Monitoring Heat Injuries in a Hazmat Environment

Heat injuries are a major problem worldwide. In a study chronicling heat deaths in the U.S. from 1979 to 1999, a total of 8,015 deaths were associated with excessive heat exposure.¹ Weather conditions caused 3,829 (48%) deaths, and manmade conditions (kitchens, vehicles, boiler rooms, etc) caused 377 (5%) deaths, particularly for those wearing protective clothing.¹

Military members who wear combat gear are especially vulnerable to heat injuries, but none more so than members who wear personal protective equipment (PPE). In this review, PPE is defined as self-contained breathing apparatus protective equipment (SCBA) levels B or C. The challenge of PPE is the inability of the individual to dispel heat through radiation, convection, and evaporation. The only close approximation of the PPE environment is combat and football protective equipment. In 2011, CDC reported that football players in uniforms, which resemble PPE for the purpose of this discussion, experienced heat injury at a rate 10 times higher than the average rate for other sports.² These heat injuries in football players occurred most often during August.² The injuries could be due to the application of protective clothing and the lack of the participants’ acclimatization. Protective clothing impedes the wearer’s ability to balance heat production with heat dissipation.

In 2010, Armstrong and colleagues suggested that the weight of a football uniform increases heat production.³ And the insulation provided by a football uniform reduces heat dissipation to the surrounding air, decreasing heat loss.³ Additionally, this same study indicated that the more protective gear the subject used, the greater the heat stress.³ The most challenging environment for heat injury is PPE due to the inability to facilitate any heat loss. In 2011, Caldwell and colleagues observed that wearing torso armor increased body temperature 10.8% faster than that of the control group, and those wearing full armor increased body temperature 38% faster than that of the control group.⁴ And it was proposed that 60% of this heat effect was from wearing the combat helmet.⁴

The inability to dissipate heat, particularly in protective gear, results in degradation of the effectiveness of the individual and, if left unchecked, may lead to death. Methods exist for health care providers to assess, intervene, and treat populations with heat injuries. These methods include but are not limited to vital signs (blood pressure [BP], body temperature, respiration rate), history of previous heat injury, medications (over-the-counter and prescription), and mental status.

Heat Injuries

Heat injuries are generally divided into 3 categories defined by their severity: heat stress, heat exhaustion, and heat stroke. Heat injuries are due to the individual’s inability to dissipate heat. As the severity of the heat exposure continues, the individual will experience heat stress, and if decompensation continues, the individual will progress to heat exhaustion and finally heat stroke.

If the individual’s physiology is limited or if compensatory mechanisms are compromised, heat stress may occur. Heat compensation can be retarded by any number of the following (including but not limited to): humidity, previous heat injury, lack of sleep, medications, sedentary lifestyle, obesity, caffeinated energy drinks, and dehydration.

In the early phases of heat stress, an individual’s vital signs will increase to compensate for the increase in body heat. Heat exchange is dependent on gradients of temperature and humidity, and as temperature and humidity increase, the ability to transfer heat decreases and becomes less efficient. Failure to accommodate for the increased heat generated and transferred will inevitably result in heat injury.

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Working in a hazmat environment in PPE is the worst possible heat transfer scenario due to the inability to use evaporation, the primary means by which heat is released from the body. In this scenario, heat injuries can become dangerous and even fatal if monitoring of vital signs and uncompensated heat production is allowed to continue. As the heat insult progresses from heat stress to heat exhaustion and heat stroke, the core temperature, heart rate, and BP continue to increase. Also, during the progression of heat injury, mental status changes often begin to occur. In 2012, Morley and colleagues found that firefighters wearing protective clothing demonstrated a neurocognitive decline after 50 minutes of treadmill exercise, but these performance declines were not noted until 1 hour or more following the exercise.⁵

Mental status change is a key diagnostic factor that indicates the progression of the patient from heat stress to heat exhaustion and from heat exhaustion to heat stroke. As the hyperthermia progresses, vital signs increase, and the patient’s mental status will begin to deteriorate. If the hyperthermia advances from heat exhaustion to heat stroke, hospitalization is required to reverse the condition. If homeostasis is not restored, the patient may die.