Abstract

In 2021, there were 488 incident cases of heat stroke and 1,864 incident cases of heat exhaustion among active component service members of the U.S. Armed Forces. The unadjusted annual rates of incident heat stroke and heat exhaustion peaked in 2018 and then declined in 2019 and 2020. Between 2020 and 2021, the rate of incident heat stroke was relatively stable (0.37 cases per 1,000 person-years [p-yrs]) while the rate of heat exhaustion increased slightly (1.40 cases per 1,000 p-yrs). In 2021, subgroup-specific rates of incident heat stroke and heat exhaustion were highest among male service members, those less than 20 years old, Marine Corps and Army members, recruit trainees, and those in combat-specific occupations. During 2017–2021, a total of 312 heat illnesses were documented among service members in the U.S. Central Command (CENTCOM) area of responsibility (AOR); 6.4% (n=20) were diagnosed as heat stroke. Commanders, small unit leaders, training cadre, and supporting medical personnel must ensure that the military members whom they supervise and support are informed about the risks, preventive countermeasures, early signs and symptoms, and first-responder actions related to heat illnesses.

What are the new findings?

From 2020 to 2021, the rate of incident heat stroke was relatively stable while the rate of heat exhaustion increased slightly. The annual numbers of heat illnesses diagnosed in the CENTCOM AOR have generally trended downward since 2017. Sizeable proportions of heat stroke and heat exhaustion cases were not identified by way of mandatory reports through the Disease Reporting System internet (DRSi).

What is the Impact on Readiness and Force Health Protection?

Heat illness can degrade operational readiness by causing considerable morbidity, particularly during training of recruits and of soldiers and Marines in combat arms specialties. This analysis demonstrates again the magnitude of risks of heat illnesses among active component service members and the enhanced risks associated with sex, age, location of assignment, and occupational categories. Recognition of these risk factors should inform the preventive measures that military leaders, trainers, and service members routinely employ.

Background

Heat illness refers to a group of disorders that occur when the elevation of core body temperature surpasses the compensatory limits of thermoregulation.1 Heat illness is the result of environmental heat stress and/or exertion and represents a set of conditions that exist along a continuum from less severe (heat exhaustion) to potentially life threatening (heat stroke).

Heat exhaustion is caused by the inability to maintain adequate cardiac output because of strenuous physical exertion and environmental heat stress.^1,2 Acute dehydration often accompanies heat exhaustion but is not required for the diagnosis.3 The clinical criteria for heat exhaustion include a core body temperature greater than 100.5ºF/38ºC and less than 104ºF/40ºC at the time of or immediately after exertion and/or heat exposure, physical collapse at the time of or shortly after physical exertion, and no significant dysfunction of the central nervous system. If any central nervous system dysfunction develops (e.g., dizziness or headache), it is mild and rapidly resolves with rest and cooling measures (e.g., removal of unnecessary clothing, relocation to a cooled environment, and oral hydration with cooled, slightly hypotonic solutions).^1–4 

Heat stroke is a debilitating illness characterized clinically by severe hyperthermia (i.e., a core body temperature of 104ºF/40ºC or greater), profound central nervous system dysfunction (e.g., delirium, seizures, or coma), and additional organ and tissue damage.^1,4,5 The onset of heat stroke should prompt aggressive clinical treatments, including rapid cooling and supportive therapies such as fluid resuscitation to stabilize organ function.^1,5 The observed pathologic changes in several organ systems are thought to occur through a complex interaction between heat cytotoxicity, coagulopathies, and a severe systemic inflammatory response.^1,5 Multiorgan system failure is the ultimate cause of mortality due to heat stroke.⁵

Timely medical intervention can prevent milder cases of heat illness (e.g., heat exhaustion) from becoming severe (e.g., heat stroke) and potentially life threatening. However, even with medical intervention, heat stroke may have lasting effects, including damage to the nervous system and other vital organs and decreased heat tolerance, making an individual more susceptible to subsequent episodes of heat illness.6–8 Furthermore, the continued manifestation of multiorgan system dysfunction after heat stroke increases patients’ risk of mortality during the ensuing months and years.^9,10 

Strenuous physical activity for extended durations in occupational settings as well as during military operational and training exercises exposes service members to considerable heat stress because of high environmental heat and/or a high rate of metabolic heat production.^11,12 In some military settings, wearing needed protective clothing or equipment may make it biophysically difficult to dissipate body heat.^13,14 The resulting body heat burden and associated cardiovascular strain reduce exercise performance and increase the risk of heat-related illness.^11,15 

Over many decades, lessons learned during military training and operations in hot environments as well as a substantial body of research findings have resulted in doctrine, equipment, and preventive measures that can significantly reduce the adverse health effects of military activities in hot weather.^16–22 Although numerous effective countermeasures are available, heat-related illness remains a significant threat to the health and operational effectiveness of military members and their units and accounts for considerable morbidity, particularly during recruit training in the U.S. military.^11,23,24 Moreover, with the projected rise in the intensity and frequency of extreme heat conditions associated with global climate change, heat-related illnesses will likely represent an increasing challenge to the military.^25–28

In the U.S. Military Health System (MHS), the most serious types of heat-related illness are considered notifiable medical events. Notifiable cases of heat illness include heat exhaustion and heat stroke. All cases of heat illness that require medical intervention or result in change of duty status are reportable.⁴ 

This report summarizes reportable medical events of heat illness as well as heat illness-related hospitalizations and ambulatory visits among active component service members during 2021 and compares them to the previous 4 years. Episodes of heat stroke and heat exhaustion are summarized separately.

Methods

The surveillance period was January 2017 through December 2021. The surveillance population included all individuals who served in the active component of the Army, Navy, Air Force, or Marine Corps at any time during the surveillance period. All data used to determine incident heat illness diagnoses were derived from records routinely maintained in the Defense Medical Surveillance System (DMSS). These records document both ambulatory health care encounters and hospitalizations of active component service members of the U.S. Armed Forces in fixed military and civilian (if reimbursed through the MHS) treatment facilities worldwide. In-theater diagnoses of heat illness were identified from medical records of service members deployed to Southwest Asia or the Middle East and whose health care encounters were documented in the Theater Medical Data Store. Because heat illnesses represent a threat to the health of individual service members and to military training and operations, the Armed Forces require expeditious reporting of these reportable medical events through any of the service-specific electronic reporting systems; these reports are routinely transmitted and incorporated into the DMSS. 

For this analysis, a case of heat illness was defined as an individual with 1) a hospitalization or outpatient medical encounter with a primary (first-listed) or secondary (second-listed) diagnosis of heat stroke (International Classification of Diseases, 9th Revision [ICD-9]: 992.0; International Classification of Diseases, 10th Revision [ICD-10]: T67.0*) or heat exhaustion (ICD-9: 992.3–992.5; ICD-10: T67.3*–T67.5*) or 2) a reportable medical event record of heat exhaustion or heat stroke.²⁹ Because of an update to the Disease Reporting System internet (DRSi) medical event reporting system in July 2017, the type of reportable medical events for heat illness (i.e., heat stroke or heat exhaustion) could not be distinguished using reportable medical event records in DMSS data. Instead, information on the type of reportable medical event for heat illness during the entire 2017–2021 surveillance period was extracted directly from the records of the DRSi. It is important to note that MSMR analyses carried out before 2018 included diagnosis codes for other and unspecified effects of heat and light (ICD-9: 992.8 and 992.9; ICD-10: T67.8* and T67.9*) within the heat illness category “other heat illnesses.” These codes were excluded from the current analysis and the April MSMR analyses of 2018–2021. 

Each individual could be considered an incident case of heat illness only once per calendar year. If an individual had a diagnosis for both heat stroke and heat exhaustion during a given year, only 1 diagnosis was selected, prioritizing heat stroke over heat exhaustion. Encounters for each individual within each calendar year then were prioritized in terms of record source with hospitalizations prioritized over reportable events, which were prioritized over ambulatory visits. Incidence rates were calculated as incident cases of heat illness per 1,000 person-years (p-yrs) of active component service. Percent change in incidence was calculated using unrounded rates. 

For surveillance purposes, recruit trainees were identified as active component members who were assigned to service-specific training locations during the relevant basic training periods (e.g., 8 weeks for Navy basic training). Recruit trainees were considered a separate category of enlisted service members in summaries of heat illnesses by military grade overall. 

Records of medical evacuations from the U.S. Central Command (CENTCOM) area of responsibility (AOR) (i.e., Southwest Asia/Middle East) to a medical treatment facility outside the CENTCOM AOR were analyzed separately. Evacuations were considered case defining if affected service members had at least 1 inpatient or outpatient heat illness medical encounter in a permanent military medical facility in the U.S. or Europe from 5 days before to 10 days after their evacuation dates.

It should be noted that medical data from sites that were using the new electronic health record for the Military Health System, MHS GENESIS, between July 2017 and October 2019 are not available in the DMSS. These sites include Naval Hospital Oak Harbor, Naval Hospital Bremerton, Air Force Medical Services Fairchild, and Madigan Army Medical Center. Therefore, medical encounter data for individuals seeking care at any of these facilities from July 2017 through October 2019 were not included in the current analysis.

Results

In 2021, there were 488 incident cases of heat stroke and 1,864 incident cases of heat exhaustion among active component service members (Table 1). The crude overall incidence rates of heat stroke and heat exhaustion were 0.37 and 1.40 per 1,000 p-yrs, respectively. In 2021, subgroup-specific incidence rates of heat stroke were highest among male service members, those less than 20 years old, those of other/unknown race/ethnicity (includes American Indian/Alaska Native service members, Asian/Pacific Islander service members, and those of unknown race/ethnicity), Marine Corps and Army members, recruit trainees, and those in combat-specific occupations (Table 1). The overall rate of heat stroke among female service members was 43.1% lower than the rate among male service members. The overall rates of incident heat stroke among Marine Corps and Army members were more than 8 times the rates among Air Force and Navy members. There were only 25 cases of heat stroke reported among recruit trainees, but their incidence rate was more than 2.5 times that of other enlisted service members and officers. 

The crude overall incidence rate of heat exhaustion among female service members was slightly (8.1%) lower than the rate among males (Table 1). In 2021, compared to their respective counterparts, service members less than 20 years old, Marine Corps and Army members, recruit trainees, and service members in combat-specific occupations had notably higher overall rates of incident heat exhaustion. 

Crude (unadjusted) annual incidence rates of heat stroke increased slightly from 0.42 per 1,000 p-yrs in 2017 to 0.46 cases per 1,000 p-yrs in 2018, but then dropped to 0.36 cases per 1,000 p-yrs in 2020 before leveling off at 0.37 per 1,000 p-yrs in 2021 (Figure 1). In the last year of the surveillance period, there were fewer heat stroke-related ambulatory visits than in any of the previous 4 years. The proportions of total heat stroke cases from hospitalizations remained relatively stable during 2017–2021 (range: 23.6%–28.1%). The proportions of total heat stroke cases from reportable medical events ranged from 24.9% to 34.5% and the proportions from ambulatory visits varied between 41.8% and 48.8%.

Crude annual rates of incident heat exhaustion increased from 1.47 per 1,000 p-yrs in 2017 to a peak of 1.74 per 1,000 p-yrs in 2018, fell to 1.26 per 1,000 p-yrs in 2020, and then increased to 1.40 per 1,000 p-yrs in 2021. (Figure 2). During the 5-year surveillance period, the proportions of total heat exhaustion cases from reportable medical events fluctuated between 33.5% and 40.9% and the proportions of cases from ambulatory visits varied between 57.4% and 65.0%. However, the proportions of heat exhaustion cases from hospitalizations remained relatively stable (range: 1.4%–3.2%). 

Heat Illnesses by Location

During the 5-year surveillance period, a total of 12,477 heat-related illnesses were diagnosed at more than 250 military installations and geographic locations worldwide (Table 2). Of the total heat illness cases, 5.6% occurred outside of the U.S., including 284 in Okinawa and 418 at 59 other locations in Europe, East Asia, Southwest Asia, Africa, and Cuba. Four Army installations in the U.S. accounted for more than one-third (35.1%) of all heat illnesses during the period: Fort Benning, GA (n=2,033); Fort Bragg, NC (n=936); Fort Campbell, KY (n=792); and Fort Polk, LA (n=614). Seven other locations accounted for an additional 29.1% of heat illness events: Marine Corps Base (MCB) Camp Lejeune/Cherry Point, NC (n=1,038); Marine Corps Recruit Depot Parris Island/Beaufort, SC (n=530); Naval Medical Center San Diego, CA (n=526); MCB Camp Pendleton, CA (n=457); Fort Hood, TX (n=407); MCB Quantico, VA (n=340); and Joint Base San Antonio-Lackland Air Force Base, TX (n=331). Of these 11 locations with the most heat illness events, 7 are located in the southeastern U.S. The 21 locations with more than 100 cases of heat illness accounted for over three-quarters (78.1%) of all active component cases during 2017–2021.

Heat Illnesses in the CENTCOM AOR

During the 5-year surveillance period, a total of 312 heat illnesses were diagnosed and treated in the CENTCOM AOR (i.e., Southwest Asia/Middle East) (Figure 3). Of the total cases of heat illness, 6.4%(n=20) were diagnosed as heat stroke. Deployed service members who were affected by heat illnesses were most frequently male (n=241; 77.2%); non-Hispanic White (n=183; 58.7%); 20–24 years old (n=168; 53.8%); in the Army (n=138; 44.2%); enlisted (n=301; 96.5%); and in repair/engineering (n=101; 32.4%) or communications/intelligence (n=65; 20.8%) occupations (data not shown). During the surveillance period, 2 service members were medically evacuated for heat illnesses from the CENTCOM AOR; 1 of the evacuations took place in spring (May 2017) and 1 in November 2020.

Editorial Comment

This annual update of heat illnesses among service members in the active component documented that the unadjusted annual rates of incident heat stroke and heat exhaustion peaked in 2018 and then declined in 2019 and 2020. Between 2020 and 2021, the rate of incident heat stroke was relatively stable while the rate of heat exhaustion increased slightly. 

There are significant limitations to this update that should be considered when interpreting the results. Similar heat-related clinical illnesses are likely managed differently and reported with different diagnostic codes at different locations and in different clinical settings. Such differences undermine the validity of direct comparisons of rates of nominal heat stroke and heat exhaustion events across locations and settings. Also, heat illnesses during training exercises and deployments that are treated in field medical facilities are not completely ascertained as cases for this report. In addition, recruit trainees were identified using an algorithm based on age, rank, loca­tion, and time in service. This method is only an approximation and likely resulted in some misclassification of recruit train­ing status. Moreover, it should be noted that the guidelines for mandatory reporting of heat illnesses were modified in the 2017 revision of the Armed Forces guidelines and case definitions for reportable medical events and carried into the 2020 revision.⁴ In this updated version of the guidelines and case definitions, the heat injury category was removed, leaving only case classifications for heat stroke and heat exhaustion. To compensate for such possible variation in reporting, the analysis for this update, as in previous years, included cases identified in DMSS records of ambulatory care and hospitalizations using a consistent set of ICD-9/ICD-10 codes for the entire surveillance period. However, it also is important to note that the exclusion of diagnosis codes for other and unspecified effects of heat and light (formerly included within the heat illness category “other heat illnesses”) in the current analysis precludes the direct comparison of numbers and rates of cases of heat exhaustion to the numbers and rates of “other heat illnesses” reported in MSMR updates before 2018. 

As has been noted in previous MSMR heat illness updates, results indicate that a sizable proportion of cases identified through DMSS records of ambulatory visits did not prompt mandatory reports through the reporting system.²³ However, this study did not directly ascertain the overlap between hospitalizations and reportable events and the overlap between reportable events and outpatient encounters. It is possible that cases of heat illness, whether diagnosed during an inpatient or outpatient encounter, were not documented as reportable medical events because treatment providers were not attentive to the criteria for reporting or because of ambiguity in interpreting the criteria (e.g., the heat illness did not result in a change in duty status or the core body temperature measured during/immediately after exertion or heat exposure was not available). Underreporting is especially concerning for cases of heat stroke because it may reflect insufficient attentiveness to the need for prompt recognition of cases of this dangerous illness and for timely intervention at the local level to prevent additional cases. 

In spite of its limitations, this report demonstrates that heat illnesses continue to be a significant and persistent threat to both the health of U.S. military members and the effectiveness of military operations. Of all military members, the youngest and most inexperienced Marine Corps and Army members (particularly those training at installations in the southeastern U.S.) are at highest risk of heat illnesses, including heat stroke, exertional hyponatremia, and exertional rhabdomyolysis (see the other articles in this issue of the MSMR). 

Commanders, small unit leaders, training cadre, and supporting medical personnel—particularly at recruit training centers and installations with large combat troop populations—must ensure that the military members whom they supervise and support are informed regarding the risks, preventive countermeasures (e.g., water consumption), early signs and symptoms, and first-responder actions related to heat illnesses.^16–22,^30–32 Leaders should be aware of the dangers of insufficient hydration on the one hand and excessive water intake on the other; they must have detailed knowledge of, and rigidly enforce countermeasures against, all types of heat illnesses. 

Policies, guidance, and other information related to heat illness prevention and sun safety among U.S. military members are available online through the Army Public Health Center website at: https://phc.amedd.army.mil/topics/discond/hipss/Pages/default.aspx.

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