To Mist or Not to Mist: That is the Question

By David Jaslow

AP Photo/Tim Larsen
A firefighter cools himself off after helping battle a five-alarm fire in New Jersey last August.
Summertime is prime time for fire rehab in most parts of the United States. Daytime temperatures may rise to the high 90s and beyond, and nights are sticky and humid. It is essential that fire rehab sectors have the capability to allow emergency responders to rest, shed their personal protective equipment, rehydrate and allow for heat transfer from their body to the atmosphere.

But when the ambient temperature is near, at or exceeds body temperature, this can be a challenging task especially if air conditioning is not immediately available. A question that I have often been asked is whether commercially available misting devices or attachments which allow one to convert a positive pressure fan into a misting unit have any utility or value in the rehab sector.

The best way to answer that question is to explore the physiology of thermoregulation in the body and briefly discuss the emergency medicine approach to heat exhaustion. Heat stroke is unusual in firefighters since this condition is typically manifested over hours (the exertional variety) in athletes who do not rest at all or in the elderly who have pre-existing medical conditions (the classic variety).

The primary goal in the medical monitoring and rehabilitation sector of public safety personnel at any emergency incident or training exercise is to rapidly decrease the heat load that is created by heavy physical exertion. And, at the same time, making a determination whether the individuals have developed any incident-related illness or injury or exhibit any signs and symptoms of pre-existing illness, injury or baseline disease process. 

Preventable illness
Why is the rush to relieve heat stress so important? Heat illness is a major cause of preventable morbidity worldwide especially in regions characterized by high ambient temperatures such as the United States. The risk of death and injury from heat is directly related to peak temperature, duration of exposure and the acclimatization period. The term acclimatization is defined as the process of an organism adjusting to changes in its environment, typically temperature, altitude, etc. Over days to weeks, this adaptive mechanism includes earlier onset of sweating, increased sweat volume, more dilute sweat, enhanced cardiovascular performance, and a host of other features.

In firefighting other than wildland conditions, there is virtually no acclimatization period and the body goes from normal condition to extremely stressful conditions instantaneously. Additionally, peak temperatures begin to rise in bunker gear as the duration of exposure lengthens. Those with underlying cardiovascular disease are especially at risk because three of the prominent physiologic responses to a heat load are increases in heart rate, blood pressure and oxygen consumption. Eventually, cardiac ischemia can overwhelm its victim.

In terms of the pathophysiology of heat stress, heat is produced by the body and it can be gained from the environment. In fact, heat increase due to the work of muscles — by product of muscle fibers moving back and forth — can be 15 times that of basal metabolic contributions to the heat load. The rate of heat rise may be further worsened during high environmental temperatures and humidity. The body must always maintain homeostasis — a state of balance. Thus, it must get rid of heat somehow.

Heat transfer from the body occurs via one of four mechanisms:

• Conduction: transfer via direct physical contact with a cooler object (2%)
• Convection: dissipation of heat from the body to the air and water vapor surrounding the body (10%)
• Radiation: transfer of heat to the environment via electromagnetic waves (65%; occurs only if there is a temperature gradient between the body and the air).
• Evaporation: transfer of heat by transformation of perspiration and saliva into a vapor (30%; does not work in high humidity environments)

When air temperature exceeds body temperature, convection ceases and the body gains heat energy. Furthermore, radiation ceases once the ambient temperature exceeds 95 F. Conduction clearly can’t occur if the body is in contact with an object that is at or in excess of body temperature. Thus, at that point when the ambient temperature is almost to 100 F, evaporation becomes the only means of heat loss.

Is this the case inside bunker gear during a fire or rescue incident? You bet! This is why heat stroke occurs once the body losses its ability to sweat. Then, evaporation is gone and you’re cooked, literally.

A rise in body temperature activates heat receptors in both the hypothalamus and the periphery, which results in increased shunting of blood to the periphery. Increased minute ventilation and sweating maximize evaporative heat loss.

These responses may be blunted in chronically ill patients or patients with cardiovascular disease. Firefighters or other emergency responders who take medications that cause salt and water depletion or impair physiologic cardiovascular responses — such as beta blockers —  re at an increased susceptibility to heat injury. When compensatory mechanisms fail, hyperthermia develops. For more information about physiologic responses to heat stress, look at the table on this web page:

When it comes to misting devices, the real question we should be asking ourselves in the fire service is not really whether we want to spend the money on the devices but what methods of heat reduction will optimally cool the exhausted firefighter who may be near or on the brink of developing heat exhaustion?

Cooler object
Placing a cooler object such as water on the body to increase conduction will have minimal effects. Similarly, the increase in convection even if greater amounts of water vapor were produced by sprinkling water on the body would only account for 10 percent of the heat load transferred.

We need to maximize radiation, which is only accomplished by shedding bunker gear down to the ankles, and maximize evaporation. The most important means to enhance evaporation is via fans which blow air across the victim or patient. Clearly, positive pressure fans found on the fire apparatus are ideal for this situation. Ice packs to the groin or axilla may help lower core body temperature and make the individual feel better.

 Placing water on the patient to enhance evaporation of a heat load is only recommended in the emergency medicine literature for those patients who have clinical heat stroke. Otherwise, use of fans and/or placement in an air-conditioned environment is good enough.

In summary, misters may feel good — who doesn't like a water park in the summertime? — but the degree of cooling that they contribute to those who do not suffer from clinical heat stroke and who are already sitting in front of a large fan or in an air-conditioned apparatus cab is unlikely to influence the clinical outcome of the firefighter.

There are no randomized controlled trials to prove this point, but it is generally considered to be the correct approach in the peer-reviewed literature for ED patients and should be directly applicable to the field. Misters won’t hurt you, but that money could probably be better spent on other rehab sector items, medical equipment or training for EMS personnel on how to implement the NFPA 1584 standard to protect our nation's fire and rescue personnel.

For further information on heat illnesses, see the eMedicine chapter Heat Exhaustion and Heat Stroke online at

David Jaslow, MD, MPH, FAAEM is a board certified emergency physician who is fellowship trained in EMS and disaster medicine. He is the director of the Division of EMS and Disaster Medicine within the Department of Emergency Medicine at Albert Einstein Medical Center in Philadelphia. Dr. Jaslow is a state-certified Firefighter I and he is credentialed by the Pennsylvania Department of Health as a pre-hospital physician. He functions as a chief officer in several suburban Philadelphia fire and EMS agencies and provides medical oversight as the lead physician for the Bucks County Technical Rescue Task Force as well as Pa. Task Force-1 Urban Search and Rescue.

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