What is Carboxyhemoglobin

Carboxyhemoglobin: Overview and Practice Questions (2024)

by | Updated: May 16, 2024

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.

Carboxyhemoglobin Practice Questions

1. What is the definition of carboxyhemoglobin?
Carboxyhemoglobin is a complex formed when carbon monoxide binds with hemoglobin in the blood, reducing the blood’s ability to carry oxygen.

2. What is the normal CO ppm in the air?
Normal CO ppm in the air is 0.5 – 5 ppm.

3. What are the normal carboxyhemoglobin values for the sensor?
Normal carboxyhemoglobin values are 0-5% (less than 12% is normal for smokers).

4. At what carboxyhemoglobin value should you use high-flow oxygen?
Use high-flow oxygen at a carboxyhemoglobin value of 10%.

5. At what CO level in the air would you expect to see symptoms?
Symptoms, such as headaches, are expected at 35 ppm of CO in the air.

6. At what CO level in the air would you expect to see death in 30 minutes?
Death in 30 minutes can occur at 3200 ppm of CO in the air.

7. How much more affinity does carbon monoxide have to hemoglobin compared to oxygen?
Carbon monoxide has 200 to 240 times more affinity to hemoglobin than oxygen.

8. What does the “rainbow” carboxyhemoglobin sensor measure?
The “rainbow” carboxyhemoglobin sensor measures carbon monoxide bound to hemoglobin.

9. At what CO level in the air would you expect to see nausea and seizures?
Nausea and seizures can be expected at 800 ppm of CO in the air.

10. At what CO level in the air would you expect to see death in 2 hours?
Death in 2 hours can occur at 1,600 ppm of CO in the air.

11. At what carboxyhemoglobin level would you expect to see loss of consciousness (LOC) or fainting?
Loss of consciousness or fainting can occur at 40-50% carboxyhemoglobin levels.

12. At what carboxyhemoglobin level would you expect to see death?
Death can occur at 80% carboxyhemoglobin levels.

13. At what carboxyhemoglobin level would you expect to see seizures or apnea?
Seizures or apnea can occur at 60-70% carboxyhemoglobin levels.

14. At what carboxyhemoglobin concentration in nonsmokers and smokers is carbon monoxide poisoning indicated?
A carboxyhemoglobin concentration higher than 3% in nonsmokers and higher than 10% in smokers indicates some degree of carbon monoxide poisoning.

15. What is the treatment for carbon monoxide poisoning?
The treatment for carbon monoxide poisoning is breathing in 100% oxygen at more than 1.4 atmospheres within a hyperbaric chamber.

16. What is the maximum value in the oxyhemoglobin dissociation curve?
The maximum value in the oxyhemoglobin dissociation curve is 97%.

17. How does carboxyhemoglobin result in decreased oxygen transport and tissue ischemia?
Carboxyhemoglobin binds to cytochrome oxidase and impairs mitochondrial function.

18. How does carboxyhemoglobin affect muscle function?
Carboxyhemoglobin binds to myoglobin.

19. Where is CO rapidly absorbed?
CO is rapidly absorbed in the pulmonary endothelium.

20. What are the two factors CO elimination is dependent on?
CO elimination is dependent on the degree of oxygenation and minute ventilation.

21. What is the half-life of CO on room air, with high flow oxygen, and in a hyperbaric chamber?
The half-life of CO is 300 minutes on room air, 90 minutes with high flow oxygen, and 30 minutes in a hyperbaric chamber.

22. How does methylene chloride affect the body?
Methylene chloride is inhaled or ingested and then converted to CO by the liver.

23. What is a late sign of CO poisoning?
Cherry red skin is a late sign of CO poisoning.

24. What is delayed neuropsychiatric syndrome (DNS)?
Delayed neuropsychiatric syndrome (DNS) is a symptom of CO poisoning that arises days, weeks, or months after CO poisoning recovery and is characterized by personality changes, movement disorders, and neurological defects.

25. Why is pulse oximetry not reliable in CO poisoning cases?
Pulse oximetry is not reliable because it cannot differentiate between carboxyhemoglobin and oxyhemoglobin.

26. Why order a head CT in CO poisoning cases?
A head CT is ordered to rule out other causes of neurologic deficits.

27. What are the indications for hyperbaric oxygen therapy?
Indications for hyperbaric oxygen therapy include CO levels greater than 25%, CO levels greater than 25% in pregnant women, severe metabolic acidosis, loss of consciousness, and end organ ischemia as indicated by ECG.

28. What is the function of a hyperbaric oxygen chamber?
A hyperbaric oxygen chamber increases O2 pressure to 2-3 times the amount of atmospheric O2.

29. Does carbon monoxide poisoning occur in a specific time of year?
No, it can happen at any time.

30. What are the physical properties of carbon monoxide?
Carbon monoxide is colorless, odorless, tasteless, non-irritating, and a gas.

31. What are the most common sources of CO in deaths?
The most common sources of CO in deaths are fires, automobile exhaust, defective heaters, and incomplete combustion of burning products.

32. When is CO produced?
CO is produced whenever organic materials are burned with an inadequate supply of oxygen necessary to produce complete combustion.

33. What are common sources of CO in the home?
Common sources of CO in the home include furnaces, charcoal grills, ranges, water heaters, autos in closed garages, fireplaces, and gas clothes dryers.

34. How many deaths caused by CO occur in the US per year?
Approximately 2700 deaths (excluding fire deaths).

35. How many CO deaths per year in the US are due to suicide?
2,000 deaths

36. How many CO deaths per year in the US are due to accidents?
700 deaths

37. Are CO deaths more commonly due to suicide or accidents?
Suicide is more likely.

38. Most suicides due to CO poisoning involve what method?
Inhalation of automobile exhaust.

39. What is the mechanism of action for CO poisoning?
CO produces tissue hypoxia by competing with oxygen for binding sites on hemeproteins (hemoglobin, myoglobin), displacing oxygen atoms and preventing hemoglobin from delivering oxygen to the cells.

40. What molecule does CO compete with O2 for binding?
Hemeproteins (hemoglobin, myoglobin)

41. Which has a greater affinity for hemoglobin, CO or O2?

42. How much greater is the affinity of CO for hemeproteins compared to O2?
30 to 500 times greater

43. How much greater is the affinity of CO for hemoglobin compared to O2?
250 to 300 times greater

44. How is CO a toxin?
CO is believed to have a toxic effect at the cellular level by impairing mitochondrial respiration.

45. What is the percent saturation of CO (% COHb)?
The percentage of hemoglobin combined with CO in the form of carboxyhemoglobin (COHb).

46. Do you need a lot of CO in the atmosphere to produce high carboxyhemoglobin levels?
False; you only need a little.

47. What are the normal levels of carboxyhemoglobin for smokers?
Normal is 5-6%, can commonly reach 10%, and can exceed 15%.

48. Are smokers more likely to have more CO in their system than nonsmokers?

49. What are the symptoms at 10% COHb?
No symptoms. Heavy smokers can have as much as 10 to 15% COHb.

50. What are the symptoms at 15% COHb?
Mild headache

51. What are the symptoms at 25% COHb?
Nausea and serious headache. Fairly quick recovery after treatment with oxygen and/or fresh air.

52. What are the symptoms at 30% COHb?
Symptoms intensify. Potential for long-term effects, especially in infants, children, the elderly, victims of heart disease, and pregnant women.

53. What are the symptoms at 40 to 60% COHb?
Mental confusion, weakness, loss of coordination, and unconsciousness.

54. What are the symptoms at 60%+ COHb?

55. At what COHb percentage will you die from CO poisoning?

56. What is the most common cause of CO poisoning resulting in death?

57. What is the second most common cause of CO poisoning resulting in death?
Inhalation of vehicle exhaust fumes, most often due to suicide.

58. Why do CO poisoning deaths sometimes have skin slippage?
CO diverted into a vehicle or other small area can significantly raise the temperature of this space, causing postmortem skin slippage in some cases.

59. What factors can affect the carboxyhemoglobin levels of persons who die from CO poisoning?
Source of the CO, circumstances surrounding the death, and the health of the individual.

60. What is the gold-standard treatment for CO poisoning?
Hyperbaric oxygen

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.

John Landry, BS, RRT

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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