Definition and epidemiology
Carbon monoxide poisoning results from inhalation of CO, a colorless, odorless, non-irritating gas produced by incomplete combustion of carbon-based fuels. Because it is undetectable by human senses, exposure often occurs without warning, which is why CO is sometimes called the "silent killer."
CO poisoning is one of the most common causes of poisoning death globally, with higher incidence in colder months due to indoor heating and generator use during power outages. Smoke inhalation from house fires is the leading cause of CO poisoning in many countries, followed by malfunctioning furnaces, water heaters, and portable generators used indoors or in poorly ventilated spaces. Intentional exposure (suicide attempts using vehicle exhaust or charcoal burning) also accounts for a notable proportion of cases in some regions.
Mortality and long-term neurologic sequelae are concentrated in cases with prolonged exposure, high ambient CO concentration, or delayed recognition, underscoring the importance of early suspicion in at-risk settings.
- CO binds hemoglobin with 200-250x the affinity of oxygen, forming carboxyhemoglobin
- Standard pulse oximetry is falsely normal; co-oximetry is required to measure COHb
- "Cherry red" skin is a classic board fact but rarely seen clinically
- Headache is the most common early symptom; cluster cases (whole household) are a key clue
- Treatment is immediate 100% oxygen; hyperbaric oxygen for severe/high-risk cases
- Consider concomitant cyanide toxicity in smoke inhalation with elevated lactate
- Fetal hemoglobin has even higher CO affinity than adult hemoglobin, worsening fetal risk
- Delayed neuropsychiatric sequelae can appear days to weeks after apparent recovery
Etiology and risk factors
Common sources
- Smoke inhalation (house fires)
- Faulty or unvented furnaces, boilers, and water heaters
- Portable generators used indoors or near open windows
- Charcoal grills or hibachis used indoors or in enclosed spaces (tents, garages)
- Motor vehicle exhaust in closed garages
- Improperly vented gas appliances (stoves, space heaters)
- Industrial exposure (foundries, steel production)
Risk factors for severe toxicity
- Enclosed or poorly ventilated space
- Prolonged exposure duration
- Extremes of age (infants, elderly)
- Pregnancy (fetal hemoglobin has even higher CO affinity than adult hemoglobin)
- Pre-existing cardiac or pulmonary disease
- Co-exposure to cyanide (common in structure fires from burning plastics)
Cluster presentations are a key clue: multiple household members or pets presenting with similar vague symptoms (headache, nausea, dizziness) that improve when away from the home should raise strong suspicion for CO poisoning.
Pathophysiology
CO binds hemoglobin to form carboxyhemoglobin (COHb) with an affinity approximately 200-250 times greater than oxygen. This competitive binding reduces the oxygen-carrying capacity of blood and shifts the oxyhemoglobin dissociation curve to the left, impairing oxygen release to tissues even when some oxyhemoglobin remains available.
Beyond simple hypoxia, CO has direct cellular toxicity: it binds mitochondrial cytochrome c oxidase, disrupting oxidative phosphorylation and promoting oxidative stress and lipid peroxidation, particularly in the brain. CO also binds myoglobin, contributing to cardiac dysfunction and rhabdomyolysis in severe cases.
The combination of impaired oxygen delivery and direct mitochondrial poisoning explains why clinical severity often correlates poorly with measured carboxyhemoglobin level, and why delayed neurologic sequelae can occur days to weeks after apparent recovery, thought to relate to lipid peroxidation and demyelination in the globus pallidus and white matter.
Clinical features
Symptoms are nonspecific and often mimic a viral illness, which contributes to missed or delayed diagnosis.
Mild to moderate toxicity
- Headache (most common early symptom)
- Nausea, vomiting
- Dizziness, malaise
- Flu-like symptoms without fever
Severe toxicity
- Confusion, altered mental status
- Syncope
- Chest pain, dysrhythmias (especially in patients with coronary disease)
- Seizures
- Coma
- Cardiovascular collapse, death
Classic (but unreliable) findings
- "Cherry red" skin discoloration is classically taught but rarely seen clinically and should not be relied upon
- Pulse oximetry is falsely normal or near-normal because standard oximeters cannot distinguish carboxyhemoglobin from oxyhemoglobin
Delayed neuropsychiatric sequelae (memory impairment, personality change, parkinsonism, cognitive deficits) may appear 2-40 days after apparent recovery, particularly after severe or prolonged exposure.
Diagnosis
Diagnosis requires a high index of suspicion based on history and setting, confirmed by laboratory measurement of carboxyhemoglobin.
Key tests
- Co-oximetry (arterial or venous blood gas with CO-oximeter): directly measures carboxyhemoglobin percentage; this is the definitive test
- Standard pulse oximetry: unreliable, often falsely normal
- ECG: evaluate for ischemia or dysrhythmia, especially in older patients or those with chest pain
- Troponin: consider in patients with cardiac risk factors or significant exposure, as myocardial injury can occur
- Lactate: elevated lactate suggests concomitant cyanide toxicity (consider in smoke inhalation victims)
- Head CT/MRI: not routine, but may show globus pallidus changes in severe or delayed presentations
Interpreting carboxyhemoglobin level
| COHb level | Interpretation |
|---|---|
| Up to 3% | Normal in nonsmokers |
| Up to 10-15% | Normal in smokers |
| 10-30% | Mild-moderate toxicity, headache, nausea |
| 30-50% | Moderate-severe, confusion, syncope |
| Over 50% | Severe, coma, high risk of death |
COHb level does not reliably predict clinical severity or long-term outcome, especially if blood is drawn after oxygen therapy has already begun or if exposure ended well before sampling. Clinical findings and exposure history remain central to decision-making.
Differential diagnosis
- Viral syndrome / influenza (lacks fever classically, consider in cluster presentations)
- Cyanide poisoning (often co-occurs in smoke inhalation; look for elevated lactate and refractory shock)
- Methemoglobinemia (also causes falsely altered pulse oximetry, but typically reads low around 85%, not normal)
- Migraine or primary headache disorder
- Acute coronary syndrome (CO can also precipitate true ischemia)
- Stroke or other causes of altered mental status
- Food poisoning / gastroenteritis
- Alcohol or other toxic ingestion
Management
Immediate steps
- Remove the patient from the source of exposure immediately
- Administer high-flow 100% supplemental oxygen via non-rebreather mask or endotracheal tube as soon as poisoning is suspected, even before confirmatory testing
- Continuous cardiac monitoring and ECG given risk of dysrhythmia and ischemia
Hyperbaric oxygen therapy (HBOT)
HBOT increases the rate of CO elimination from hemoglobin and tissues and may reduce the risk of delayed neurologic sequelae. Indications vary somewhat by guideline but generally include:
- Loss of consciousness at any point
- Significant neurologic findings (confusion, seizures, abnormal exam)
- Cardiovascular dysfunction or ischemia
- Pregnancy with significant exposure (due to higher fetal hemoglobin CO affinity)
- Markedly elevated carboxyhemoglobin (thresholds vary by institution, commonly cited around 25% or higher)
The evidence for mortality benefit from HBOT is mixed, and access/logistics vary, so normobaric 100% oxygen remains the universal first step while HBOT eligibility is assessed. Decisions should follow local toxicology/poison control guidance.
Supportive care
- IV fluids and treatment of rhabdomyolysis if present
- Seizure management with standard anticonvulsants if needed
- Empiric treatment for cyanide co-toxicity (hydroxocobalamin) if smoke inhalation with hemodynamic instability or significantly elevated lactate
- Address underlying source (carbon monoxide detector installation, appliance repair, fire department involvement) to prevent re-exposure
All suspected cases, even mild, warrant observation and consideration of poison control consultation given the risk of delayed sequelae.
Complications and prognosis
Most mild to moderate cases recover fully with prompt oxygen therapy. However, significant morbidity can occur:
- Delayed neuropsychiatric syndrome: cognitive decline, memory loss, personality changes, parkinsonism, occurring days to weeks after apparent recovery in up to a meaningful minority of moderate-to-severe cases
- Cardiac injury: myocardial ischemia or infarction, dysrhythmias, which independently predict increased long-term mortality
- Rhabdomyolysis and acute kidney injury
- Fetal demise or injury in pregnant patients due to high fetal hemoglobin affinity for CO and prolonged fetal CO elimination half-life
- Death, particularly with high-level or prolonged exposure, suicide attempts, or delayed recognition
Prognosis is generally better with rapid removal from the source and early initiation of oxygen therapy. Follow-up neuropsychological assessment is reasonable after moderate to severe poisoning given the risk of delayed sequelae.
COHb "HEADACHE": Headache, Enclosed space exposure, Altered mental status, Dizziness, Angina/arrhythmia, Cherry-red skin (rare), Hyperbaric therapy if severe, Elevated COHb on co-oximetry
Practice questions
1. A 34-year-old woman is brought to the emergency department by her husband after he found her confused in their garage with the car engine running and the garage door closed. Vital signs show oxygen saturation of 99% on room air by pulse oximetry. She is lethargic but arousable. Which of the following is the most appropriate next diagnostic step?
2. A family of four presents to the emergency department in winter with headache, nausea, and dizziness that began shortly after they turned on a portable generator inside their home during a power outage. Their dog has also been vomiting. Which finding, if present, would best support concomitant cyanide toxicity requiring additional antidotal therapy beyond oxygen?
3. A 58-year-old man with a history of coronary artery disease is treated for carbon monoxide poisoning after a house fire, with an initial carboxyhemoglobin level of 32% and a brief loss of consciousness at the scene. After receiving high-flow 100% oxygen, his mental status improves and his repeat carboxyhemoglobin level is 8%. Which of the following is the most appropriate next step in management?
Frequently asked questions
What causes carbon monoxide poisoning?
Carbon monoxide poisoning is caused by inhaling CO gas produced by incomplete combustion of fuels, most often from house fires, faulty furnaces or water heaters, portable generators used indoors, charcoal grills in enclosed spaces, or vehicle exhaust in closed garages. Because CO is colorless and odorless, exposure typically goes unnoticed until symptoms develop, making poorly ventilated spaces with combustion sources the highest-risk setting.
What are the first symptoms of carbon monoxide poisoning?
The earliest symptoms are typically headache, nausea, dizziness, and malaise, often mimicking a mild viral illness but without fever. A key diagnostic clue is multiple people or pets in the same household becoming ill simultaneously, with symptoms improving after leaving the building. As exposure continues, confusion, chest pain, and loss of consciousness can follow.
How is carbon monoxide poisoning diagnosed?
Diagnosis is confirmed by measuring carboxyhemoglobin level using co-oximetry on an arterial or venous blood gas sample, since standard pulse oximetry cannot distinguish carboxyhemoglobin from normal oxyhemoglobin and often reads falsely normal. Diagnosis also relies heavily on a suspicious exposure history, such as a heating malfunction, generator use, or smoke exposure, combined with compatible symptoms.
Is carbon monoxide poisoning curable?
Yes, most cases are fully reversible with prompt removal from the source and high-flow oxygen therapy, which accelerates clearance of carbon monoxide from hemoglobin. Severe cases may require hyperbaric oxygen therapy and carry risk of lasting complications, including delayed neuropsychiatric symptoms, cardiac injury, or death if treatment is delayed or exposure was prolonged and severe.
Carbon monoxide poisoning vs cyanide poisoning — what is the difference?
Both impair cellular oxygen use and commonly co-occur in smoke inhalation from fires, but CO poisoning works by binding hemoglobin and mitochondrial cytochrome oxidase, while cyanide primarily poisons cytochrome oxidase directly, causing more severe lactic acidosis and refractory shock. Elevated carboxyhemoglobin suggests CO toxicity, while markedly elevated lactate out of proportion to COHb level suggests concomitant cyanide toxicity requiring specific antidotal therapy.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: T58
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.