What is Carboxyhemoglobin

What is Carboxyhemoglobin? (2023)

by | Updated: Nov 25, 2023

Carboxyhemoglobin, a complex molecule formed by the binding of carbon monoxide (CO) to hemoglobin, is a critical and often dangerous byproduct of CO exposure.

Characteristically known for its affinity to hemoglobin that surpasses oxygen by more than 200 times, carboxyhemoglobin results in reduced oxygen delivery to organs and tissues, leading to various degrees of tissue hypoxia and potential organ failure.

This article provides an overview of the formation, effects, and clinical implications of carboxyhemoglobin, underscoring its significance in both environmental and occupational health contexts.

What is Carboxyhemoglobin?

Carboxyhemoglobin (COHb) is a compound formed when carbon monoxide (CO) binds to the hemoglobin in red blood cells. This binding impairs the blood’s ability to carry oxygen, leading to reduced oxygen supply to the body’s tissues and organs, and can result in serious health effects.

Carboxyhemoglobin Carbon Monoxide Poisoning Illustration

Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless, and tasteless gas, making it difficult to detect. It’s produced by the incomplete combustion of carbon-containing materials like fossil fuels.

Highly toxic, CO can bind with hemoglobin in the blood more effectively than oxygen, leading to potentially fatal health complications such as impaired oxygen transport and tissue damage.


Hemoglobin is a crucial protein in red blood cells, responsible for transporting oxygen from the lungs to the body’s tissues and organs, and returning carbon dioxide back to the lungs for expulsion.

It’s the iron in hemoglobin that binds oxygen, giving blood its characteristic red color.

The functionality of hemoglobin is essential for sustaining life and ensuring the efficient operation of bodily tissues and organs.

Affinity of Hemoglobin for Carbon Monoxide

The affinity of hemoglobin for carbon monoxide is remarkably high, about 200-250 times greater than its affinity for oxygen.

This strong attraction means that even small concentrations of carbon monoxide in the air can lead to significant formation of carboxyhemoglobin.

When carbon monoxide binds to hemoglobin, it prevents oxygen from doing so, drastically reducing the amount of oxygen transported through the bloodstream.

This process can quickly lead to tissue hypoxia, a dangerous condition where body tissues are deprived of adequate oxygen, and can result in severe health consequences, including organ damage and potentially death.

What is Carbon Monoxide Poisoning?

Carbon monoxide poisoning occurs when carbon monoxide (CO) is inhaled and binds to hemoglobin in the blood, forming carboxyhemoglobin.

This binding reduces the blood’s capacity to carry oxygen, leading to a decrease in oxygen supply to the body’s tissues and organs.

Symptoms can range from mild, such as headache and dizziness, to severe, including confusion, loss of consciousness, and even death.

CO poisoning is particularly insidious because the gas is odorless and colorless, making it difficult to detect until symptoms manifest.

Prolonged or high-level exposure can result in significant and sometimes irreversible damage to the heart and brain.

Risks of Carboxyhemoglobin

The risks of carboxyhemoglobin formation in the blood primarily stem from its ability to significantly impair oxygen transport and delivery to the body’s tissues and organs.

Elevated levels of carboxyhemoglobin can lead to a range of health issues, including:

  • Tissue Hypoxia: By replacing oxygen in the hemoglobin, carboxyhemoglobin reduces the amount of oxygen that reaches body tissues, leading to hypoxia (oxygen deficiency at the tissue level).
  • Organ Damage: Vital organs, particularly the brain and heart, which are highly dependent on oxygen, can suffer acute or chronic damage due to oxygen deprivation.
  • Neurological Effects: Symptoms such as headaches, dizziness, confusion, and in severe cases, loss of consciousness and seizures may occur, reflecting the sensitivity of brain tissues to oxygen deprivation.
  • Cardiovascular Stress: The heart must work harder to circulate the reduced amount of available oxygen, potentially leading to chest pain, arrhythmias, and in extreme cases, myocardial infarction (heart attack).
  • Chronic Health Issues: Prolonged exposure to low levels of carbon monoxide can result in long-term neurological and cardiac complications, including chronic fatigue, memory problems, and even heart disease.
  • Fatal Outcomes: High levels of carboxyhemoglobin can be life-threatening, leading to coma or death, especially without prompt treatment.

Note: The risks are particularly severe in enclosed or poorly ventilated spaces where carbon monoxide can accumulate unnoticed. Vulnerable populations, such as the elderly, pregnant women, children, and those with pre-existing respiratory or cardiovascular conditions, are at greater risk of adverse effects.

Treatment for Carboxyhemoglobin

Treatment for elevated levels of carboxyhemoglobin primarily focuses on rapidly reducing the amount of carbon monoxide in the blood and restoring oxygen-carrying capacity.

Key treatment strategies include:

  • Immediate Exposure Removal: The first and most critical step is to remove the patient from the carbon monoxide source to a location with fresh air, thereby halting further CO inhalation.
  • Oxygen Therapy: Administering high concentrations of oxygen is crucial. This is typically done through a tight-fitting mask, which helps to quickly displace carbon monoxide from hemoglobin, reducing carboxyhemoglobin levels and restoring oxygen transport.
  • Hyperbaric Oxygen Therapy (HBOT): In severe cases, or when neurological symptoms are present, hyperbaric oxygen therapy may be employed. HBOT involves breathing pure oxygen in a pressurized chamber, which can hasten the replacement of CO from hemoglobin and enhance oxygen delivery to deprived tissues.
  • Supportive Care: Depending on the severity of symptoms, additional treatments such as fluids, medications to alleviate symptoms like headaches and nausea, and monitoring for heart and neurological issues may be necessary.
  • Monitoring and Follow-Up: Prolonged monitoring for any delayed neurological effects is crucial, as some complications from carbon monoxide poisoning can appear days or even weeks after the initial exposure.
  • Education and Prevention: Post-treatment, educating the patient and family about the dangers of CO exposure and ways to prevent future incidents, such as installing CO detectors and ensuring proper ventilation, is important for long-term safety.

Note: Immediate medical attention is essential for suspected carbon monoxide poisoning, as the condition can rapidly become life-threatening.

FAQs About Carboxyhemoglobin

What Does a High Carboxyhemoglobin Mean?

A high carboxyhemoglobin level indicates significant exposure to carbon monoxide.

This condition means that a large proportion of hemoglobin in the blood is bound with carbon monoxide rather than oxygen, leading to reduced oxygen delivery to the body’s tissues and organs.

High levels can result in symptoms ranging from headaches and dizziness to more severe effects like unconsciousness and, in extreme cases, death.

What is Carboxyhemoglobin Also Known As?

Carboxyhemoglobin is also commonly referred to as COHb.

This term is used in medical and scientific contexts to denote the complex formed when carbon monoxide binds with hemoglobin in the blood, hindering the blood’s ability to transport oxygen.

How Do You Flush Carbon Monoxide Out of Your Body?

Flushing carbon monoxide from the body primarily involves inhaling high concentrations of oxygen.

This can be done through normal breathing in a fresh-air environment for mild cases or, more effectively, through administering 100% oxygen using medical equipment.

This process accelerates the replacement of carbon monoxide with oxygen in the blood, aiding the body in expelling the toxin through the normal respiratory process.

What Disease is Caused by Carboxyhemoglobin?

Carboxyhemoglobin itself doesn’t directly cause a specific disease but is a key marker and mediator of carbon monoxide poisoning.

This condition can lead to a range of health issues, from mild neurological symptoms (like headache and dizziness) to severe outcomes such as cardiac arrest, brain damage, and even death, depending on the exposure level and duration.

What are Normal Carboxyhemoglobin Levels?

Normal carboxyhemoglobin levels vary slightly based on individual factors and exposure to environmental CO.

In non-smokers, typical levels are usually below 1-2%, while smokers may have levels around 5-10% due to inhaling carbon monoxide present in cigarette smoke.

Levels exceeding these ranges can indicate dangerous exposure to carbon monoxide.

How Do You Measure Carboxyhemoglobin Levels?

Carboxyhemoglobin levels are commonly measured using a blood test known as a carboxyhemoglobin test.

This test requires a blood sample, typically drawn from a vein, which is then analyzed to determine the percentage of hemoglobin that is bound with carbon monoxide.

The test is crucial for diagnosing carbon monoxide poisoning and assessing its severity.

How Does Carboxyhemoglobin Affect Pulse Oximetry?

Carboxyhemoglobin can significantly affect the readings of pulse oximetry, a non-invasive method used to measure the oxygen saturation level of the blood.

Since carboxyhemoglobin and oxyhemoglobin have similar absorption spectra, standard pulse oximeters can mistake carboxyhemoglobin for oxyhemoglobin, falsely indicating normal or near-normal oxygen levels.

This limitation means pulse oximetry can be unreliable in diagnosing carbon monoxide poisoning, as it may not reflect the actual reduction in oxygen-carrying capacity of the blood.

Final Thoughts

The study of carboxyhemoglobin is vital in understanding the pathophysiological effects of carbon monoxide poisoning.

The high affinity of carbon monoxide for hemoglobin not only impairs oxygen transport and delivery but also triggers a cascade of inflammatory responses and cellular damage.

Understanding these mechanisms is crucial for the development of effective treatment strategies and preventive measures in both clinical and public health settings.

Awareness and early detection play a pivotal role in mitigating the potentially lethal consequences of carbon monoxide exposure, emphasizing the importance of continuous research and public education on this topic.

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.


  • Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
  • Palmeri R, Gupta V. Carboxyhemoglobin Toxicity. [Updated 2023 Apr 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.

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