Cardiac & Resuscitation, Operations, Patient Care, Training

Can We Stand the Heat?

Issue 5 and Volume 33.

Firefighters face unique risk factors as a result of several inherent job stressors. These include sudden surges in the sympathetic nervous system caused by unexpected alarms; rapid shifts from low to high levels of exertion; the requirement to carry, lift and wear heavy protective gear and equipment; prolonged exposure to high temperatures and excessive fluid loss. Each of these factors has been shown to contribute to dehydration and a rapid rise in core body temperature, which may lead to heat stress and insufficient blood flow to the heart. This combination can cause cardiovascular strain.1

To further investigate the connection between cardiovascular strain and heat stress, the Orange County (Calif.) Fire Authority (OCFA) undertook a research study to examine certain factors — hydration status, exertion level, core body temperature and post-incident cooling techniques — that may contribute to heat-related illnesses. The study was conducted through the OCFA’s Wellness and Fitness Program, which was created by OCFA management and labor in an effort to improve the health and well-being of its firefighters.

The purpose of this study was to gain a better understanding of firefighter job demands in order to provide recommendations to help minimize the inherent risks associated with the firefighting profession, resulting in a healthier, safer and more productive workforce. This article summarizes our study findings and shares our recommendations with a wider audience. We believe EMS agencies that provide post-incident rehab services may find our recommendations insightful and worth adding to their rehab protocols.

Study methods
In August 2007, the OCFA conducted the “Hydration, Core Body Temperature and Post-Incident Rehabilitation Study.” The study involved the careful monitoring and recording of vital signs on participants during a simulated structure fire, strenuous firefighting-related activities, and the rehab and recovery stage.

Volunteers for the study included 101 OCFA firefighters of different ages, body types, physical fitness levels and firefighting-experience levels. On arrival at the study location, each participant was assigned an identification number and given the equipment necessary for data collection during the study. Equipment included a CorTemp ingestible core body temperature capsule, a Suunto t6 wrist-top heart rate monitoring device and a plastic cup with a lid (for a urine sample). All equipment was labeled with each participant_s corresponding identification number.

Prior to commencing the drill, all participants ingested the CorTemp capsule, which monitored their core body temperature via a radio signal transmitted from the non-digestible, silicone-coated capsule to an external data recorder. Participants were asked to ingest the capsule with as little water as comfortably possible and also refrain from consuming any food or liquid until the end of the drill.

Each participant then provided a pre-incident urine sample. The sample was analyzed using a refractometer to determine each participant’s hydration level. Participants were then weighed wearing undergarments only. Next, each participant’s vital signs were measured and recorded. Finally, all participants were advised on the proper use of the Suunto device immediately prior to dressing out in full personal protective equipment (PPE).

Core body temperature readings were taken at specific times during firefighting activities. Heart rate was continuously monitored using the Suunto device. All other measurements (such as blood pressure) were taken at specific times — including before, during and after the study phases.

During Phase One of the study, participants performed two 15-minute drills. The first drill involved a live, simulated fire. All units were dispatched to a reported structure fire with persons trapped. During the simulation, no specific tactics or strategies were given. Each company or chief officer was instructed to allocate resources based on the incident priority. Participants were assured they were not being rated on performance nor evaluated on the amount of time required to rescue the victim or extinguish the fire.

Participants spent 15 minutes tactically advancing hose lines for rescue, extinguishing the fire and coordinating with a truck company for vertical ventilation. After 15 minutes, the first drill in Phase One was terminated. All participants exited the building and had their core body temperature and tympanic temperatures measured and recorded.

Once all temperatures were recorded, the same participants were deployed for a second 15-minute drill. During this drill, no smoke or fire hazards were presented, and participants wore their SCBA tanks but removed their SCBA masks. They performed 15 minutes of continuous firefighting-related movements, such as stair climbing and overhaul. Once the participants the second drill, they again exited the drill tower and had their core body temperature and tympanic temperatures measured and recorded.

Phase Two of the study was the recovery phase and involved ambient environmental conditions only. In this phase, participants were randomly assigned to one of four cooling stations: misting fans, wet towels, forearm submersion or ambient air (peripheral cooling). The cooling stations, which included chairs for all participants, were set up under a canopy to provide protection from the sun. This phase began once each participant was at their assigned cooling station and had removed all PPE (other than pants and boots). This phase continued for 20 minutes.

The misting fans used in the study were connected to a standard 3/4″ garden hose and had misting nozzles angled out from a center panel to direct water into the air. The wet towels were kept in ice water until participants placed them on areas of the body with abundant blood supply, such as around the back of the neck.

The forearm submersion method involved a chair containing plastic bags filled with water and positioned on the arm rests. During rehab, the participant’s forearms were immersed in the water. The last cooling station involved evaporation from ambient air; participants at this station were provided only a chair under a canopy.

None of the participants were allowed to consume water or other liquids until this phase was completed in order to accurately monitor the effect of the cooling measure alone.

During the 20-minute recovery, core body temperature and vital signs were measured and recorded at five-minute intervals. At the end of the 20-minute recovery phase, participants were asked to report to the locker room so their post-activity body weight could be determined (wearing the same apparel as when they initially weighed in).

Because dehydration can have detrimental effects on firefighter health and performance, OCFA sought to determine what percentage of firefighters were in a dehydrated state prior to beginning the drills, which would indicate hydration status prior to a fire. Although the majority of study volunteers reported regular exercise and good overall health, a urine analysis revealed 91% were dehydrated to some extent prior to commencing the study.

Participants’ heart rates were continuously monitored via the Suunto t6 device throughout the study for two reasons: 1) to analyze the exertion level required for firefighting skills and 2) to monitor recovery heart rate.

During the live fire drill in Phase One of this study, the average heart rate for all participants was 164 beats per minute (bpm). When data for active fire suppression participants was studied separately from personnel not involved in highly physical roles during the live fire drill (such as engineers and incident commanders), the average heart rate during the live fire drill increased to 173 bpm during the 15-minute period. Similarly, when data for less physically active participants was excluded from the calculation of average maximum heart rate during Phase One, the average maximum rate increased from 180 bpm to nearly 190.

The goal of measuring core body temperature was also twofold: 1) to determine how much a firefighter_s core body temperature rose during a simulated fire and 2) to analyze the effectiveness of common post-incident cooling measures in order to improve OCFA’s post-incident rehabilitation protocols.

During the study, participants’ core body temperature ranged from a low of 98 degrees F to a high of 106 degrees F. The majority of participants’ peak core body temperatures were in the range of 100 degrees F to 102 degrees F, with 44% of all participants reaching a core body temperature of 102.1 degrees F or higher.

Interestingly, in the majority of participants, core body temperature did not peak until five„minutes into rehab (during Phase Two). This finding indicates core body temperature may continue to rise even after the cessation of physical activity. We also noted that even after 20 minutes of rest, few core body temperatures had returned to starting temperatures.

Tympanic temperatures were measured during the study to determine if this method could be used in the field to reliably determine when core body temperature rises to an unsafe level. Tympanic temperatures were measured at the same intervals as the highly reliable CorTemp core body temperature readings. On average, tympanic temperature readings were more than two degrees lower than core body temperature readings.

The data also suggests that as the participants’ temperatures increased, the difference between core body temperature readings and tympanic temperature readings increased. This finding merits more research to determine whether the discrepancies between core body temperature and tympanic temperature truly increase as core body temperature rises.

The study examined some of the most common cooling measures used during rehab, the recovery period that follows physically and mentally demanding incidents. The goals of rehab are to reduce heart rate, lower core body temperature, hydrate, and provide sufficient calories to maintain the necessary energy for a rescuer to continue working.

The wet towels and forearm submersion chairs had a similar rate of core body temperature reduction. A further analysis also revealed thatƒalthough core body temperature continued to rise in all participants during the rehabilitation periodƒoverall, the group using the wet towels as a cooling method had the least increase in core body temperature. During the 20-minute rehabilitation period, the wet towels proved to be more practical; they required less space and set-up and were less expensive than the cooling chairs, while still providing similar results.

Interestingly, when asked which cooling measure seemed most appealing, most participants stated the wet towels appeared to be the most refreshing and effective. Further analysis may reveal added benefit from the psychological effects of this cooling method.

During the OCFA study, 59 of the 101 participants lost at least 2% of their body weight during the 30 minute drill. The average weight loss among participants was slightly more than 3 lbs., and the highest individual loss was 7 lbs.

Most participants were aware they had lost a significant amount of fluid; however, none knew of the adverse effects a relatively small percentage of weight loss could have on their bodies. Losing as little as 1% of body weight could affect performance and hinder the body’s ability to cool itself.„The potential effects increase as the percentage of fluid loss increases, negatively affecting physical performance and decision making, both of which are critical components of the firefighting profession.

This study was conducted to investigate the connection between cardiovascular strain and heat stress. The OCFA sought to examine modifiable risk factors for heat stress and cardiovascular strain, including hydration status, exertion level, physical fitness level and post-incident cooling techniques.

These factors were examined to gain a better understanding of what can be done to minimize the increase in core body temperature during an incident and to improve post-incident rehab for firefighters. The following areas were recognized as needing improvement:

Take a proactive stance: Overall, the study results suggest that fire departments, along with the„EMS agencies and personnel who provide rehab services, must become more proactive in researching and implementing strategies to ensure firefighter health and safety. For example, rather than focusing only on re-hydrating firefighters after an incident, departments should formally educate firefighters on the signs of dehydration by distributing informational materials, conducting educational classes and addressing the topic in required multi-company classes in order to help decrease the incidence of dehydration. And, as new evidence becomes available regarding cardiovascular health and the effectiveness of rehab, it’s our responsibility to ensure that information is considered and applied under appropriate medical direction.

Improve rehab protocols: The effectiveness of the various rehab and post-incident cooling techniques is perhaps the most important finding of this study. Fire departments should develop or adopt comprehensive rehab protocols that include rest, hydration, active cooling, medical monitoring and refueling.

It can’t be stressed enough that adequate hydration is essential for optimal physical performance. This study shows rapid fluid loss can result from just 30 minutes of physical exertion. Due to the potentially serious adverse health consequences, rehab protocols should specify that firefighters must drink approximately 24 oz. of water for each pound of body weight lost.

A rehab protocol should take into consideration that certain nutrientsƒsuch as sodium, potassium and calciumƒare also lost through sweat and that the loss of these nutrients can be detrimental to performance (e.g., loss of sodium can increase the risk of muscle cramps). Depending on an individual’s sweat loss, sweat sodium rate, exertion level and duration of exertion, replenishment of these nutrients may be required.

Caution must be taken to ensure firefighters do not reach a critical core body temperature before reducing their physical output or terminating their exposure to high temperatures. Firefighters should apply cooling measures as soon as possible after physical activitiesƒespecially tasks that involve intense firefighting — to possibly lower the eventual peak core body temperature. Additionally, medical monitoring of firefighters in the rehab sector affords„EMS providers the ability to detect dangerous signs and symptomsƒincluding sustained increase in heart rate, core body temperature and carbon monoxide.

Depending on the incident type and duration, carbohydrate intake also becomes increasingly important. Performance is, in part, a function of fuel availability. Carbohydrates are the body’s most important fuel source for energy production. Insufficient carbohydrate availability may limit exercise performance by decreasing the time for the body to reach a level of fatigue. Carbohydrate intake can have a significant impact on a rescuer’s ability to conduct strenuous work activities for longer periods of time; therefore, proper and appropriate carbohydrate intake should be included in a thorough rehab protocol.

Educate responders on the benefits of overall fitness: Although most firefighters understand that their profession is physically demanding, not all of them actually understand how the body responds to the physical demands of their job, particularly how these demands can affect health and performance. Educational sessions should focus on how fitness level affects a person’s risk for heat illness and heart strain, as well as how appropriate exercise as part of a fitness plan can prepare them for these physical demands. This study enabled our department to quantify the physical demands of the job by measuring intensity via heart rate monitors and to tailor workouts that mimicked normal workloads.

Establish regular medical screenings: For fire departments and„EMS agencies, particularly those located in areas of high ambient temperatures, identifying individuals at increased risk will minimize the likelihood of heat illness. Research has shown heat-related incidents are not random and unpredictable; the patterns are usually consistent. Educating firefighters on the risk factors for increased core body temperature and heat illness is the first step in helping prevent incidents. Additionally, establishing thorough medical screening and departmental protocols on heat and hydration are crucial steps in achieving individualized risk reduction among firefighters.

Develop a wellness program: All fire departments and„EMS agencies should develop or adopt fitness programs customized to the demands and movements of their jobs. Appropriately trained and qualified professionals should design physical training programs to ensure the programs are safe, scientifically valid and effective. Accurately documenting fitness data is an important element in determining a program’s effectiveness, assessing injury risks and fitness levels, and measuring cost effectiveness.

Although exposure to certain risk factors for heat stress is unavoidable for firefighters, much can be done to prepare for such conditions. Dehydration, low fitness levels, and excess body weight are all modifiable risk factors for heat illness. Individuals who are overweight are less effective at losing heat and are at a higher risk for increased core body temperature and heat strain. This study demonstrated that physically fit individuals are better able to tolerate an increase in core body temperature and are less likely to experience heat stress.

Developing and implementing a comprehensive program for educating crews on health and fitness can result in healthier employees, reduced time off due to injuries and lower worker_s compensation costs. Creating a culture that values well-being can improve morale and result in a positive outlook on job performance. Fit and healthy firefighters are able to perform their job requirements more effectively, thus providing a higher level of service to their communities.

Not all health problems can be predicted and prevented, but educating firefighters and all emergency responders on the importance of proper fitness, hydration, and safety and rehabilitation protocolsƒand ensuring strict compliance to those protocolsƒare important steps toward maintaining optimal health, maximizing job performance and job satisfaction, and ultimately improving the overall quality of our lives.

Nancy Espinoza, MS, is the exercise physiologist at the Orange County Fire Authority. For more information, contact Nancy at [email protected], or Fire Captain Mike Contreras, OCFA’s Wellness and Fitness coordinator, at [email protected]

Acknowledgments: The OCFA would like to extend sincere thanks to all the personnel who volunteered to participate in the study and to Local 3631 for their continuous support and partnership.

The study was funded by Orange County Fire Authority’s Wellness and Fitness (WEFIT) program with approval from the WEFIT Oversight Committee, which is made up of labor and management.


Gonzolez-Alonso J, Mora-Rodroguez R, Below PR, et al: “Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise.” Journal of Applied Physiology. 82(4):1229-1236, 1997.