Carbon Monoxide Poisoning

Etiology

COHb is produced through both exogenous carbon monoxide exposure and endogenous heme catabolism. Clinically meaningful elevations arise almost exclusively from inhalational exposure to carbon monoxide in settings involving incomplete fuel combustion and inadequate ventilation. Common sources include malfunctioning furnaces, gas stoves, obstructed chimneys, vehicle exhaust, portable generators, and smoke from residential or structural fires. Occupational exposure is well documented among firefighters, mechanics, welders, and industrial workers with potential exposure to carbon monoxide–generating processes in enclosed or poorly ventilated environments.[7]

Tobacco smoke is the most prevalent source of chronic low-level carbon monoxide exposure. Nonsmokers typically demonstrate COHb levels below 1.5%, whereas smokers commonly exhibit levels between 3% and 15%, depending on intensity and frequency of use. Even secondhand smoke exposure may elevate COHb above baseline nonsmoker values and, in some settings, double or triple background concentrations.

Certain industrial chemicals may elevate COHb through metabolic conversion. As mentioned, methylene chloride (dichloromethane), present in paint strippers, solvents, aerosolized products, and degreasing agents, is metabolized by hepatic cytochrome P450 2E1 to carbon monoxide. Owing to lipophilicity, methylene chloride accumulates in adipose tissue and releases carbon monoxide gradually, producing delayed or prolonged toxicity. COHb levels may continue to rise despite removal from the exposure source and administration of oxygen, warranting extended clinical observation.[8]

Endogenous carbon monoxide arises from heme degradation, primarily mediated by heme oxygenase. This physiologic process, driven largely by erythrocyte turnover, accounts for approximately 80% of baseline carbon monoxide production and results in normal COHb levels of 0.4% to 1.5% in healthy adults. Mild elevations, approximately 3% to 4%, may occur in conditions associated with accelerated heme breakdown or inflammation, including hemolytic anemia, malaria, sepsis, and disorders characterized by upregulation of inducible heme oxygenase 1.[9] Such elevations rarely produce toxicity but may complicate clinical interpretation.

Epidemiology

COHb toxicity, most often resulting from carbon monoxide exposure, is a significant global public health concern and a leading cause of poisoning-related morbidity and mortality. In the US, carbon monoxide exposure accounts for 20,000 to 30,000 emergency department visits, approximately 14,000 hospitalizations, and an estimated 1,000 deaths each year, encompassing both accidental and intentional exposures.[10] Among unintentional, non–fire-related cases, approximately 400 to 500 deaths occur annually, with reported case-fatality rates ranging from 1% to 3%.[11] In contrast, intentional carbon monoxide exposures, often associated with suicide attempts, carry substantially higher mortality, with fatality rates 5 to 10 times greater than those observed in accidental cases.[12]

Seasonal and geographic trends reflect the influence of environmental and socioeconomic factors. Incidence increases during winter months and in colder climates, largely due to increased use of indoor heating sources, generators, and combustion appliances in enclosed or poorly ventilated spaces.[13] Severe weather events, power outages, and improper use of portable generators further contribute to episodic surges in cases.

Globally, COHb toxicity is substantially underreported, particularly in low-resource regions where indoor biomass combustion, inadequate ventilation, and limited diagnostic capability are common.[14] The true worldwide burden is difficult to quantify because of inconsistent surveillance and the nonspecific nature of carbon monoxide–related symptoms. The World Health Organization estimates that unintentional carbon monoxide poisoning contributes to tens of thousands of deaths annually, with disproportionately high impact in low- and middle-income countries. Ambient carbon monoxide concentrations vary widely by environment, typically ranging from 0.05 to 0.17 mg/m3 (0.04 to 0.15 ppm) globally, but reaching higher levels in urban or industrial regions characterized by dense combustion activity.[15][16]

While carbon monoxide from combustion is the predominant source of COHb elevation, rare but notable cases result from exposure to methylene chloride. Through metabolic biotransformation to carbon monoxide, this compound has been associated with at least 85 deaths in the US between 1980 and 2018, involving both occupational and accidental exposures.[17] Although infrequent, such cases underscore the importance of recognizing noncombustion sources of COHb toxicity.

Pathophysiology

Carbon monoxide rapidly diffuses across the alveolar-capillary membrane and binds hemoglobin with an affinity approximately 200 times greater than oxygen, forming COHb. This process reduces blood oxygen-carrying capacity and induces a leftward shift of the oxyhemoglobin dissociation curve, impairing oxygen release to tissues. The resulting cellular hypoxia contributes to metabolic acidosis and widespread organ dysfunction. Tissue injury in carbon monoxide poisoning extends beyond hypoxic mechanisms and involves a complex interplay of mitochondrial dysfunction, oxidative stress, inflammation, and lipid peroxidation.

Carbon monoxide binds to mitochondrial cytochrome c oxidase, disrupting the electron transport chain and inhibiting aerobic metabolism.[18] Impaired oxidative phosphorylation reduces adenosine triphosphate (ATP) production and promotes anaerobic metabolism, contributing to lactic acidosis. Carbon monoxide exposure also triggers abnormal interactions between platelets and neutrophils, leading to neutrophil activation and degranulation. Release of myeloperoxidase, proteases, and reactive oxygen species amplifies oxidative injury and cellular damage.

In endothelial cells, oxidative stress promotes conversion of xanthine dehydrogenase to xanthine oxidase, increasing reactive oxygen species generation and impairing intrinsic antioxidant defenses. Carbon monoxide–induced myeloperoxidase activity contributes to lipid peroxidation, degrading unsaturated fatty acids and altering myelin structure. These processes promote inflammation and delayed demyelination of white matter in the central nervous system (CNS), a hallmark of delayed neurologic sequelae (DNS).[19]

Carbon monoxide toxicity is multifactorial, resulting from combined tissue hypoxia, direct mitochondrial inhibition, inflammatory activation, and oxidative damage. Reoxygenation may further exacerbate injury through reperfusion mechanisms, generating partially reduced oxygen species that damage proteins, nucleic acids, and cell membranes, similar to reperfusion injury observed in other ischemic conditions.

Organs with the highest oxygen demand, particularly the brain and heart, are most susceptible. In the cardiovascular system, compensatory increases in cardiac output may initially preserve oxygen delivery. These compensatory mechanisms eventually fail with rising COHb levels and declining arterial oxygen content, resulting in demand ischemia and reduced cardiac output. The resulting hemodynamic decline exacerbates cerebral hypoxia and may precipitate loss of consciousness, a consistently recognized predictor of severe toxicity.[20] Individuals with underlying cardiac, pulmonary, or vascular disease exhibit limited physiologic reserve and may develop severe manifestations, such as acute coronary syndrome (ACS), pulmonary edema, syncope, or arrhythmias, at lower carbon monoxide concentrations.

Carbon monoxide exposure is particularly dangerous in utero. Fetal hemoglobin demonstrates higher affinity for both oxygen and carbon monoxide, producing a more pronounced leftward shift of the dissociation curve and severe impairment of oxygen delivery. Animal studies demonstrate that fetal COHb levels may continue to rise for up to 40 hours after maternal levels stabilize, and fetal COHb may ultimately exceed maternal concentrations. Owing to fetal susceptibility and prolonged elimination, most clinical guidelines recommend more aggressive treatment in pregnant patients, including early consideration of hyperbaric oxygen therapy (HBOT).[21]

Histopathology

Postmortem histopathology has demonstrated multiple CNS abnormalities. Findings include areas of ischemic or hemorrhagic necrosis in the globus pallidus and perivascular foci of demyelination within deep white matter.[22][23]

Toxicokinetics

Carbon monoxide binds hemoglobin with an affinity approximately 200 times greater than oxygen, forming COHb. Although binding is reversible, the high affinity results in a prolonged elimination half-life that varies with inspired oxygen concentration. In ambient room air, COHb half-life typically ranges from 4 to 6 hours, but decreases to 60 to 90 minutes with administration of 100% oxygen and to approximately 20 to 30 minutes with HBOT, due to increased partial pressure of oxygen facilitating dissociation of carbon monoxide from hemoglobin.

Toxicokinetics of COHb formation and clearance are commonly described by the Coburn–Foster–Kane equation, incorporating variables such as ambient carbon monoxide concentration, pulmonary ventilation, hemoglobin concentration, endogenous carbon monoxide production, and diffusion parameters. According to the Coburn–Foster–Kane model, COHb levels rise rapidly during the first several hours of exposure, approach a plateau after approximately 3 hours, and generally reach steady state after 6 to 8 hours, assuming constant environmental carbon monoxide levels. Final COHb concentration depends primarily on exposure intensity and duration, as well as individual ventilation patterns.[24]

Children may eliminate carbon monoxide more rapidly due to higher minute ventilation relative to body mass. However, increased ventilation may also result in faster accumulation at equivalent exposure levels. In contrast, the fetus exhibits markedly different kinetics. Fetal hemoglobin demonstrates higher affinity for carbon monoxide than adult hemoglobin and reduced capacity for oxygen release to tissues, resulting in a prolonged fetal COHb elimination half-life. Additionally, fetal COHb levels may continue to rise even after maternal levels begin to decline, reflecting placental transfer and diminished fetal clearance capacity, particularly in the setting of maternal hypoxemia.[25]

Overall, carbon monoxide toxicokinetics are influenced by ambient exposure, physiologic ventilation, hemoglobin characteristics, and age-specific factors. These variables are essential considerations when interpreting COHb levels, assessing severity, and determining the appropriate treatment duration.

History and Physical

The clinical manifestations of carbon monoxide toxicity are highly variable and predominantly nonspecific, making early recognition challenging. Therefore, a detailed history and physical examination are critical. The most common presenting symptom is headache, followed by dizziness, weakness, and nausea. Additional manifestations include confusion, difficulty concentrating, shortness of breath, and visual disturbances. More severe exposures may result in loss of consciousness, which, although less frequent, is strongly suggestive of significant toxicity and serves as an important prognostic indicator.[26][27] The triad of cherry-red lips, cyanosis, and retinal hemorrhages is rare and should not be relied upon for diagnosis.

Since carbon monoxide interferes with oxygen delivery, compensatory increases in cardiac output may result in tachycardia, tachypnea, or arrhythmias. Severe cases may progress to pulmonary edema or ACS, particularly in individuals with underlying cardiopulmonary disease. The condition's nonspecific symptomatology often mimics viral illness, gastroenteritis, metabolic disorders, psychiatric conditions, or influenza-like syndromes, contributing to diagnostic delay.

A thorough history should assess potential exposure sources, including recent use of gas appliances, propane heaters, kerosene stoves, or generators, particularly in enclosed spaces. Evaluation should include home heating systems, hot water heaters, and the presence of carbon monoxide detectors. Occupational exposure should also be considered, particularly among construction workers, mechanics, welders, or painters using gas-powered equipment. Clustered symptoms among household members, coworkers, or cohabitants should raise suspicion for carbon monoxide exposure.

Given that carbon monoxide toxicity disproportionately affects organs with high oxygen demand, a focused neurologic examination is essential. Subtle cognitive deficits, including impaired concentration, slowed processing speed, gait abnormalities, and mood changes, may be present even in the absence of overt neurologic signs. Early neurologic abnormalities, particularly those associated with loss of consciousness, increase the risk of long-term neuropsychiatric sequelae and warrant close follow-up.

Carbon monoxide exposure also induces inflammatory and oxidative stress pathways that potentially contribute to cardiac manifestations, including myocardial ischemia and arrhythmias. Symptoms such as chest pain or dyspnea should prompt evaluation for ischemia, even in individuals without known cardiovascular disease.

In summary, carbon monoxide poisoning frequently presents with vague, nonspecific complaints, requiring a high index of suspicion. Careful assessment of environmental context, symptom clusters, and exposure risk factors is essential for timely diagnosis and management.

Evaluation

Initial evaluation of suspected carbon monoxide toxicity begins with prompt measurement of COHb levels, ideally obtained from a blood gas sample analyzed by cooximetry. Venous and arterial COHb levels correlate closely. Therefore, arterial sampling is not required solely for COHb determination. Although noninvasive pulse CO-oximeters (carboxyhemoglobin oximeters) have been approved by the US Food and Drug Administration, multiple clinical studies demonstrate poor agreement with blood gas measurements, rendering them unreliable and unsuitable for diagnostic use except when no alternative is available.[28]

Electrocardiography (ECG) and cardiac monitoring are warranted in individuals with suspected carbon monoxide exposure to assess for ischemia or dysrhythmias. Patients with chest pain, known coronary artery disease, abnormal ECG findings, or age greater than 65 years should have cardiac biomarkers (eg, troponin, creatine kinase myocardial band isoenzyme) measured, as carbon monoxide poisoning may precipitate myocardial injury even in individuals without preexisting cardiovascular disease. A pregnancy test is essential in female patients of childbearing age, as carbon monoxide exposure poses a significant fetal risk, and a positive finding may alter management.

Blood gas analysis is useful not only for COHb measurement but also for the evaluation of metabolic acidosis, which may reflect tissue hypoxia. Elevated lactate levels are common but nonspecific and do not reliably correlate with toxicity severity. Additional toxicologic testing, including acetaminophen levels, salicylate levels, and urine drug screening, is indicated when intentional carbon monoxide exposure or coingestion is suspected.

Cyanide poisoning should also be considered in patients exposed to fire smoke. Administration of hydroxocobalamin may interfere with COHb measurement. Therefore, blood gas samples should be obtained prior to administration whenever feasible.

Neuroimaging may be obtained to evaluate unexplained changes in mental status or exclude alternative diagnoses. Computed tomography is typically normal but may occasionally reveal lesions in the globus pallidus or changes in deep white matter. Magnetic resonance imaging demonstrates higher sensitivity and may show T2 hyperintensities, basal ganglia lesions, or hippocampal abnormalities. However, these findings are nonspecific and may not correlate with clinical outcomes.

Two emerging biologic markers, neuron-specific enolase (NSE) and S100B, show promise in identifying patients at increased risk for DNS. These proteins, released during neuronal and astroglial injury, may serve as early indicators of CNS injury. A study demonstrated that S100B concentrations exceeding 0.165 mcg/L predicted DNS with high sensitivity and specificity, suggesting a potential role for these markers in risk stratification.[29]

Treatment / Management

Initial Management

The initial treatment for carbon monoxide poisoning consists of immediate removal from the exposure source and administration of high-flow 100% oxygen, typically delivered via a nonrebreather mask. Supplemental oxygen accelerates dissociation of carbon monoxide from hemoglobin, shortening the COHb half-life from 4 to 6 hours on room air to approximately 60 to 90 minutes, while improving tissue oxygen delivery. Patients with impaired mental status, airway compromise, or respiratory failure may require endotracheal intubation and mechanical ventilation.

The use of HBOT in carbon monoxide poisoning remains controversial. The 2025 American College of Emergency Physicians clinical policy concludes that evidence is insufficient to make level A or B recommendations but provides level C guidance, stating that HBOT may be considered for selected symptomatic patients based on clinical severity, resource availability, and transport logistics. This recommendation is based on class III evidence, including retrospective studies and meta-analyses, which collectively suggest modest benefit. Reported advantages of HBOT compared with normobaric oxygen include earlier hospital discharge, a lower proportion of patients with depressed mental status (number needed to treat ≈ 42), and modest improvement in activities of daily living (number needed to treat ≈ 41).[30] However, these findings require cautious interpretation due to significant methodological limitations, including heterogeneous treatment protocols, lack of blinding in most studies, and limited long-term outcome data.

Among randomized controlled trials (RCTs), only 1 rigorous, double-blind study demonstrated a significant reduction in cognitive sequelae at both 6 weeks and 12 months when HBOT was administered within 24 hours using a protocol of 3 sessions (initial 150 minutes, followed by two 120-minute sessions separated by 6 to 12 hours).[31] Conversely, other RCTs have produced mixed or inconclusive results, and systematic reviews, including the Cochrane analysis, highlight substantial heterogeneity with no consistent pooled evidence that HBOT prevents DNS.[32] A 2018 meta-analysis similarly reported variability in outcomes, with many confidence intervals overlapping, suggesting that any benefit may be small or susceptible to random variation.[33](A1)

Factors That May Influence Timing and Protocol

Recent observational studies indicate that early initiation of HBOT (ie, within 6 hours of carbon monoxide exposure) is associated with lower rates of neurocognitive impairment at follow-up, whereas delayed treatment (ie, 6 to 24 hours) correlates with worse outcomes.[34] The optimal treatment dose is unclear. Although the RCT demonstrating benefit employed 3 sessions, other observational and randomized studies have not shown a consistent advantage of multiple treatments over a single session.[35] HBOT protocols vary widely in pressure, duration, and number of sessions, limiting comparability across studies. Clinical features associated with a greater likelihood of benefit from HBOT include the following:(B2)

• Loss of consciousness
• Abnormal neurologic findings
• Severe metabolic acidosis
• Cardiovascular dysfunction
• Age at least 36 years
• Prolonged or intermittent exposure
• COHb level at least 25% [36]

Large population-based cohort studies also report associations between HBOT and reduced short- and long-term mortality, particularly among younger patients and those with acute respiratory failure. However, these findings are subject to confounding and do not establish causality.[37]

The potential adverse effects of HBOT, combined with its modest and inconsistent benefits, underscore the need for careful patient selection. Reported risks include barotrauma, oxygen toxicity that may manifest as seizures, hemodynamic instability, and limitations related to transport and hyperbaric chamber access.

Other Treatment Considerations

Extracorporeal membrane oxygenation (ECMO) has been reported in isolated cases of severe carbon monoxide poisoning complicated by cardiac arrest, refractory shock, or severe respiratory failure. ECMO supports oxygenation and perfusion while carbon monoxide is cleared and may allow recovery even in cases with extremely high COHb levels. However, available evidence is limited to case reports and small case series, and no current guidelines endorse routine ECMO use in carbon monoxide poisoning.[38]

Management must also address associated complications, including myocardial ischemia, arrhythmias, metabolic acidosis, and potential coexposures such as cyanide in fire-related smoke inhalation. Treatment is best delivered through coordinated interprofessional care involving emergency medicine physicians, toxicologists, cardiologists, and critical care clinicians when indicated.

Differential Diagnosis

COHb toxicity is clinically synonymous with carbon monoxide poisoning, and its presentation is often indistinguishable from many other causes of acute or chronic hypoxia, altered mental status, and nonspecific constitutional symptoms such as headache, dizziness, weakness, and nausea. Clinicians must consider a broad differential diagnosis, especially when exposure history is unclear, because carbon monoxide–related symptoms lack specificity.

Methemoglobinemia is one of the most important alternative diagnoses. Like carbon monoxide poisoning, this condition is a dyshemoglobinemia that impairs oxygen delivery. Patients typically exhibit cyanosis unresponsive to supplemental oxygen, low pulse oximetry readings, and normal arterial oxygen partial pressure, reflecting adequate dissolved oxygen but impaired hemoglobin function. Methemoglobinemia may result from oxidizing agents, medications (eg, nitrates, dapsone), or toxins. Importantly, methemoglobinemia and carboxyhemoglobinemia may coexist, particularly following exposure to combustion products, smoke, or mixed chemicals.[39]

Cyanide poisoning is another critical consideration, particularly in victims of fires or industrial incidents. Cyanide inhibits mitochondrial cytochrome oxidase, resulting in severe cellular hypoxia despite normal oxygen saturation or arterial partial pressure of oxygen. Key clinical features include rapidly progressive altered mental status, cardiovascular collapse, and significant lactic acidosis. These features overlap substantially with severe carbon monoxide poisoning.

Hydrogen sulfide may also inhibit oxidative phosphorylation. Toxicity may present with rapid loss of consciousness, respiratory distress, cardiac dysrhythmias, and metabolic acidosis. Exposure is commonly occupational, occurring in environments such as sewers, manure storage facilities, and petroleum refineries, and may closely mimic the mitochondrial impairment observed in carbon monoxide poisoning.

Exposure to simple asphyxiants, including methane, propane, nitrogen, argon, or helium, can produce headache, dizziness, syncope, and hypoxia through displacement of ambient oxygen, without dyshemoglobinemia or interference with hemoglobin oxygen-carrying capacity.[40] These cases often arise from confined-space or industrial exposures.

Other medical causes of hypoxia must also be considered, such as severe anemia or pulmonary embolism, as well as conditions associated with syncope or altered mental status, including ACS, heart failure, stroke, and seizure. Cardiovascular abnormalities associated with carbon monoxide poisoning may further confound differentiation from primary cardiac etiologies.[41]

The American College of Emergency Physicians emphasizes that carbon monoxide poisoning is often a diagnosis of exclusion, though it requires a high index of suspicion when environmental exposure is possible, or multiple individuals from the same setting present with similar complaints. Laboratory confirmation with elevated COHb levels is diagnostic. However, pulse oximetry cannot distinguish between hemoglobin species, and CO-oximetry is required to differentiate COHb from other dyshemoglobinemias, such as methemoglobin.

Prognosis

The prognosis of carbon monoxide toxicity varies widely and is influenced by exposure severity, development of coma, degree of organ dysfunction, and timeliness of treatment. Overall mortality ranges from 1% to 3% but may reach 14% in critically ill patients requiring intensive care or presenting with multiorgan failure.[42] Prognosis is further informed by risk scores such as SOFA (Sequential Organ Failure Assessment), FIRED (Fire-related Exposure, Respiratory failure, Encephalopathy, Duration of exposure), and ABCG (Age, Blood pressure, Consciousness, and Glucose score), as well as clinical predictors, including a Glasgow Coma Scale (GCS) score below 13, respiratory failure, and elevated blood urea nitrogen.[43][44][45][46]

Complications

Carbon monoxide poisoning can lead to significant complications. The most well-recognized long-term adverse outcome is neurologic injury, which may be classified into 1 of 2 categories. Persistent neurologic sequelae (PNS) present immediately after exposure and fail to resolve. DNS appears after a lucid interval of days to weeks. Reported symptoms include cognitive impairment, short-term memory loss, mood and personality changes, psychosis, gait instability, Parkinsonism, urinary incontinence, and visual disturbances. The estimated incidence of DNS ranges from 10% to 30%, although precise estimates are limited by variability in neuropsychological testing and lack of standardized diagnostic criteria. The risk is higher in patients with early neurologic deficits, loss of consciousness, or severe metabolic abnormalities. COHb levels alone correlate poorly with long-term outcome. However, emerging biomarkers such as neuron-specific enolase and S100B may help identify patients at increased risk for DNS.[47]

Cardiac sequelae are also common and clinically important. Carbon monoxide exposure induces tissue hypoxia, oxidative stress, and direct myocardial injury, which can precipitate arrhythmias, myocardial ischemia, and infarction, even in individuals without known coronary disease. Up to 1 out of 3 hospitalized patients may exhibit signs of myocardial injury, and elevated troponin levels in these individuals are associated with increased short- and long-term mortality. Importantly, some carbon monoxide–related cardiac dysfunction is reversible. A prospective study found that left ventricular systolic function normalized in approximately 80% of affected patients within 72 hours following exposure and supportive management.[48] Neurologic and cardiac complications contribute substantially to long-term morbidity and highlight the need for careful monitoring, early follow-up, and consideration of targeted rehabilitation in patients recovering from moderate-to-severe carbon monoxide poisoning.