Canadian Medical Association

carbon monoxide poisoning clinical advice

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In Canada’s coldest months, when fuel-burning appliances are in greater use, carbon monoxide (CO) poisoning should be top of mind. According to a 2017 study by the University of the Fraser Valley, there are more than 300 CO-related deaths and more than 200 hospitalizations per year in Canada. 75 per cent of CO-related hospitalizations occurred as a result of CO poisoning originating in the home. This deadly gas is most dangerous when people are sleeping.

CO has no colour, odor or taste ― the only way to detect its presence is with a carbon monoxide alarm.

Here are some resources on carbon monoxide poisoning selected by Joule’s Ask a Librarian team using a variety of tools included with a CMA membership.

Clinical summary from DynaMed

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Carbon Monoxide Toxicity

  • Common risk factors include:
    • use of charcoal, coke, gas, petroleum, or wood-burning heaters and combined heater/cooking devices in poorly ventilated areas
    • smoke inhalation secondary to fireplace or building fires
    • inhaling motor vehicle exhaust fumes (either intentionally or unintentionally)
    • exposure to methylene chloride, which is converted in the liver to carbon monoxide; sources of exposure include
      • industrial (solvent)
      • household items; found in several products such as paint thinner or remover
  • Alcohol or drug use may increase risk for unintentional CO poisoning.


  • Suspect the diagnosis in patients with recent history of potential carbon monoxide (CO) exposure and symptoms suggestive of CO poisoning (such as headache, nausea, dyspnea, or altered mental status).
  • The diagnosis is confirmed by elevated carboxyhemoglobin (COHb) levels, but low or normal levels do not rule out carbon monoxide toxicity.
    • Checking for elevated COHb levels can be performed using portable CO-oximetry devices, but do not use regular pulse oximetry.
    • Definitive testing is usually done with arterial blood gas sample, but can be performed with venous blood sampling as COHb levels are similar with both arterial and venous blood.
    • COHb levels consistent with CO toxicity include:
      • ≥ 3%-4% for nonsmokers
      • ≥ 10% for smokers
  • Check serum lactate levels to screen for possible concurrent cyanide toxicity (typically characterized by lactate > 10 mmol/L) if the source of the CO was a fire.
  • Perform chest x-ray to check for other causes of dyspnea.
  • Perform electrocardiogram and measure cardiac enzymes due to possibility of myocardial injury in patients with moderate to severe CO poisoning (looking for myocardial ischemia, infarction, or arrhythmias) (Strong recommendation).
  • Consider checking a toxicology panel if the CO poisoning was intentional.


  • Treat all patients with suspected carbon monoxide (CO) exposure with oxygen inhalation (after drawing blood for carboxyhemoglobin levels).
    • use nonrebreather mask to deliver 100% oxygen until patient is asymptomatic and carboxyhemoglobin levels are
      • ≤ 3%-4% in nonsmokers
      • ≤ 10% in smokers
    • Consider hyperbaric oxygen for patients with CO poisoning (Weak recommendation).
      • Definitive criteria or evidence for hyperbaric oxygen therapy is lacking.
      • Experts often suggest hyperbaric oxygen therapy in the following clinical scenarios:
        • loss of consciousness
        • persistent abnormal neurologic findings, such as mental status alteration
        • pregnancy
        • high carboxyhemoglobin levels (> 25%)
        • prolonged exposure (> 24 hours)
        • cardiovascular dysfunction
        • methylene chloride as the source of CO poisoning
  • The elimination half-life of CO is 4-5 hours when breathing room air in most patients, and 13 hours in patients with CO poisoning due to methylene chloride exposure.
    • Administration of 100% oxygen by nonrebreather face mask decreases half-life to about
      • 1 hour in most patients
      • 6 hours in patients with methylene chloride exposure
    • Hyperbaric chamber at 2.5 atmospheres decreases half-life to 20 minutes.
    • Patients with CO poisoning due to methylene chloride exposure may need longer therapy due to longer half-life of CO from this source.
  • Perform clinical follow-up at 1-2 months after the event for all patients with acute accidental CO poisoning to assess for delayed neurologic sequelae.

Preview (CMA members can access full summaries via this link)

Clinical summary from Essential Evidence Plus

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Carbon monoxide poisoning



  • Need to rely on a careful history and physical exam.
  • Use of diagnostic testing to evaluate the severity of poisoning.
  • Check levels of COHb.
  • Use of neuroimaging to exclude other diagnoses.

History and Physical

  • Obtain history and objective proof, if possible, of the inhaled poison and the duration of the exposure. Need to concentrate on neurologic complaints, cardiovascular complaints and other symptoms such as abdominal pain and muscle cramps.
  • Among the more common symptoms are headache, weakness, muscle cramps, nausea/vomiting, abdominal pain, altered mental status, dizziness, chest pain, dyspnea and memory loss.
  • Check for housemates with similar symptoms.
  • Check for history of smoking and other possible exposures from automobiles or other sources of CO.
  • On physical exam, look for signs and symptoms of tissue hypoxia including neurologic deficits and cardiovascular abnormalities.
  • Some of the more common signs include ataxia, dysrhythmias, coma, skin blistering.

Diagnostic testing

  • Order CBC, electrolytes, renal function tests, urinalysis and arterial blood gas analyses with co-oximetry tests to include COHb levels. Pulse oximetry is of little value as current technology is unable to distinguish COHb from oxygenated hemoglobin.
  • COHb levels are elevated chronically in cigarette smokers; they also become less reliable the longer the delay from exposure, and do not always correlate with symptoms or outcomes.
  • Heart-type fatty acid-binding protein may be useful as a biomarker for acute CO poisoning, with a cutoff of >1.5 ng/ml. 6
  • Order a chest x-ray in patients with cardiovascular or respiratory signs or symptoms. Consider neuroimaging for patients who are unconscious or have other neurologic symptoms.
  • CK-MB and cardiac troponin can identify patients with acute cardiac damage due to CO poisoning. 9
  • Pregnancy test is mandatory of women of childbearing age as the unborn fetus is at greater risk than the mother because fetal Hb has a much higher affinity for CO than adult Hb. 1

CMA members can access full summaries via this link

Further reading

Book chapters and clinical summaries via ClinicalKey (CMA members only):

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Clinical Overview - Carbon monoxide toxicity, Updated September 20, 2019. © 2019.


Key Points

  • Carbon monoxide toxicity results from exposure to carbon monoxide, a colorless, odorless gas produced as a byproduct of incomplete combustion of carbon-based products (eg, gas, coal) 1
    • Exposure can be either unintentional (eg, poorly ventilated combustion sources, house fires) or intentional (suicide attempts)
  • Presents with nonspecific signs and symptoms including headache, dizziness, nausea, dyspnea, chest pain, and altered mental status; easily misdiagnosed as other conditions
  • Diagnosis is made on basis of exposure history, clinical presentation, and elevated serum carboxyhemoglobin level
  • First line treatment consists of 100% normobaric oxygen via face mask or endotracheal intubation, airway management, and cardiovascular support
  • Hyperbaric oxygen is indicated in selected patients, including those with severe poisoning (eg, loss of consciousness, neurologic or cardiac manifestations) and pregnant patients


  • Easily misdiagnosed in the absence of a clear exposure history; maintain high index of suspicion during winter months
  • Toxic effects on fetus are more severe than those on mother; therefore, pregnant patients should be treated with oxygen (possibly hyperbaric oxygen) even if clinical symptoms are mild

CMA members can access full summaries via this link

Articles via CMAJ and CMAJ Open:

Five things to know about...: Carbon monoxide poisoning, Peter E. Wu and David N. Juurlink

CMAJ May 13, 2014 186 (8) 611

Mortality and hospital admission rates for unintentional nonfire-related carbon monoxide poisoning across Canada: a trend analysis,  Eric Lavigne, Scott Weichenthal, Joan Wong, Marc Smith-Doiron, Rose Dugandzic, and Tom Kosatsky, cmajopen 3:E223-E230; published online May 25, 2015.

Free information sources

Carbon Monoxide Poisoning - Hospitalizations and Deaths in Canada, University of the Fraser Valley, 2017.

Carbon monoxide poisoning, Government of Canada, Date modified: 2019-08-21.

Carbon Monoxide Poisoning, HealthLink BC, Current as of: December 13, 2018.

Carbon Monoxide, Canada Safety Council.

Carbon Monoxide Poisoning Centers for Disease Control and Prevention, Page last reviewed: January 3, 2020.

Need more information or have a more focused clinical question on carbon monoxide poisoning (or any other clinical topic of interest)? Contact the Ask a Librarian team to request a literature search.

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