.

Over the past two years, CFA’s (Country Fire Authority, Victoria, Australia) occupational health and safety department, in conjunction with Deakin University, have conducted a number of heat stress trials. These trials aimed to assess increases in firefighters’ core temperatures following simulated firefighting tasks while wearing structural PPC, and the effectiveness of 20-minute hand and lower arm immersion in water to alleviate increase to firefighters core temperature. One hundred and eighty volunteer firefighters participated in these trials conducted at Mt Hotham, Echuca, Lara, Wangaratta and Penshurst. Based on the trial’s results, recommendations for the rehabilitation of firefighters after wearing structural PPC have been made.

Firefighters were required to enter a heated building while wearing structural PPC and breathing apparatus. A 20-minute recovery period was selected to meet the NFPA 1584 rehabilitation standards. During this time all subjects removed excess clothing, rehydrated and rested in a seated position. Members of the active rehabilitation group also immersed their hands and lower arms in Kor-Kool Chairs filled with tap water.

Subjects were fitted with an Equivital Life Monitor heart rate monitor worn around the chest to measure heart rate, respiration rate, skin temperature and activity, during the exercise and recovery period, and tympanic temperature (temperature of eardrum) was measured from the subjects’ left ear by the same researcher to ensure consistent collection technique, using the Braun Thermoscan Pro 4000 tympanic thermometer. Core temperature was recorded by the Equivital Life Monitor, using the radio receiverwhich received radio measurements from the ingested pill. Blood pressure measurements were taken using MP2 monitors, and subjects reported on thermal sensation using the Young et al. 1987 Thermal Sensation scale. Each measure was taken at the beginning of recovery and every five minutes thereafter.

Findings

Core temperature reductions between 0.1 and 2.9 ºC were evident in the rehabilitation group compared to a range of -0.12 to 0.69 ºC in the control group after 20 minutes recovery, with significant differences between the two groups at each time point. Further data represented in Figure 1 demonstrated a clear trend whereby tympanic temperature did not accurately represent core temperature, with variance between the two measures of up to 1.2°C. The findings support current research, which warns that tympanic temperature devices cannot accurately predict core temperature.

How does hand and lower arm immersion work?

All mammals have specific regions of the body’s surface designed for dissipating excess heat from the core to the environment. In humans some of these areas are found in the soles of feet and palms of hands. When core temperature is elevated peripheral blood flow increases in these regions by up to 1300%, allowing for heat to be transferred from the body to the environment through convective heat transfer. Immersing the hands and lower arms in water reduces the temperature of the blood located at the surface, which then returns to the core, reducing the temperature of the central blood supply. This cooling process is dependent on water temperature, core-to-surface temperature gradient and peripheral blood flow. A number of studies have shown the optimal water temperature to be approximately 10ºC for forearm and/ or hand immersion. House et al. (1997) investigated the effectiveness of hand immersion in water at 10ºC, 20ºC and 30ºC and found core temperature reductions within ten minutes was more effective in colder water. This temperature allows for a suitable core-to-water thermal gradient, aiding quick heat dissipation. At higher water temperatures >20ºC both hands and forearms should be immersed to increase the surface area for heat dissipation. Iced water is not recommended as vasoconstriction of the peripheral blood vessels can occur and cause a reverse reaction whereby the body’s heat is trapped in the core.

Hand and forearm immersion is preferred over conventional cooling methods, such as misting fans, ice packs or pouring cold water over the body, as these methods may make one ‘feel’ cooler, but are generally ineffective at cooling the body’s core. Studies of passive cooling techniques such as opening one’s jacket and resting in a shaded area have shown benefits when the environment is at a moderate temperature (>22-24ºC), however these natural cooling techniques are limited when the environment is hot (>35ºC, 50% relative humidity). In a previous study in the heat (>35ºC, 50% relative humidity) firefighters wearing protective clothing performed intermittent work for 50 minutes followed by 20 minutes of passive or active cooling. The study found forearm submersion blunted the rise in core temperature and extended work time compared with passive cooling. Another study found that adding the forearm with hand immersion produced a lower core temperature than passive cooling and hand immersion alone, as a result of the greater surface area for heat exchange.

Recommendations

Based on current research and the present study it can be concluded that hand and lower arm immersion during a 20-minute recovery period following exposure to a hot environment, while wearing structural PPC, Is an effective cooling strategy to stimulate heat dissipation. This cooling strategy may be the only viable option in comparison to passive cooling methods and the use of ice vests alone. Hand and lower arm immersion decrease core temperature during the 20-minute recovery period and aiding heat transfer from the body. This active cooling modality will help to reduce heat strain associated with wearing structural PPC.

Key recommendations for the management of heat stress:

•           Hand and lower arm immersion is a practical cooling strategy as firefighters can remove their jacket during rest periods and easily immerse the hands and forearms in cool/cold water.

•           Cool/ cold water is readily available at the fire ground.

•           10ºCis the optimal water temperature for heat dissipation when using hand and/or lower arm immersion technique

•           This method works extremely well in hot environments (35ºC, 50% relative humidity), or when an individual has suffered heat stress (hyperthermia).

•           Hand and lower arm immersion should also be used during bush fire suppression when the body’s natural cooling technique (evaporation) is limited in a hot environment.

•            Educate firefighters in prevention, recognition and management of heat related illness and planning for incidents on extreme days. Refer to heat stress chart and rehabilitation recommendations.

•            Extended firefighting incidents should consider increasing number of personnel available and allow for longer or more frequent recovery time.

•            Further education of firefighters to improve pre-incident hydration status. Firefighters should recognise that adequate hydration helps to prolong work time and reduce incidence of heat illness.

•            Education of firefighters to maintain a good level of fitness.

•            Education of incident controllers and crew leaders in rehabilitation requirements.

  • Operation Florian

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