Abstract

Over the 5-year surveillance period, from July 2019 through June 2024, the crude incidence rate of any cold weather injury was 31.1 per 100,000 person-years (p-yrs) for the active component and 6.4 per 100,000 persons for the reserve component. From July 2023 through June 2024, a total of 456 members of the active (n=403) and reserve (n=53) components of the U.S. Armed Forces had at least 1 cold weather injury. During the 2023-2024 cold season, the rates of any cold weather injury increased slightly for the active (31.1 per 100,000 p-yrs) and reserve (6.4 per 100,000 persons) components compared to the previous cold season. The rate of any cold weather injury varied among the services for the 2023-2024 cold season, with higher rates observed in the Army and the Marine Corps. Over the entire surveillance period, active component service member cold weather injury rates were higher among males, non-Hispanic Black or African Americans, and those under 20 years old.

What are the new findings?

The crude incidence rate for cold weather injuries among all active component service members (31.1 per 100,000 p-yrs, 2023-2024), increased by 8.4% from the injury rate observed last season (28.7 per 100,000 p-yrs, 2022-2023) but remains unchanged from the rate observed during the 5-year surveillance period (31.1 per 100,000 p-yrs, 2019-2024). During the 2023-2024 cold season, active component Air Force personnel experienced their highest rate of any cold weather injury of the 5-year surveillance period.

What is the impact on readiness and force health protection?

Despite the terminology, cold weather injuries can occur in a variety of conditions, and in much warmer temperatures than expected, particularly when operating or training in wet or aquatic environments. It is essential that both service members and their leadership understand the hazards in their environments, the health risks those hazards pose, and prevention strategies to combat them (e.g., weather-appropriate clothing, clean and dry socks and footwear, and proper protective gear for extremities).

Background

Cold weather injuries are of significant military concern due to potential effects on service members (e.g., morbidity and potential disability) and the total force (e.g., adverse impacts on operations and costs of treatment).^1,2 In response, the U.S. Armed Forces have developed, and are continually improving, their training, doctrine, procedures, and protective equipment and clothing to counter the threat of cold environments.^3-6 Although these measures are effective when properly implemented, cold weather injuries continue to affect hundreds of service members each cold season due to exposures to both cold and wet environments.^7,8

Cold weather injuries can be broadly categorized in two major groups: those with a central effect, and those primarily affecting the body’s periphery. Hypothermia occurs if the body cannot maintain a core temperature at or above 95°F. If skin temperatures reach 95°F, the body’s physiological response is initiated to minimize loss of heat and maintain the core temperature for vital organ protection.^9,10 This response is achieved by decreasing blood flow to the extremities and redistributing warm blood to the core.^9-11 Lack of blood flow to the extremities, even before a drop in core temperature, is the leading cause of peripheral cold injuries.

Initially, hypothermia may impair cognition (e.g., confusion, slurred speech, memory loss), heart rate, and breathing. Severe hypothermia can lead to loss of consciousness, pulmonary edema, coma, ventricular arrhythmias (including ventricular fibrillation), and asystole.^10,12,13 Freezing atmospheric temperatures are not required to produce hypothermia, particularly when water immersion is involved. Because heat loss occurs 2 to 5 times faster in water compared to air, core body temperature can start to drop in water temperatures as warm as 80°F.¹⁰

Peripheral cold injuries mainly affect the hands, feet, and face, and can be further classified as either freezing injuries, such as frostbite, or non-freezing injuries, such as immersion foot. Freezing peripheral injury is defined as the damage sustained by tissues when skin temperatures fall below freezing, most frequently affecting tissues of the ears, nose, cheeks, chin, fingers, and toes.^10,11,14-16 A substantial proportion of patients with peripheral frostbite experience permanent changes in their microcirculation and disruption of localized nerve functions (e.g., reduced sensation in the affected area).¹⁵ Although most frostbite damage is minor, severe injury may lead to impaired functioning and inability to perform occupational tasks due to cold hypersensitivity, chronic ulceration, vasospasm, localized osteoarthritis, or chronic pain.^11,15,17

Non-freezing peripheral injury includes a spectrum of localized injuries to the soft tissues, nerves, and vasculature of distal extremities that result from prolonged exposure to wet, cold (generally 32-59°F) conditions; the injury process is generally slower in warmer water.^10,11,14,18 Although most non-freezing peripheral injuries involve feet, any body part can be affected by the condition, including hands.¹⁹ When immersion foot occurs, the foot becomes hyperemic (i.e., increased blood flow), painful, and swollen with continuous exposure; progression to blistering, decreased blood flow, ulceration, and gangrene is gradual.^11,18,20

Environmental factors that increase risk of cold weather injury include prolonged outdoor exposure to temperatures 40°F and lower, wind speeds exceeding 5 miles per hour, high altitudes, geographic location, wet conditions due to rain or snow, and submersion in water.¹⁹ Situational factors that increase risk of cold weather injury include type of physical activity, inadequate shelter, and inappropriate clothing, including—specifically for non-freezing peripheral injuries of the foot—immobility, wet socks, and constrictive boots.^20-22 Individual risk factors vary and include prior cold weather injury, dehydration, fatigue, improper acclimatization, inadequate nutrition, alcohol use, smoking, chronic disease (e.g., peripheral vascular disease, diabetes), and medications that impair compensatory responses (e.g., oral antihyperglycemics, beta-blockers, general anesthetic agents).^{10,11,16,20-22}

Continuous surveillance of cold weather injuries is essential to understand the magnitude of the risk they pose, inform prevention efforts, and remind leaders of the hazards of training and operating in wet and cold environments. Department of Defense guidelines for reportable medical events require reporting of cases of hypothermia, freezing peripheral injuries (e.g., frostbite), and non-freezing peripheral injuries (e.g., immersion injuries, chilblains).²³

Since 2004, MSMR has published annual updates on the incidence of cold weather injuries affecting U.S. Armed Force members for the five most recent cold seasons.²⁴ The timing of these annual updates is intended to call attention to the recurring risks of such injuries as winter approaches in the Northern Hemisphere, where most members of the U.S. Armed Forces are assigned. This 2024 report addresses the occurrence of frostbite, immersion hand and foot injuries, and hypothermia during the cold seasons from July 2019 through June 2024.

Methods

This surveillance population included all individuals who served in the active or reserve components of the U.S. Armed Forces at any time during the surveillance period of July 1, 2019 through June 30, 2024. For analysis purposes, a cold season was defined as July 1 through June 30 intervals so complete cold weather seasons could be represented in annual summaries and comparisons. Service members in the Space Force were classified separately from the Air Force for the 2022-2023 and 2023-2024 cold seasons as a result of complete data availability for the newly formed service; for previous cold seasons they were classified as Air Force.

Records of cold weather injuries for freezing peripheral injuries (i.e., frostbite), non-freezing peripheral injuries (i.e., immersion hand and foot injuries), and hypothermia were identified from two sources: 1) RMEs submitted to the Disease Reporting System internet and 2) diagnostic codes from inpatient and outpatient medical encounters in the Defense Medical Surveillance System and in-theater records from the Theater Medical Data Store. A cold weather injury case was defined by the presence of an RME or one of any qualifying International Classification of Diseases, 9th or 10th revision (ICD-9/ICD-10) codes in the first diagnostic position of a record of a health care encounter (Table 1).

To estimate the number of unique individuals who experienced a cold weather injury each cold season, and to avoid counting follow-up health care encounters, only one cold weather injury per individual per season was included in the counts of ‘any cold weather injury’. To count specific types of cold weather injury, namely frostbite, immersion hand and foot, and hypothermia cases, one of each type of cold weather injury per individual per season could be included in the counts of ‘all cold weather injuries’. For example, if an individual was diagnosed or reported with an immersion injury at one point during a cold season and then with frostbite later in the same cold season, each of those different injury types would be included in the injury-specific calculations. If a service member had multiple medical encounters for the same cold weather injury, only one encounter was used for analysis. The hospitalization encounter was prioritized over the ambulatory visit.

Annual seasonal incidence rates of cold weather injuries among active component service members were calculated as incident cold weather injury diagnoses per 100,000 person-years of service. Annual seasonal incidence rates of cold weather injuries among reservists were calculated as cases per 100,000 persons using the total number of reserve component service members for each cold season of the surveillance period. Counts of persons in the reserves were used as the denominator in these calculations because information on the start and end dates of active duty service periods of reserve component members is unavailable, so person-time cannot be accurately calculated.

Cold weather injuries are summarized by the locations where service members were treated for those injuries, identified by a Defense Medical Information System Identifier of a health care encounter. Because such injuries can occur during field training, temporary duty, or outside usual duty stations, DMIS ID was used as a proxy for the location where the cold weather injury occurred.

Results

2023-2024 cold season

From July 2023 through June 2024, a total of 456 members of the active (n=403) and reserve (n=53) components had at least 1 cold weather injury (Table 2, 1 per person per cold season). In the active component, soldiers had the highest rate of any cold weather injury (n=233, 52.6 per 100,000 p-yrs) during the 2023-2024 cold season, followed by members of the Marine Corps (n=68, 40.5 per 100,000 p-yrs), Air Force (n=67, 21.4 per 100,000 p-yrs), and Navy (n=30, 9.2 per 100,000 p-yrs). One active component Space Force member (11.1 per 100,000 p-yrs) and four active component Coast Guard members (10.2 per 100,000 p-yrs) were affected by cold weather injuries during the 2023-2024 cold season (Table 2, Figure 1). Within the reserve component, Army personnel accounted for over half of the cases (n=29, 5.3 per 100,000 persons) in 2023-2024 (Table 2, Figure 2), although Reservists in the Marine Corps (n=10, 26.7 per 100,000 persons) had higher rates of cold weather injuries.

Within the different services in 2023-2024, frostbite was the most common type of cold weather injury among active component Army (n=125, 45.5%), Navy (n=20, 66.7%), and Air Force (n=47, 67.1%) members (Tables 3a–3c), whereas immersion injury was the most common type of cold weather injury among the Marine Corps active component (n=30, 44.1%) (Table 3d).

Five cold seasons: July 2019–June 2024

For all five cold seasons, the crude incidence rate of any cold weather injury for all ACSMs was 31.1 per 100,000 p-yrs (Table 2). For the most recent cold season, the crude incidence rate of any cold weather injury for all ACSMs increased by 8.4% (from 28.7 per 100,000 p-yrs in 2022-2023 to 31.1 per 100,000 p-yrs in 2023-2024) compared to the prior cold season (Table 2). Throughout the surveillance period, cold weather injury rates remained consistently higher among ACSMs in the Army and Marine Corps (Figure 1). During the 5-year surveillance period, the crude incidence rate of any cold weather injury for the reserve component was 6.4 per 100,000 persons (Table 2).

During the most recent current cold season, the crude incidence rates of any cold weather injury increased by 9.1% (from 5.9 per 100,000 persons in 2022-2023 to 6.4 per 100,000 persons in 2023-2024) compared to the prior cold season (Table 2).

Frostbite was the most common cold weather injury type among ACSMs during the first three cold seasons of the surveillance period, except among active component sailors, for whom hypothermia resulted in the highest cold weather injury rate during the 2020-2021 and 2021-2022 seasons (Tables 3a–3e). The rate of immersion injury increased above the rate of frostbite injury for active component members of the Marine Corps during the 2022-2023 and 2023-2024 cold seasons (Table 3d). The rate of immersion injury also increased for Army active component members during the last two cold seasons, although the immersion injury rate in 2023-2024 (26.4 per 100,000 p-yrs) remained just below the rate of frostbite (28.2 per 100,000 p-yrs) (Table 3a).

During the 5-year surveillance period, overall rates of all cold weather injuries in the active component were generally higher among service members who were male, non-Hispanic Black or African American, and in the two youngest age groups (<20 and 20-24 years old) (Tables 3a–3d). When specific types of cold weather injury were considered, male and non-Hispanic Black or African American personnel had higher rates of frostbite in comparison to other types of cold weather injury (Tables 3a–3d). Among all cold weather injury cases reported within the active component during the 5-year period, the Marine Corps demonstrated the highest recruit cold weather injury rate (182.7 per 100,000 p-yrs). In all services, enlisted personnel had higher rates of cold weather injury compared to officers (Tables 3a–3e).

Throughout the 5-year surveillance period, a total of 37 ACSMs (1.8% of the total) were hospitalized. Of the 37 active component hospitalizations, hypothermia (n=18) and frostbite (n=18) were equally represented, while only one hospitalization was due to immersion injury. The Army (n=25) and Marine Corps (n=7) accounted for a majority (86.5%) of hospitalized cases (data not shown).

Cold weather injuries during deployments

During the 5-year surveillance period a total of 65 cold weather injuries were diagnosed among service members deployed outside the U.S. (data not shown), of which 25 (38.5%) were frostbite, 30 (46.2%) were immersion injuries, and 10 (15.4%) were hypothermia. Approximately one-third (n=22) of all 65 deployment-associated cold weather injuries were diagnosed during the 2023-2024 cold season. Immersion injuries accounted for over three-quarters (n=18, 81.8%) of the cold weather injuries identified in service members deployed outside the U.S. during the 2023-2024 cold season.

Cold weather injuries by geographic location

During the 5-year surveillance period, 17 military locations reported at least 25 incidents of cold weather injury (1 per person per cold season) among ACSMs. Figure 3 charts the 2023-2024 seasonal number of cold weather injuries (1 per person per year) in addition to the median case numbers for the previous four cold seasons for each of those 17 locations. The highest 5-year counts of incident cold weather injuries for seasons 2019-2024 were recorded at Fort Wainwright, AK (n=282), Joint Base Elmendorf-Richardson, AK (n=181), Fort Carson, CO (n=87), Marine Corps Base Camp Lejeune, NC (n=79), and Fort Moore, GA (n=75) (data not shown).

Discussion

The overall rate of any cold weather injury in 2023-2024 for the active and reserve components increased by 8.4% and 9.1%, respectively, from the previous cold season. The rate increase for the current cold season was most pronounced in the Marine Corps active component (21.3%) and reserve component (163.3%). The Coast Guard and Space Force average less than five cases per year among their ACSMs, thus, small changes in the numbers of cases annually will result in abnormally large fluctuations in the injury rate. During the 2023-2024 cold season, the Air Force experienced its highest rate of any cold weather injury (21.4 per 100,000 p-yrs) over the 5-year surveillance period.

For the first 3 years of the 5-year surveillance period, the most common cold weather injury observed in ACSMs overall was frostbite, but in the last 2 years of the surveillance period immersion injury rates were higher for Marine Corps service members. This change in injury type prevalence could indicate a shift in environmental risk factors, but it does not signal a shift in injury severity. The long-term complications of non-freezing injury are similar to, and equally debilitating as, those produced by frostbite: hypersensitivity to cold, chronic pain, and severe pain induced by walking.^17,18,20

Similar to previous MSMR reports, the highest cold weather injury rates were observed among male, younger age group, and non-Hispanic Black or African American service members.^8,24 Higher rates of cold weather injury have also been noted among service members in the United Kingdom military with similar demographic characteristics.^21,25,26 Differences in physiological responses to cold stress have been observed between different racial and ethnic groups, with individuals of African descent demonstrating greater vasoconstriction responses compared to individuals of Asian or Caucasian descent.^10,15,27 Furthermore, signs and symptoms of cold weather injuries (e.g., skin redness, blotchy skin, waxy and white skin) may initially be more difficult to see on service members with skin of color.^28,29 Service members, leadership, and medical personnel should be educated on the early signs and symptoms of cold weather injuries for a wide range of skin types.

When examining the service-specific demographic groups with increased rates, it should be noted that there were differences in the most frequently observed cold weather injury type. Younger marines had higher rates of hypothermia and younger soldiers had higher rates of frostbite compared to other cold weather injury types. These differences could indicate different situational risk factors for cold weather injury within the services, for example, training activities, occupational tasks, and geographic region. A study of U.K. service personnel noted that the most common situational risk factors for non-freezing peripheral injury were standing guard, as well as wet socks and boots.²¹ Unit leaders must be able to assess environmental, situational, and individual risk factors of their training and operational settings and understand how those factors increase risk of cold weather injuries for service members in their charge.

It should be noted that this analysis of cold weather injuries was unable to distinguish between injuries sustained during official military duties (e.g., training or operations) and those associated with unrelated or personal activities. In addition, the personnel files from the Defense Manpower Data Center used to calculate the population estimates for the active and reserve components as well as the demographic data presented in Tables 3a–3e for the active component were unavailable for May and June 2024; the duty statuses of all service members, active and reserve, in April 2024 were assumed to be their May and June 2024 statuses. It is likely that some individuals in the U.S. Armed Forces both joined and left service during those months, and those movements are unaccounted for in the population estimates. Likewise, it is possible some time-varying demographics (e.g., age and rank) changed for individuals in May and June 2024, compared to April, and those shifts in categories are unaccounted. In all instances, however, the effect on the rates shown throughout the report should be minimal due to the large population size.

Cold weather injuries can be prevented by ensuring proper clothing, including layers that can be added or removed according to environmental conditions and physical activity, along with footwear that is non-constrictive, dry, and regularly changed if wet.^9,10,22 Maintenance of proper hydration and nutrition, avoidance of long periods of sedentary or immobile positions, and planning for appropriate shelter and opportunities for re-warming are also important.

Military training or mission requirements in cold and wet weather conditions can preclude immediate warm or dry shelter, ability to change wet or damp clothing, or even healthy physical activity.^2,3,11 To prepare for all circumstances posing a threat for cold weather injury, service members should be cognizant of, and able to identify, signs of cold weather injury in addition to environmental, individual and situational risk factors. Service members should also be aware of protective measures for themselves and their fellow service members, whether during training, operations, combat, or recreational activities in wet or freezing conditions.

Authors’ Affiliation

Disease Epidemiology Program, Defense Centers for Public Health–Aberdeen, Defense Health Agency, Aberdeen Proving Ground, MD

Disclaimer

The views expressed in this presentation are those of the authors and do not necessarily reflect the official policy of the Department of Defense, Defense Health Agency, nor the U.S. Government.

References

1. Candler WH, Freedman MS. Military medical operations in cold environments. In: Pandolf KB, Burr RE, eds. Medical Aspects of Harsh Environments, Volume 1. Office of the Surgeon General;2001:553-566.
2. Paton BC. Cold, casualties, and conquests: the effects of cold on warfare. In: Pandolf KB, Burr RE, eds. Medical Aspects of Harsh Environments, Volume 1. Office of the Surgeon General;2001:313-349.
3. Department of the Army Headquarters. Technical Bulletin Medical 508. Technical Bulletin: Prevention and Management of Cold-Weather Injuries. U.S. Dept. of Defense. Apr. 1, 2005. Accessed Oct. 23, 2024. https://apps.dtic.mil/sti/tr/pdf/ADA433435.pdf
4. Department of the Army, U.S. Army Training and Doctrine Command Headquarters. TRADOC Regulation 350-29. Training: Prevention of Heat and Cold Casualties. U.S. Dept. of Defense. Jun. 15, 2023. Accessed Oct. 23, 2024. https://adminpubs.tradoc.army.mil/regulations/tr350-29.pdf
5. Department of the Army, U.S. Army Training and Doctrine Command Headquarters. TRADOC Regulation 350-6. Training: Enlisted Initial Entry Training Policies and Administration. U.S. Dept. of Defense. Aug. 9, 2019. Accessed Oct. 2024. https://adminpubs.tradoc.army.mil/regulations/tr350-6.pdf
6. Castellani JW, O’Brien C, Baker-Fulco C, Sawka MN, Young AJ. Sustaining Health & Performance in Cold Weather Operations. Technical Note TN/02-2. U.S. Army Research Institute of Environmental Medicine. Oct. 2001. Accessed Oct. 23, 2024. https://apps.dtic.mil/sti/pdfs/ADA395745.pdf
7. DeGroot DW, Castellani JW, Williams JO, Amoroso PJ. Epidemiology of U.S. Army cold weather injuries, 1980–1999. Aviat Space Environ Med. 2003;74(5):564-570.
8. Armed Forces Health Surveillance Branch. Update: cold weather injuries, active and reserve component, U.S. Armed Forces, July 2013–June 2018. MSMR. 2018;25(11):10-17.
9. Bubnis MA, Hulsopple C. Human performance and injury prevention in cold weather environments. Curr Sports Med Rep. 2022;21(4):112-116. doi:10.1249/jsr.0000000000000946
10. Haman F, Souza SCS, Castellani JW, et al. Human vulnerability and variability in the cold: establishing individual risks for cold weather injuries. Temperature (Austin). 2022;9(2):158-195. doi:10.1080/23328940.2022.2044740
11. Kowtoniuk RA, Liu YE, Jeter JP. Cutaneous cold weather injuries in the U.S. military. Cutis. 2021;108(4):181-184. doi:10.12788/cutis.0363
12. Jolly BT, Ghezzi KT. Accidental hypothermia. Emerg Med Clin North Am. 1992;10(2):311-327.
13. Biem J, Koehncke N, Classen D, Dosman J. Out of the cold: management of hypothermia and frostbite. CMAJ. 2003;168(3):305-311.
14. Imray CHE, Oakley EHN. Cold still kills: cold-related illnesses in military practice freezing and non-freezing cold injury. J R Army Med Corps. 2005;151(4):218-222. doi:10.1136/jramc-151-04-02
15. Ervasti O, Hassi J, Rintamaki H, et al. Sequelae of moderate finger frostbite as assessed by subjective sensations, clinical signs, and thermophysiological responses. Int J Circumpolar Health. 2000;59(2):137-145.
16. Harirchi I, Arvin A, Vash JH, Zafarmand V. Frostbite: incidence and predisposing factors in mountaineers. Br J Sports Med. 2005;39(12):898-901. doi:10.1136/bjsm.2004.016097
17. Handford C, Thomas O, Imray CHE. Frostbite. Emerg Med Clin North Am. 2017;35(2):281-299. doi:10.1016/j.emc.2016.12.006
18. Hall A, Sexton J, Lynch B, et al. Frostbite and immersion foot care. Mil Med. 2018;183(suppl 2):168-171. doi:10.1093/milmed/usy085
19. van Dongen TTCF, Berendsen RR, de Jong FJM, et al. Frostbite: a systematic review on freezing cold injuries in a military environment. BMJ Mil Health. 2023:e002171. doi:10.1136/military-2022-002171
20. Imray C, Grieve A, Dhillon S, Caudwell, Xtreme Everest Research Group. Cold damage to the extremities: frostbite and non-freezing cold injuries. Postgrad Med J. 2009;85(1007):481-488. doi:10.1136/pgmj.2008.068635
21. Kuht JA, Woods D, Hollis S. Case series of non-freezing cold injury: epidemiology and risk factors. J R Army Med Corps. 2019;165(6):400-404. doi:10.1136/jramc-2018-000992
22. Imray CHE, Richards P, Greeves J, Castellani JW. Nonfreezing cold-induced injuries. J R Army Med Corps. 2011;157(1):79-84. doi:10.1136/jramc-157-01-14
23. Armed Forces Health Surveillance Branch, Defense Health Agency, U.S. Air Force School of Aerospace Medicine, Army Public Health Center, Navy and Marine Corps Public Health Center. Armed Forces Reportable Medical Events: Guidelines and Case Definitions. U.S. Dept. of Defense. Oct. 2022. Accessed Nov. 19, 2024. https://www.med.navy.mil/Portals/62/Documents/NMFA/NMCPHC/root/Documents/program-and-policysupport/DRSI/Armed-Forces-Reportable-Medical-Events-Oct-2022.pdf
24. Army Medical Surveillance Activity. Cold injuries, active duty, U.S. Armed Forces, July 1999–June 2004. MSMR. 2004;10(5):2-10.
25. Burgess JE, Macfarlane F. Retrospective analysis of the ethnic origins of male British Army soldiers with peripheral cold weather injury. J R Army Med Corps. 2009;155(1):11-15. doi:10.1136/jramc-155-01-04
26. Heil KM, Oakley EHN, Wood AM. British Military freezing cold injuries: a 13-year review. J R Army Med Corps. 2016:162(6):413-418. doi:10.1136/jramc-2015-000445
27. Maley MJ, Eglin CM, House JR, Tipton MJ. The effect of ethnicity on the vascular responses to cold exposure of the extremities. Eur J Appl Physiol. 2014;114(11):2369-2379. doi:10.1007/s00421-014-2962-2
28. Taylor SC. Diagnosing skin diseases in skin of color. Dermatol Clin. 2023;41(3):xiii-xv. doi:10.1016/j.det.2023.03.001
29. Ohanenye C, Taliaferro S, Callendar VD. Diagnosing disorders of facial erythema. Dermatol Clin. 2023:41(3):377-392. doi:10.1016/j.det.2023.02.004