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Carbon Monoxide Poisoning, Active and Reserve Component Service Members and Non-Service Member Beneficiaries of the Military Health System, U.S. Armed Forces, July 2009–June 2019

Combustion fumes from a car exhaust pipe. (iStock photo). Combustion fumes from a car exhaust pipe. (iStock photo).

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Medical Surveillance Monthly Report

ABSTRACT

Morbidity and mortality from carbon monoxide (CO) poisoning are important public health problems, but their full impact is difficult to estimate. The current analysis used the 2018 Council of State and Territorial Epidemiologist criteria and International Classification of Diseases (ICD) codebased data to classify CO poisoning cases by intent, source of exposure, and degree of certainty that poisoning was CO related. During July 2009–June 2019, there were 1,288 CO poisoning cases classified as confirmed/probable among active component service members, 366 among reserve component members, and 4,754 among non-service member beneficiaries. Service members working in repair/engineering occupations accounted for the greatest proportion of confirmed CO poisoning cases among active component members and the second greatest proportion among reserve component members. Compared to suspected cases, confirmed/probable cases were more often associated with intentional self-harm and undetermined causes of injury, whereas suspected cases were more often coded as unintentional. Confirmed/probable active component and non-service member beneficiary cases were more likely than their respective suspected case counterparts to receive care in inpatient settings. The need for improvements in ICD coding to reduce the percentage of CO poisoning cases coded with unknown injury intent and/or unknown CO poisoning source is discussed.

WHAT ARE THE NEW FINDINGS? 

During July 2009–June 2019, there were 1,288 confirmed/probable cases of CO poisoning among active component service members and 366 among reserve component service members. Twenty-four active component service member deaths were ascertained within 1 week of CO poisoning-related hospitalizations. Work in repair/engineering was the most frequent specific occupation associated with confirmed CO poisoning cases among active and reserve component service members.

WHAT IS THE IMPACT ON READINESS AND FORCE HEALTH PROTECTION?
Military personnel encounter unique and potentially lethal sources of significant CO exposure. Primary medical care providers (including unit medics and emergency medical technicians) should be knowledgeable of and sensitive to the diverse and nonspecific early clinical manifestations of CO intoxication. Service members, unit leaders, and supervisors at all levels should be aware of the dangers of CO poisoning and should employ appropriate preventive measures.

BACKGROUND

Carbon monoxide (CO) is a toxic, colorless, odorless, tasteless, and nonirritating gas that results from the incomplete combustion of carbon-based fuels such as wood, charcoal, gasoline, heating oil, kerosene, methane, propane, and butane.1 Exposure to excessive amounts of this gas is entirely preventable; nonetheless, CO poisoning is one of the most common causes of unintentional poisoning deaths in the U.S., accounting for hundreds of deaths and thousands of emergency department visits annually.2–5

Clinical CO toxicity is due to a combination of tissue hypoxia-ischemia resulting from carboxyhemoglobin (COHb) formation and direct CO-mediated damage at the cellular level.1 CO binds to hemoglobin with approximately 240 times the affinity of oxygen, and the resulting COHb critically impairs oxygen transport and utilization.6 The amount of COHb formed depends on the concentration of CO in the inspired air, the duration of exposure to the CO, and the alveolar ventilation (the amount of inspired air available for gas exchange each minute).7 CO also interferes with peripheral oxygen utilization at the mitochondrial level by binding to intracellular proteins including myoglobin, cytochromes, and nicotinamide adenine dinucleotide phosphate reductase(NADPH).8–10 In addition, CO can trigger an inflammatory cascade that results in lipid peroxidation affecting myelin proteins, apoptosis in neurons, and delayed neurologic sequelae.9,11 Other mechanisms of CO toxicity include increased nitrous oxide production and oxidative free radical damage within the vasculature as well as increased amino acid levels (particularly glutamate).9

Because the clinical effects of mild to moderate CO poisoning are diverse and largely nonspecific (e.g., dizziness, headache, malaise, fatigue, disorientation, nausea, and vomiting), a high index of suspicion is crucial for an appropriate and timely diagnosis.1,9,10 Extremely high CO exposure can produce more pronounced and severe symptoms such as altered mental status, confusion, syncope, seizures, acute stroke-like syndromes, and coma.12,13 Muscular activity that increases oxygen demand exacerbates the symptoms of CO exposure; however, individuals at rest may experience no other symptoms before losing consciousness.9 Cardiovascular and metabolic manifestations of CO poisoning may include hypotension, dysrhythmia, ischemia, infarction, and severe lactic acidosis.8,9 Moreover, myocardial injury after moderate to severe CO poisoning is associated with increased long-term mortality.14 Results of a prospective study of over 200 CO poisoning cases demonstrated a mortality rate of 24% within 7.6 years of follow-up among those cases with acute myocardial injury.14 Among cases with evidence of myocardial injury, the observed mortality rate was triple the death rate expected for a comparable, unpoisoned cohort.14 Finally, CO poisoning may result in persistent or delayed neurologic effects, including reduced memory, cognitive impairment, confusion, seizures, and attention disorders.1,15,16

In the U.S., CO poisoning generally results from inhalation of fumes produced by fires, motor vehicles (e.g., automobiles, trucks, tractors, forklifts, motorboats), malfunctioning and/or inadequately ventilated heating or cooking devices (e.g., furnaces, fireplaces, stoves, barbecues, water heaters), and gasoline-powered tools (e.g., pumps, compressors, power generators).17,18 CO from these sources can accumulate and reach dangerous levels in enclosed or partially enclosed spaces.19 The Centers for Disease Control and Prevention (CDC) found that the most frequently recorded locations for CO exposures reported to poison control centers between 2000 and 2009 were “residence” and “workplace” (77.6% and 12.0%, respectively).3

By their natures, many military activities, materials, and settings pose CO hazards.20–22 In 2009, the Department of Defense (DoD) removed CO poisoning from the list of reportable medical events.23 However, healthcare providers within the Military Health System (MHS) are still encouraged to report any patient diagnosed with CO poisoning to their local preventive medicine service or public health detachments.22 Because case definitions for CO poisoning vary by state, clinicians are encouraged to report any suspected cases.22

MSMR surveillance reports on episodes of CO poisoning among members of the U.S. Armed Forces date back to 2001.24–28 The current analysis updates and expands on these earlier reports by including non-service member beneficiaries of the MHS, using International Classification of Diseases, 9th and 10th Revision (ICD-9 and ICD-10, respectively) codes to classify cases by degree of certainty that poisoning was CO related, intent, and source of exposure. Counts and locations of CO poisoning cases classified as confirmed are also reported for the period from July 2009 through June 2019.

METHODS

The surveillance period was 1 July 2009 to 30 June 2019. The surveillance population included all individuals who served in the active or reserve component of the Army, Navy, Air Force, or Marine Corps at any time during the surveillance period. The non-military surveillance population consisted of other beneficiaries of the MHS including retired service members, family members, other dependents of service members and retirees, and other authorized government employees and their family members. Diagnoses indicative of CO poisoning were ascertained from electronic records of all medical encounters of individuals who received care in fixed (i.e., not deployed or at sea) medical facilities of the MHS or in civilian facilities in the Purchased CareThe TRICARE Health Program is often referred to as purchased care. It is the services we “purchase” through the managed care support contracts.purchased care system documented in the Defense Medical Surveillance System (DMSS). Healthcare encounters of deployed service members are maintained in the Theater Medical Data Store (TMDS), which is incorporated into the DMSS. For service members, cases were classified as active or reserve component using component at the time of the case-defining diagnosis.

For the current analysis, an incident case of CO poisoning was defined by inpatient, outpatient, or TMDS medical encounters with a qualifying ICD-9 or ICD-10 diagnosis code in any diagnostic position. Qualifying diagnosis codes were drawn from the 2018 Council of State and Territorial Epidemiologists (CSTE) criteria for defining a case of CO poisoning from administrative records and are presented in Table 1.29 For the purpose of identifying incident CO poisoning cases, TMDS records were treated as outpatient encounters.

Drawing on the 2018 CSTE criteria, cases were classified by degree of certainty that poisoning was CO related (Table 1).29 A case of CO poisoning was considered “confirmed” if the medical encounter was coded with ICD-9 code 986 (toxic effect of CO), ICD-10 code T58.* (toxic effect of CO), or an ICD-9 external cause of injury code (E code) explicitly indicating CO exposure (E868.3, E868.8, E868.9, E952.1, E982.1). A “probable” case of CO poisoning was defined as any inpatient or outpatient encounter with an ICD-9 E code explicitly indicating motor vehicle exhaust exposure (E868.2, E952.0, E982.0) and no ICD-9 code 986. “Suspected” cases were defined as any inpatient or outpatient encounter with an ICD-9 diagnosis or E code inclusive of CO poisoning from exposure to utility gas, conflagration, vehicular, or other sources (Table 1) and no ICD-9 code 986. The ICD-9 code set used in the analysis to classify cases as suspected was modified to exclude the following E codes: E962.9 (assault by poisoning, unspecified), E972 (injury due to legal intervention by gas), and E978 (legal execution). The ICD-10 codes used to classify cases as suspected are not included in the 2018 CSTE list (Table 1). Classification by degree of certainty was based on the first-occurring diagnostic position containing a qualifying diagnosis code, with prioritization of codes classified as confirmed over those classified as probable over those classified as suspected within the given encounter.

Cases were then categorized separately by intent as “undetermined,” “assault,” "unintentional,” or “intentional self-harm” based on ICD-9 E codes or specific ICD-10 codes pertaining to the nature of the CO poisoning exposure (Table 2).29 Categorization of intent was based on the first-occurring case-qualifying diagnosis code that was not ICD-9 code 986, 987.0, 987.1, or 987.9. If any of these 4 codes occurred alone, the case was categorized as undetermined intent (Table 2). It is important to note that because of the coding logic, the diagnosis code used for the classification of degree of certainty (confirmed/probable/suspected) could have been different from the diagnosis code used in the categorization of intent type.

To exclude follow-up encounters for single CO intoxication episodes, only 1 episode per individual per year was included in the analysis. Also, for ICD-10 codes T58.*, T59.89*, T59.81*, T59.9*, only initial encounters as indicated by a seventh digit of “A” were included in the analysis (Table 1). Because CO poisonings are more frequent in the fall and winter, a surveillance year was defined as 1 July through 30 June for analysis purposes. Only 1 poisoning episode per individual per July–June period (cold year) was included in the analysis. If multiple encounters occurred in the same cold year, a 2-step prioritization was used: cases classified as confirmed were prioritized over those classified as probable, which were prioritized over those classified as suspected; inpatient encounters were prioritized over TMDS encounters, which were prioritized over outpatient encounters.

The disposition of active and reserve component CO poisoning cases was examined and coded as returned to duty with no limitations, returned to duty with limitations, not returned to duty, or death. If there was no indication of disposition in the record of the medical encounter (14.3% of active component cases and 45.0% of reserve component cases), then the service member was assumed to be returned to duty with no limitations. In addition, for active and reserve component inpatient cases, casualty data for active duty and activated reserve component members were checked for any deaths within +/- 7 days of any CO poisoning-related hospitalizations. Finally, the diagnostic position of the casedefining diagnosis (first through second, third though fourth, fifth +) was also examined for active and reserve component CO poisoning cases.

RESULTS

Active component

During the 10-year surveillance period, there were 24,223 incident diagnoses that met the 2018 CSTE criteria for confirmed, probable, or suspected CO poisoning among active component service members (Table 3). The vast majority (n=22,935; 94.7%) of these diagnoses were classified as suspected, 5.2% (n=1,269) were classified as confirmed, and 0.08% (n=19) were classified as probable. More than four-fifths (85.1%; n=20,611) of the total incident diagnoses were recorded during outpatient encounters, 7.0% (n=1,685) during inpatient encounters, and 8.0% (n=1,927) during TMDS encounters. A total of 24 active component service member deaths were ascertained from casualty data within +/- 7 days of CO poisoning-related hospitalizations.

Among the 1,288 confirmed/probable cases of CO poisoning, 613 (47.6%) were classified as having unintentional intent, 538 (41.8%) as having undetermined intent, 136 (10.6%) as having self-harm intent, and 1 (0.1%) as due to assault (Table 3). More than four-fifths (81.4%) of incident CO poisoning diagnoses classified as confirmed/probable were recorded in an outpatient setting. In terms of disposition, the vast majority of confirmed/probable cases were categorized as having returned to duty with no limitations (88.5%), 9.2% were not returned to duty, and 1.9% were categorized as having returned to duty with limitations. Among the 22,935 suspected cases, roughly seven-eighths (87.4%) were associated with unintentional causes of injury and 11.7% were associated with undetermined intent (Table 3). Less than 1% of suspected cases were associated with intentional self-harm (0.6%; n=141) or assault (0.3%; n=64) intent. The majority (85.3%) of suspected CO poisoning diagnoses among active component members were recorded in outpatient settings.

The source of CO poisoning was “other or unspecified” for 913 (71.9%) of the confirmed active component cases based on associated ICD-9 or ICD-10 codes (data not shown). Motor vehicle exhaust was the second most common source of confirmed cases (n=216; 17.0%) and accounted for all of the probable cases (n=19) (data not shown). Less frequent sources of CO among confirmed cases included utility gas (n=73; 5.8%), domestic fuel (n=54; 4.3%), smoke/fumes from fire (n=12; 0.9%), and liquefied petroleum gas (n=1; 0.1%). For suspected cases, motor vehicle exhaust was the most common source of CO poisoning, accounting for more than four-fifths (82.2%; n=18,849) of the cases. The next most frequent source was “other and unspecified” (n=3,071; 13.4%) followed by smoke/fumes from fire (n=938; 4.1%); domestic fuel, utility gas, and liquefied petroleum gas each accounted for 0.1% of suspected cases (data not shown).

Approximately three-fifths (60.2%; n=776) of the case-defining diagnoses of confirmed/probable cases were recorded as ICD-9 codes (data not shown). Of these confirmed/probable ICD-9 cases, 460 (59.3%) were coded with ICD-9 diagnosis code 986 and no qualifying E codes, 234 (30.3%) were coded with 986 and a qualifying E code, and 82 (10.6%) were coded with a confirmed/probable E code alone (i.e., no 986 code) (data not shown).

During July 2009–June 2019, the majority of active component service members classified as confirmed CO poisoning cases were male (80.5%), 29 years old or younger (68.2%), and non-Hispanic white (60.8%) (Table 4). Army members accounted for the greatest proportion of confirmed CO poisoning cases (41.4%), and Marine Corps members the lowest (8.4%). Enlisted service members made up almost seven-eighths (86.9%) of the confirmed CO poisoning cases. Service members in repair/engineering occupations accounted for the greatest proportion of confirmed CO poisoning cases (30.2%), and those working in motor transport accounted for the lowest proportion (3.2%).

Annual counts of confirmed/probable CO poisoning cases among active component service members fluctuated between 107 (July 2009–June 2010 and July 2017–June 2018) and 170 (July 2016–June 2017) (Figures 1a, 1b). Examination of cases by year and month showed that the greatest monthly count of confirmed/probable CO poisoning cases occurred in January 2017 (n=61). Cumulative monthly counts of confirmed/probable cases increased from late summer through early fall, were highest in late fall and early winter, and were lowest in late spring and early summer (Figure 2). Cumulative monthly counts of suspected cases followed a broadly similar trend.

Active component confirmed CO poisoning cases were widely distributed among units and installations in the U.S. and overseas. During the 10-year surveillance period, confirmed CO poisoning cases were diagnosed at more than 190 military installations and geographic locations worldwide. Fourteen U.S. military installations accounted for more than one-third (35.0%) of total confirmed cases (Table 5). The 2 installations with the most confirmed cases during the surveillance period included Fort Carson, CO (n=60; 4.7%), and Naval Medical Center (NMC) San Diego, CA (n=52; 4.1%). Less than 5% (4.1%; n=52) of all confirmed cases affected service members assigned outside of the U.S., and 13.8% of cases (n=175) were diagnosed in unknown locations. The known locations with the largest clusters of confirmed CO poisoning cases within the same calendar weeks included Seymour Johnson Air Force Base (AFB), NC (n=20; December 2015); Joint Base Langley-Eustis, VA (n=18; January 2014); NMC San Diego, CA (n=15; November 2017); Keesler AFB, MS (n=14; January 2013); Landstuhl Regional Medical Center, Germany (n=12; September 2016), and Fort Wainwright, AK (n=11; September 2014) (data not shown).

Reserve component

From July 2009 through June 2019, there were 4,046 incident diagnoses that met the 2018 CSTE criteria for confirmed, probable, or suspected CO poisoning among reserve component service members (Table 6). The vast majority (n=3,680; 91.0%) of these diagnoses were classified as suspected, 9.0% (n=361) were classified as confirmed, and 0.1% (n=5) were classified as probable. Roughly five-sixths (83.8%; n=3,389) of the total incident cases were documented in outpatient settings, and 4.6% (n=186) were treated in inpatient settings; 11.6% of cases were derived from TMDS encounters (n=471). No deaths were ascertained from reserve component casualty data within +/- 7 days of CO poisoning-related hospitalizations.

Of the 366 confirmed/probable cases of CO poisoning among reserve component members, 180 (49.2%) were classified as having unintentional injury intent, 160 (43.7%) were classified as having undetermined intent, and 26 (7.1%) were classified as having intentional self-harm intent; there were no cases classified as having assault intent (Table 6). The vast majority (91.0%) of incident CO poisoning diagnoses classified as confirmed/probable were recorded in outpatient settings. In terms of disposition, 96.4% of confirmed/probable cases were categorized as having returned to duty with no limitations. Among the 3,680 suspected cases, more than three-quarters (78.8%) were associated with unintentional causes of injury and 20.7% were associated with undetermined intent (Table 6). Less than 1% of suspected cases were associated with intentional self-harm (0.4%; n=15) or assault (0.2%; n=7) intent. The majority (83.0%) of suspected CO poisoning diagnoses among reserve component members were recorded in outpatient settings.

Among reserve component members, the source of CO poisoning was “other or unspecified” for 296 (82.0%) of confirmed cases (data not shown). Motor vehicle exhaust was the second most common source of CO among confirmed cases (n=42; 11.6%) and accounted for all of the probable cases (n=5) (data not shown). Less frequent sources of CO among confirmed cases included utility gas (n=16; 4.4%), domestic fuel (n=5; 1.4%), and liquefied petroleum gas (n=2; 0.5%). For suspected cases, motor vehicle exhaust was the most common source of CO poisoning, accounting for more than two-thirds (68.9%; n=2,534) of the cases. The next most frequent source of CO exposure for suspected cases was “other and unspecified” sources (n=881; 23.9%) followed by smoke/fumes from fire (n=235; 6.4%); liquefied petroleum gas and utility gas each accounted for approximately 0.4% of suspected cases. Domestic fuel accounted for 0.1% of suspected cases (data not shown).

More than half (56.6%; n=207) of the case-defining diagnoses of confirmed/probable cases were recorded as ICD-9 codes (data not shown). Of these confirmed/probable ICD-9 cases, 140 (38.3%) were coded with ICD-9 diagnosis code 986 and no qualifying E codes, 55 (15.0%) were coded with 986 and a qualifying E code, and 12 (3.3%) were coded with a confirmed/probable E code alone (i.e., no 986 code) (data not shown).

Similar to the active component, the majority of reserve component service members classified as confirmed CO poisoning cases were male (73.4%), non-Hispanic white (68.1%), in the Army (74.0%), and enlisted (89.2%) (Table 7). Almost one-half (47.6%) of confirmed reserve component cases were 29 years old or younger. Service members in other/unknown and repair/engineering occupations accounted for the greatest proportions of confirmed CO poisoning cases (27.7% and 26.3%, respectively), and those working as pilots/air crew the lowest (2.2%).

Annual counts of confirmed/probable CO poisoning cases among reserve component service members fluctuated between 28 (July 2016–June 2017) and 58 (July 2018–June 2019) (Figures 3a, 3b). The greatest monthly counts of confirmed/probable CO poisoning cases occurred in May 2013 (n=15) and April 2019 (n=15). During the surveillance period, cumulative monthly counts of confirmed/probable cases among reserve component members were highest in May (n=43) and January (n=41) and lowest in December (n=20) and September (n=22) (Figure 4). Cumulative monthly counts of suspected cases were lowest in the early and late summer months and highest in the late fall and early winter months.

From July 2009 through June 2019, confirmed CO poisoning cases were diagnosed among reserve component members at more than 90 military installations and geographic locations worldwide. Less than 1% (0.6%; n=2) of all confirmed cases affected service members assigned outside of the U.S., and 7.5% (n=27) of cases were diagnosed in unknown locations. Five installations accounted for one-eighth (12.5%) of the total confirmed CO poisoning cases among reserve component members (Table 8). These locations included Fort Belvoir, VA (n=16; 4.4%); Marine Corps Base (MCB) Camp Pendleton, CA (n=12; 3.3%); Wright-Patterson AFB, OH (n=7; 1.9%); Fort Bragg, NC (n=5; 1.4%); and NMC Portsmouth, VA (n=5; 1.4%). The largest cluster of confirmed CO poisoning cases reported within the same calendar week occurred at MCB Camp Pendleton, CA (n=9), in May 2012 (data not shown).

Non-service member beneficiaries

During the 10-year surveillance period, there were 28,394 incident diagnoses that met the 2018 CSTE criteria for confirmed, probable, or suspected CO poisoning among non-service member beneficiaries of the MHS (Table 9). More than four-fifths (n=23,640; 83.3%) of these diagnoses were classified as suspected, 16.6% (n=4,714) were classified as confirmed, and 0.1% (n=40) were classified as probable. The vast majority (93.1%; n=26,442) of the total incident diagnoses were recorded during outpatient encounters, with 6.9% (n=1,952) recorded during inpatient encounters.

Among the 4,754 confirmed/probable cases of CO poisoning among non-service member beneficiaries, 2,461 (51.8%) were classified as having undetermined intent, 2,070 (43.5%) were classified as having unintentional injury intent, 215 (4.5%) were classified as having intentional self-harm intent, and 8 (0.2%) were classified as having assault intent (Table 9). The vast majority (90.0%) of incident CO poisoning diagnoses classified as confirmed/probable were recorded in outpatient settings. Among the 23,640 suspected cases, over three-quarters (75.9%) were associated with unintentional causes of injury and 22.5% were associated with undetermined intent (Table 9). Approximately 0.5% (n=124) of suspected cases were associated with assault intent, with 1.0% (n=241) associated with intentional self-harm intent. The vast majority (93.7%) of suspected CO poisoning diagnoses among non-service member beneficiaries were recorded in outpatient settings.

The most common source of CO poisoning among non-service member beneficiaries was “other or unspecified” for 3,859 (11.9%) of confirmed cases based on associated ICD-9 or ICD-10 codes (data not shown). Motor vehicle exhaust was the second most common source of confirmed cases (n=404; 8.6%) and accounted for all of the probable cases (n=40) (data not shown). Less frequent sources of CO among confirmed cases included utility gas (n=309; 6.6%), domestic fuel (n=109; 2.3%), smoke/fumes from fire (n=20; 0.4%), and liquefied petroleum gas (n=13; 0.3%). For suspected cases, motor vehicle exhaust was the most common source of CO poisoning, accounting for more than three-fifths (63.3%; n=14,972) of the cases. The next most frequent source of suspected cases was “other and unspecified” sources (n=6,136; 26.0%) followed by smoke/fumes from fire (n=2,317; 9.8%); utility gas, liquefied petroleum gas, and domestic fuel each accounted for less than 1.0% of suspected cases (data not shown).

Roughly five-eighths (62.6%; n=2,975) of the case-defining diagnoses of confirmed/probable cases were recorded as ICD-9 codes (data not shown). Of these confirmed/probable ICD-9 cases, 2,167 (45.6%) were coded with ICD-9 diagnosis code 986 and no qualifying E codes, 626 (13.2%) were coded with 986 and a qualifying E code, and 182 (3.8%) were coded with a confirmed/probable E code alone (i.e., no 986 code) (data not shown).

Among non-service member beneficiaries, nearly three-fifths of confirmed CO poisoning cases were among females (59.2%) (data not shown). More than one-quarter (26.9%) of confirmed cases were among those aged 17 or younger, with 30.6% of confirmed cases aged 18–44 years, 22.3% aged 45–65 years, and 20.2% aged 65 years or older (data not shown).

Annual counts of confirmed/probable CO poisoning cases among non-service member beneficiaries fluctuated between 397 (July 2016–June 2017) and 576 (July 2011–June 2012) (Figures 5a, 5b). Examination of cases by year and month showed that the greatest monthly count of confirmed/probable CO poisoning cases occurred in January 2015 (n=85). During the surveillance period, cumulative monthly counts of confirmed/probable cases increased from late summer through early fall, were highest in late fall and early winter, and were lowest in late spring and early summer (Figure 6). Cumulative monthly counts of suspected cases peaked in January and February and were lowest in December and August.

From July 2009 through June 2019, confirmed CO poisoning cases were diagnosed in non-service member beneficiaries at more than 195 military installations and geographic locations worldwide. Nineteen installations accounted for more than one-quarter (26.4%) of the total confirmed CO poisoning cases among this population (Table 10). The 5 locations with the greatest numbers of cases included Fort Carson, CO (n=180; 3.8%); NMC Portsmouth, VA (n=97; 2.1%); Fort Belvoir, VA (n=96; 2.0%); Joint Base San Antonio-Lackland, TX (n=90; 1.9%); and Walter Reed National Military Medical Center, MD/DC (n=82; 1.7%). Less than 1% (0.8%; n=36) of all confirmed cases affected non-service members located outside the U.S., and 6.5% (n=307) of cases were diagnosed in unknown locations. The known installations with the largest clusters of confirmed CO poisoning cases within the same calendar weeks included Navy Branch Health Clinic Earle, NJ (n=15; March 2009); and Fort Campbell, KY (n=15; April 2018) (data not shown).

EDITORIAL COMMENT

Morbidity and mortality from CO poisoning are important public health problems, but their full impact is difficult to estimate. Studies of CO-related morbidity and mortality in the U.S. have generated varying estimates using a variety of databases and have relied on different (often study-specific) methods for case ascertainment. The ability to accurately describe CO-related injury and death from administrative data depends on the type of data available and the methods used to identify and classify cases.

Previous MSMR reports on CO poisoning employed case definitions that required that the primary (first-listed) diagnosis be directly related to or likely caused by acute CO intoxication (e.g., headache, syncope). The most recent MSMR report on CO poisoning (July 1998–June 2008) applied an additional criterion for inclusion—only outpatient encounters associated with lost duty time were included as cases. The rationale for excluding ambulatory visits with dispositions of “released without limitations” was to restrict the analysis to clinically significant cases. During July 1998–June 2008, out of more than 1,000 medical encounters with “toxic effect of carbon monoxide” as a diagnosis (ICD-9 diagnosis code 986), there were 277 clinically significant cases of CO poisoning among active and reserve component service members.

The current analysis used the 2018 CSTE criteria29 and ICD code-based data to group CO poisoning cases into confirmed, probable, and suspected categories based on degree of certainty that poisoning was CO related. Cases were also classified by intent based on ICD-9 E codes or specific ICD-10 codes pertaining to the nature of the CO poisoning exposure. To date, very few of the studies that have employed this type of data for CO poisoning case finding included the more current ICD-10 coded data for comparison to ICD-9 code-based estimates.30–32 Despite the limitations of using administrative ICD code-based data for surveillance purposes, there are several important advantages of this type of data; such data can be used at national and Armed Forces-wide levels with relatively few resources and their analysis may provide useful information on trends and risk factors that can be used in targeting intervention methods and assessing the impact of prevention on the cause-specific burden of CO-related poisonings.

During the 10-year surveillance period of the current study, there were 1,288 CO poisoning cases classified as confirmed/probable among active component service members—an average of about 129 cases per year. There were approximately 37 confirmed/probable CO poisoning cases per year among reserve component members during this period. In both of these populations, pronounced differences were observed between confirmed/probable and suspected CO poisoning cases in terms of injury/intent. Compared to suspected cases, confirmed/probable cases were more often associated with intentional self-harm (10.6% vs 0.6%, active component; 7.1% vs 0.4%, reserve component) and undetermined causes of injury (41.8% vs 11.7%, active component; 43.7% vs 20.7%, reserve component), whereas suspected cases were more often coded as unintentional (87.4% vs 47.6%, active component; 78.8% vs 49.2%, reserve component). The same general pattern of differences between confirmed/probable and suspected cases was observed among non-service member beneficiaries.

Furthermore, confirmed/probable active component and non-service member beneficiary cases were more likely than their respective suspected case counterparts to receive care in inpatient settings (10.6% vs 6.7%, active component; 10.0% vs 6.3%, non-service member beneficiaries). A recent study of CO poisoning using the 2018 CSTE case definition among U.S. military veterans who received Veterans Health Administration care during 2010–2017 observed a similar pattern of results.31 However, during the course of that study, a review of a subset of over 500 confirmed/probable cases with undetermined injury intent revealed that 53% represented unintentional cases; this finding led the authors to propose that the unintentional category of injury may have been underrepresented among the confirmed/probable CO poisoning cases in their study.

In the current study, suspected CO poisoning cases were more likely to be associated with lost duty time compared to their respective confirmed/probable counterparts (20.5% vs 11.5%, active component; 12.5 vs 3.6%, reserve component). It is important to note that there was no indication of disposition in the records of the medical encounters of 14.3% of active component cases and 45.0% of reserve component cases. Moreover, examination by case classification category revealed that a considerably greater proportion of confirmed/probable cases lacked information on disposition compared to suspected cases (41.1% vs 12.8%, active component; 77.0% vs 41.8%, reserve component).

The demographic characteristics of the active and reserve component service members classified as confirmed CO poisoning cases generally reflected the demographic composition of the U.S. Armed Forces overall during a comparable time period.33,34 It is noteworthy that those working in repair/engineering occupations accounted for the greatest proportion of confirmed CO poisoning cases among active component members and the second greatest proportion among reserve component member cases. This finding warrants further analysis to examine the overall incidence rates of CO intoxication across occupations and highlights the importance of appropriate preventive measures for military personnel who repair or maintain vehicles.22,35

This report also documents that confirmed/probable cases among active component service members and non-service member beneficiaries exhibited the expected cold weather seasonal trends commonly associated with CO poisoning. This seasonal pattern generally corresponds to times of increased use of indoor heating. Suspected active component cases demonstrated a broadly similar trend. However, among reserve component members, suspected cases exhibited a trend that more closely followed the expected seasonal pattern of CO poisoning than did the confirmed/probable cases. Further investigation of the ICD-9 E codes and ICD-10 diagnosis codes associated with the relatively higher cumulative counts of confirmed/probable cases in May and June may provide insight into this pattern.

As the results of the current study show, CO poisoning-related injuries/diagnoses in the military often involve a single exposure that affects multiple personnel. For example, 21 soldiers showed CO poisoning symptoms during a multiday exercise at the Yukon Training Area near Eielson AFB in late September 2014. Among the symptomatic soldiers, 4 were admitted to Bassett Army Community Hospital and 7 were released after examination.36 Clusters of CO exposures among non-service member beneficiaries have also been reported. For example, in late August 2019, 2 service members and 3 family members from 3 different residences presented to the Womack Army Medical Center emergency department with symptoms associated with CO exposure.37 Results of laboratory testing showed that all 5 individuals had been exposed to CO.37 It was later determined that the residents of more than 80 homes within Fort Bragg housing were found to be at risk for CO exposure.38 Examination of the heating, ventilation, and air conditioner units in the laundry rooms of these homes revealed that vents were partially blocked and that closing the laundry room doors while doing laundry allowed CO concentrations in those rooms to reach dangerous levels.38 The most recent general recommendations from the CDC and the U.S. Consumer Product Safety Commission for preventing CO poisoning are presented in Table 11.39,40

Results of the current study should be interpreted in the context of several important limitations. Several factors may have resulted in the underestimation of the actual numbers of CO poisoning cases among the study populations. First, as incident CO poisoning cases were identified based on the presence of qualifying ICD-9 or ICD-10 codes recorded during a healthcare encounter, the validity of the results depends upon the accuracy of the physician-assigned coding generated by a given encounter. Because the clinical effects of mild to moderate CO poisoning are diverse and largely nonspecific, clinicians may not have considered CO poisoning when patients presented for care. In addition, cases among National Guard and Reserve members that were diagnosed in their civilian communities outside of the MHS were not included in the analysis. Furthermore, the high degree of ICD-9 986 diagnosis codes lacking qualifying ICD-9 E codes precluded the classification of a sizable proportion of cases in terms of intent and CO source and hampered the ability to make prevention recommendations based on the causes of injury. External cause code reporting is not mandatory; however, the “ICD-10-CM Official Guidelines for Coding and Reporting” encourage medical professionals to code external causes “as they provide valuable data for injury research and evaluation of injury prevention strategies.”41 Finally, for 2017, 2018, and 2019, medical data from sites that were using MHS GENESIS, the new electronic health record for the MHS, 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 during 2017–2019 were not included in the current analysis.

Other circumstances may tend to result in overestimation of the number of incident cases of CO poisoning. For example, diagnoses of CO poisoning recorded in electronic health records may represent misdiagnoses or tentative (rule-out) diagnoses that are not confirmed. While the current study attempted to limit the inclusion of follow-up cases from earlier episodes of CO poisoning, the analysis may have included some cases that did not represent incident diagnoses of CO poisoning. Conversely, limiting cases to diagnoses coded as initial encounters may have eliminated cases that represented true second exposures (e.g., repeat suicide attempts or unintentional in-home exposures where the original CO source was not addressed).
Standardization of ICD-coded case-ascertainment criteria for estimating CO-related morbidity and mortality is necessary to describe the public health burden of CO poisoning (including the various mechanisms and intent of injury), to target prevention efforts, and to evaluate the impact of interventions. However, the results of the current study highlight the need for improvements in ICD coding to reduce the percentage of cases coded with unknown injury intent and/or unknown CO poisoning source.

In summary, service members, unit leaders, and supervisors at all levels should be aware of and responsive to the dangers of CO poisoning and CO hazards related to residential, recreational, occupational, and military operational circumstances, equipment, and activities. Moreover, service members, unit leaders, and supervisors at all levels should be familiar with and employ appropriate preventive measures. Finally, primary medical care providers (including unit medics and emergency medical technicians) should be knowledgeable of and sensitive to the early clinical manifestations of CO intoxication.

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9. Kao LW, Nanagas KA. Toxicity associated with carbon monoxide. Clin Lab Med. 2006;26(1):99–125.
10. Weaver LK. Clinical practice. Carbon monoxide poisoning. New Engl J Med. 2009;360(12):1217–1225.
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12. Herman LY. Carbon monoxide poisoning presenting as an isolated seizure. J Emerg Med. 1998;16(3):429–432.
13. Mori T, Nagai K. Carbon-monoxide poisoning presenting as an afebrile seizure. Pediatr Neurol. 2000;22(4):330–331.
14. Henry CR, Satran D, Lindgren B, Adkinson C, Nicholson CI, Henry TD. Myocardial injury and long-term mortality following moderate to severe carbon monoxide poisoning. JAMA. 2006;295(4):398–402.
15. Choi IS. Delayed neurologic sequelae in carbon monoxide intoxication. Arch Neurol. 1983;40(7):433–435.
16. Kwon OY, Chung SP, Ha YR, Yoo IS, Kim SW. Delayed postanoxic encephalopathy after carbon monoxide poisoning. Emerg Med J. 2004;21(2):250–251.
17. Hampson NB, Stock AL. Storm-related carbon monoxide poisoning: lessons learned from recent epidemics. Undersea Hyperb Med. 2006;33(4):257–263.
18. Wilbur S, Williams M, Williams R, et al. Toxicological Profile for Carbon Monoxide. Atlanta, GA: Agency for Toxic Substances and Disease Registry; 2012:55–75.
19. Centers for Disease Control and Prevention. Carbon monoxide (CO) poisoning prevention. https://www.cdc.gov/features/copoisoning/index.html. Accessed 10 September 2019.
20. Klette K, Levine B, Springate C, Smith ML. Toxicological findings in military aircraft fatalities from 1986–1990. Forensic Sci Int. 1992;53(2):143–148.
21. White MR, McNally MS. Morbidity and mortality in U.S. Navy personnel from exposures to hazardous materials, 1974–85. Mil Med. 1991;156(2):70–73.
22. Smith SR, Downs JW, Louwers TD, Martin GJ, Weyandt TB, Ridgeley CD. Chapter 24. Carbon monoxide. In: Mallon TM, ed. Occupational Health and the Service Member. Fort Sam Houston, TX: Office of the Surgeon General, Borden Institute, U.S. Army Medical Department Center and School, Health Readiness Center of Excellence; 2019:477–502.
23. Armed Forces Health Surveillance Center. Tri-Service Reportable Events. Guidelines and Case Definitions. Silver Spring, MD: AFHSC; June 2009.
24. Army Medical Surveillance Activity. Carbon monoxide poisoning in active duty soldiers, 1998–1999. MSMR. 2001;7(2):13–14.
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37. Associated Press. Carbon monoxide problem discovered at Fort Bragg housing. Army Times. 15 August 2019. https://www.armytimes.com/news/your-army/2019/08/15/carbon-monoxide-problemdiscovered-at-fort-bragg-housing/. Accessed 18 September 2019.
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39. U.S. Consumer Product Safety Commission. Protect your family from carbon monoxide poisoning. https://www.cpsc.gov/safety-education/safety-education-centers/carbon-monoxide-information-center/protect-your-family-from-carbonmonoxide-poisoning. Accessed 2 December 2019.
40. Centers for Disease Control and Prevention. Carbon monoxide (CO) poisoning prevention. https://www.cdc.gov/features/copoisoning/index.html. Accessed 2 December 2019.
41. The Centers for Medicare and Medicaid Services and the National Center for Health Statistics. ICD-10-CM Official Guidelines for Coding and Reporting. FY 2018. https://www.cms.gov/Medicare/Coding/ICD10/Downloads/2018-ICD-10-CM-Coding-Guidelines.pdf. Accessed 15 August 2019.

Incident confirmed/probable CO poisoning cases, by month, active component, U.S. Armed Forces, July 2009–June 2019

Incident confirmed/probable CO poisoning cases, by year, active component, U.S. Armed Forces, July 2009–June 2019

Cumulative numbers of incident confirmed/probable and suspected CO poisoning cases, by month of diagnosis, active component, U.S. Armed Forces, July 2009–June 2019

 Incident confirmed/probable CO poisoning cases, by month, reserve component, U.S. Armed Forces, July 2009–June 2019

Incident confirmed/probable CO poisoning cases, by year, reserve component, U.S. Armed Forces, July 2009–June 2019

Cumulative numbers of incident confirmed/probable and suspected CO poisoning cases, by month of diagnosis, reserve component, U.S. Armed Forces, July 2009–June 2019

Incident confirmed/probable CO poisoning cases, by month, non-service member beneficiaries, July 2009–June 2019

Incident confirmed/probable CO poisoning cases, by year, non-service member beneficiaries, July 2009–June 2019

Cumulative numbers of incident confirmed/probable and suspected CO poisoning cases by month of diagnosis, non-service member beneficiaries, July 2009–June 2019

 ICD-9 and ICD-10 diagnostic codes and ICD-9 E codes used to identify cases of CO poisoning and to classify them as confirmed, probable, or suspected

Classification of qualifying ICD-9/ICD-10 codes and ICD-9 E codes by intenta

Active component cases with confirmed/probable versus suspected CO poisoning, July 2009–June 2019

Incident cases of confirmed CO poisoning and percentages, by demographic and military characteristics, active component, U.S. Armed Forces, July 2009–June 2019

 Confirmed CO poisoning cases, by location of diagnosis/report (with at least 20 cases during the period), active component, U.S. Armed Forces, July 2009–June 2019

Reserve component cases with confirmed/probable versus suspected CO poisoning, July 2009–June 2019

Incident confirmed cases of CO poisoning and percentages, by demographic and military characteristics, reserve component, U.S. Armed Forces, July 2009–June 2019

Confirmed CO poisoning cases, by location of diagnosis/report (with at least 5 cases during the period), reserve component, U.S. Armed Forces, July 2009–June 2019

Non-service member beneficiary cases with confirmed/probable versus suspected CO poisoning, July 2009–June 2019

Confirmed CO poisoning cases, by location of diagnosis/report (with at least 35 cases during the period), non-service member beneficiaries, U.S. Armed Forces, July 2009–June 2019

General recommendations to prevent CO poisoning

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