Heat Stress in the Fire Service

Keywords: heat stress, performance, active cooling, rehabilitation, hydration

HOW DOES A HOT ENVIRONMENT AFFECT MY PERFORMANCE?

The nature of firefighting results in exposures to increased temperatures from fire but also to high temperatures in other emergency scenarios where the wearing of PPE does not allow for adequate heat loss from the body1. The result of increasing body heat storage is a constant rise in body temperature until one of two situations occurs: 1) exhaustion or fatality, or 2) change in the environmental conditions.

These environmental changes can include: i) removing clothing layers on the individual thereby increasing the efficiency of evaporative heat loss, ii) stopping the work activity or exercise, iii) moving into a shaded location or a place with a cooler ambient temperature2,3.

There are three main factors that determine an individual’s tolerance time during an uncompensable heat stress (UHS) situation, which causes a continuous increase in core temperature due to the body’s inability to get rid of the heat being produced within the body: 1) their initial Core temperature at the start of heat stress trial, 2) the Core temperature that they are able to tolerate before exhaustion, and 3) the rate of increase in Core temperature throughout the duration of the heat stress trial4. Understanding the main factors that limit exercise tolerance in the heat will allow for better recommendations to counter the negative effects on your body.

Firefighting activities require firefighters to remain alert, maintain high levels of mental function, be aware of their surrounding environment, and make split second decisions all while under conditions of extreme heat and psychological stress5,6,7,8,9. Heat stress, and the associated heat illnesses, can be life‑threatening when core temperature rises above 40°C.

The increased exposure to high temperatures along with the elevated levels of cardiovascular and respiratory stress while responding to an emergency cause an increased core temperature that may result in dehydration and psychological stress. These negative responses have been shown to impair cognitive function9,10. Firefighting can impose a situation of profuse sweating for the firefighter with body mass reductions being reported around 2% during simulated victim search and rescue tasks11. This level of decrease in body mass can negatively impact cognitive function, specifically mental concentration and working memory (more commonly referred to as short‑term memory)12. It is thought that when an individual works in a hot environment, part of the brain’s capacity is required to attend to decreasing the accumulation of heat in the body thereby reducing a person’s ability to clearly make decisions13,14,15,16,17.

The findings in this study have potential application for Incident Commanders (IC) at emergency scenarios. Research conducted more than a decade ago examined the physical work limits of firefighters in the same Fire Service and found that at the same moderate‑intensity workload, firefighters were able to tolerate 54 minutes ± 3.5 minutes and 65.4 minutes ± 3.7 minutes of a simulated work task in 35°C and 30°C with 50% humidity conditions18. Based on this data, the “Incident Commander’s Guide” heat stress wheel was developed19 which provides ICs the ability to input outside work intensity, temperature and humidity and determine the average work duration of specific job tasks at an emergency scenario before a firefighter’s core temperature reaches 38.5°C (which is at a level of caution where any further work in the heat would potentially place the firefighter in a state of heat illness). From this heat stress wheel, moderate‑intensity work (similar to primary search, overhaul, aerial and ground ladder setup, and vehicle extrication) at 35°C and 50% humidity would result in a core temperature of 38.5°C in 53 minutes. This type of information is critical for a firefighter especially if fighting fires in the summer months or conducting an auto extrication in July or August. The impairments in cognition will begin before an individual starts to feel any physiological impacts (such as heat exhaustion and heat stroke). This data suggests that when firefighters are working in a hot environment (either in a fire or summer temperatures) and are unable to seek shade or cooler temperatures, active cooling rehabilitation is of extreme importance (this will be discussed later). If you were to perform multiple search and rescue tasks, reaching 53 minutes of work could be easily done, potentially leading to impairments in your decision-making skills. Rehabilitation and hydration are critical!

KEY POINTS

• Some aspects of cognition may be impaired when a firefighters core temperature reaches 38.5C

• This level of core temperature (38.5°C) can be reached in as little as 53 minutes and resembles firefighter tasks such as primary search, overhaul, aerial and ground ladder setup, and auto extrication

• Rehabilitation (i.e. cooling) and hydration are your best defence against increased core temperature

POST‑INCIDENT REHABILITATION

HOW DOES ACTIVE COOLING HELP IN REHABILITATION?

When firefighters work in the heat they run the risk of increasing their core temperature to levels that can produce heat illnesses. It is critical that a firefighter “cools down” following work in the heat, but the question for many years is what is the best and most practical method. Research has concluded that an extremely efficient, and very practical, method is active cooling by submerging your hands and forearms into a cool water bath20,21,22. The reason why this is so efficient is that the body contains arteriovenous anastomoses (AVA), located in the palms of the hands and fingertips and soles of the feet and toes, which are very effective at transferring heat from the body’s core when immersed in cold water. These AVAs open up when the local tissue temperature in the extremities decrease below a certain threshold. Research has shown that an individual’s core temperature will continue to decrease to resting levels when hands and forearms are submerged in water leading to the conclusion that AVAs are a very effective mechanism for transferring heat from core temperature20,21,22.

The use of hand and forearm immersion to assist with the management of heat strain for firefighters has been demonstrated in a previous research study20. Fire services may require their personnel to go to a rehabilitation station after using one or two cylinders of air. During this rehabilitation period, most of the firefighting PPE is removed and the 15 to 20 min of rest can be used to actively cool the firefighter. Research has shown that tolerance time was increased 65% when the hands and forearms were immersed in cool water of 18°C, which was the in‑line hose temperature available to the fire service20. This study also demonstrated that 70% of the total cooling occurred during the first 10 minutes of immersion when the thermal gradient between core temperature and the water bath was the greatest. Thus, very short cooling periods may still promote effective cooling to lower core temperature prior to the start of the next work period. Immersion in colder water will promote even greater heat loss, especially if the forearms are also immersed to maintain the reduced temperature of the blood returning from the hands to the core23,24.

KEY POINTS

• Hand and forearm immersion in a cool water bath can reduce core temperature dramatically in as little as 10 minutes.

• Hand and forearm immersion is efficient in reducing core temperature due to the arteriovenous anastomoses (AVA)in the hand and fingertips.

• Short periods of cooling rehabilitation following work in a hot environment and prior to the next work period will help lower core temperature, reducing the risk of heat illness and cognitive impairments for firefighters.

HOW MUCH SHOULD I DRINK BEFORE AN INCIDENT?

In a compensable heat stress environment (CHS), where the body is able to dissipate the heat produced from exercise/work, fluid replacement has been demonstrated to have a major influence on cardiovascular and thermal responses during constant‑intensity exercise. The current hydration consensus has shifted slightly toward maintaining dehydration at less than 2% body weight during exercise, and to plan for fluid ingestion across a range of 400 to 800 mL.h‑1 (litres per hour) depending on the individual (e.g., mass and acclimation status) and situational (environment and exercise intensity) factors25.

In situations of uncompensable heat stress (UHS), i.e. when core temperature continues to increase due to the body’s inability to eliminate heat accumulated due to exercise, and wearing PPE, where airflow is typically minimal due to the presence of clothing, hydration status may be much more critical to maintaining body water and performance. Whenever possible, individuals should begin exercise in a euhydrated (i.e. hydrated at the beginning of exercise/work) state, as fluid replacement alone cannot overcome the thermal disadvantage stemming from a state of hypohydration (i.e. dehydrated prior to exercise/work)41. Previous research has shown that both a state of euhydration and fluid ingestion are critical components for maximizing exercise capacity in PPE. Note that with sweat rates of 1.2 to 1.3 kg·h−1 during light exercise, some level of dehydration is unavoidable even with the fluid ingestion conditions. Note that fluid ingested does not necessarily correlate with changes in plasma volume due to the time delay between ingestion and intestinal absorption26.

Apart from understanding that fluid replacement with exercise in PPE is critical27, the optimal fluid replacement rate is relatively unknown. Previous research has suggested that replacing 78% and 63% of the body water that is lost during exercise can improve tolerance time and working time in a hot environment when compared to no hydration at all28.

Gastric emptying and intestinal absorption is finite at approximately 0.8 to 1.2 L·h−1 (liters per hour), and higher rates of fluid ingestion may be counterproductive by leading to a sensation of bloating, potential nausea, risk of hyponatremia, and increased urination26. Moderate fluid replacement to maintain body weight loss at levels <1.5% to 2.0% might form a starting basis for occupational guidelines26.

This information on hydration is critical to combat the negative effects of working in the heat and emphasizes the need to arrive at an emergency scene in a hydrated state. Although it is not possible to drink water or a sports drink during job tasks, it is important to replace the fluids that you have lost during rehabilitation. The hotter the environment and the greater the intensity of the work, the greater the hydration required during rehabilitation.

KEY POINTS

• Hydration is critical in combatting the effects of working in the heat

• Replace at least 63% of the fluid that you lose through sweat from performing work tasks

• Replace fluids so that body weight loss is less than 1.5% after performing work tasks

GUIDELINES FOR OPTIMAL STRESS REDUCTION

The recommendations for reducing the impact of environmental stress on firefighting performance include26:

• Arrive at work in a hydrated state

• Lower resting core temperature through heat acclimation or precooling prior to beginning work in a hot environment

• Decrease body fatness to reduce the susceptibility to heat illnesses

• Improve endurance training in order to tolerate increased core temperature and improve heat dissipation mechanisms

• Replace at least 63% of fluid losses from sweating during or after firefighting job tasks to limit body weight loss to less than 1.5%

• Following an emergency in the heat, drinking 1.5 L of water during a 10-minute rest period and 90 minute recovery period restores plasma volume to pre‑work values (note: 1.5 L of water in a short time period is an aggressive hydration strategy)

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