• Neurological examination
• Depression, anxiety or post-traumatic stress syndrome
• Vocational assessment
• Patient self-reports of CO-related problems
• We will follow patients at 6 months to categorize long-term outcomes, but the prime outcome follow-up period is 6 weeks after CO poisoning.

This randomized trial will investigate important clinical outcomes of patients with acute carbon monoxide poisoning randomized to receive either one or three hyperbaric oxygen treatments.

All patients presenting with acute carbon monoxide poisoning will receive one hyperbaric oxygen treatment (barring contraindications for hyperbaric oxygen therapy). After this treatment, eligible patients who provide consent will be randomly allocated to receive two sham sessions, or two additional hyperbaric oxygen sessions administered in a double-blind fashion.

Outcome measures will be administered at 6 weeks and 6 months.

Inclusion Criteria:

• Symptomatic CO poisoning (headache, nausea, vomiting, dizziness, fatigue, muscle aches, slowed mentation, confusion, or loss of consciousness).
• Carboxyhemoglobin (COHb) levels > 10% or confirmation of poisoning with ambient levels > 50 ppm in a patient with symptoms compatible with CO poisoning and no other reasonable explanation for their signs and symptoms.
• Less than 24 hours from removal from the source of CO exposure and study enrollment.
• Accidental poisoning

Exclusion Criteria:

• Pregnancy
• Age < 18 years or > 79 years
• Complication during first hyperbaric oxygen session precluding subsequent hyperbaric oxygen.
• Intentional CO poisoning
• Unable to obtain informed consent
• Moribund patient
• Concomitant smoke inhalation with cyanide poisoning
• Bleomycin use within two weeks of study enrollment
• Intracardiac defibrillator that cannot be deactivated
• Non-English speaking
• Unlikely to return at 6 weeks