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
• History of central nervous system (CNS) disease (i.e., Alzheimer's, Parkinson's, dementia, demyelinating disease (MS), etc.)
• History of prior brain injury (i.e., stroke, traumatic brain injury)
• Presence of chronic debilitating disease likely to result in death within 12 months (i.e., kidney failure on dialysis, heart failure)
• Subject has a significant medical condition or conditions that would interfere with the treatment, safety, or compliance with the protocol.
• Intubated subjects
• Subjects requiring greater than 50% oxygen by non-rebreather facemask or exhibiting evidence of respiratory compromise or heart failure
• Subjects who, in the opinion of the investigator, are unable to comply with the requirements of the study or are unsuitable for any reason. The investigator and the Sponsor, prior to enrolling the subject on a case-by-case basis, must approve and document any waiver of these inclusion and exclusion criteria.