Carbon monoxide poisoning


Carbon monoxide (CO) poisoning occurs when carbon monoxide gas is inhaled. CO is a colorless, odorless, highly poisonous gas that is produced by incomplete combustion. It is found in automobile exhaust fumes, faulty stoves and heating systems, fires, and cigarette smoke. Other sources include wood-burning stoves, kerosene heaters, improperly ventilated water heaters and gas stoves, and blocked or poorly maintained chimney flues. CO interferes with the ability of the blood to carry oxygen. The result is headache, nausea , convulsions, and finally death by asphyxiation.


Carbon monoxide, sometimes called coal gas, has been known as a toxic substance since the third century B.C. It was used for executions and suicides in early Rome.

Anyone who is exposed to CO becomes sick, and the entire body is involved in CO poisoning. A developing fetus can also be poisoned if a pregnant woman breathes CO gas. Infants, people with heart or lung disease, or those with anemia may be more seriously affected. People such as underground parking garage attendants who are exposed to car exhausts in a confined area are more likely to be poisoned by CO. Firemen also run a higher risk of inhaling CO.


Carbon monoxide is the leading cause of accidental poisoning in the United States. Experts estimate that 1,500 Americans die each year from accidental exposure to CO and another 2,300 from intentional exposure ( suicide ). An additional 10,000 people seek medical attention after exposure to CO and recover.

Causes and symptoms

Normally when a person breathes fresh air into the lungs, the oxygen in the air binds with a molecule called hemoglobin (Hb) that is found in red blood cells. This process allows oxygen to be moved from the lungs to every part of the body. When the oxygen/hemoglobin complex reaches a muscle where it is needed, the oxygen is released. Because the oxygen binding process is reversible, hemoglobin can be used over and over again to pick up oxygen and move it throughout the body.

Inhaling carbon monoxide gas interferes with this oxygen transport system. In the lungs, CO competes with oxygen to bind with the hemoglobin molecule. Hemoglobin prefers CO to oxygen and accepts it more than 200 times more readily than it accepts oxygen. Not only does the hemoglobin prefer CO, it holds on to the CO much more tightly, forming a complex called carboxyhemoglobin (COHb). As a person breathes CO contaminated air, more and more oxygen transportation sites on the hemoglobin molecules become blocked by CO. Gradually, there are fewer and fewer sites available for oxygen. All cells need oxygen to live. When they do not get enough oxygen, cellular metabolism is disrupted and eventually cells begin to die.

The symptoms of CO poisoning and the speed with which they appear depend on the concentration of CO in the air and the rate and efficiency with which a person breathes. Heavy smokers can start off with up to 9 percent of their hemoglobin already bound to CO, which they regularly inhale in cigarette smoke. This makes them much more susceptible to environmental CO. The Occupational Safety and Health Administration (OSHA) has established a maximum permissible exposure level of 50 parts per million (ppm) over eight hours.

With exposure to 200 ppm for two to three hours, a person begins to experience headache, fatigue, nausea, and dizziness . These symptoms correspond to 15 to 25 percent COHb in the blood. When the concentration of COHb reaches 50 percent or more, death results in a very short time. Emergency room physicians have the most experience diagnosing and treating CO poisoning.

The symptoms of CO poisoning in order of increasing severity include the following:

  • headache
  • shortness of breath
  • dizziness
  • fatigue
  • mental confusion and difficulty thinking
  • loss of fine hand-eye coordination
  • nausea and vomiting
  • rapid heart rate
  • hallucinations
  • inability to execute voluntary movements accurately
  • collapse
  • lowered body temperature (hypothermia)
  • coma
  • convulsions
  • seriously low blood pressure
  • cardiac and respiratory failure
  • death

In some cases, the skin, mucous membranes, and nails of a person with CO poisoning are cherry red or bright pink. Because the color change does not always occur, it is an unreliable symptom to count on for diagnosis.

Although most CO poisoning is acute, or sudden, it is possible to suffer from chronic CO poisoning. This condition exists when a person is exposed to low levels of the gas over a period of days to months. Symptoms are often vague and include (in order of frequency) fatigue, headache, dizziness, sleep disturbances, cardiac symptoms, apathy, nausea, and memory disturbances. Little is known about chronic CO poisoning, and it is often misdiagnosed.

When to call the doctor

A healthcare professional should be consulted whenever more than passing exposure to carbon monoxide is suspected. While waiting for help to arrive, a potentially affected person should be moved outdoors.


The main reason to suspect CO poisoning is evidence that fuel is being burned in a confined area, for example, a car running inside a closed garage, a charcoal grill burning indoors, or an unvented kerosene heater in a workshop. Under these circumstances, one or more persons suffering from the symptoms listed above strongly suggests CO poisoning. In the absence of some concrete reason to suspect CO poisoning, the disorder is often misdiagnosed as migraine headache, stroke , psychiatric illness, food poisoning , alcohol poisoning, or heart disease.

Concrete confirmation of CO poisoning comes from a carboxyhemoglobin test. This blood test measures the amount of CO that is bound to hemoglobin in the body. Blood is drawn as soon after suspected exposure to CO as possible.

Other tests that are useful in determining the extent of CO poisoning include measurement of other arterial blood gases and pH; a complete blood count; measurement of other blood components such as sodium, potassium, bicarbonate, urea nitrogen, and lactic acid; an electrocardiogram (ECG); and a chest x ray.


Immediate treatment for CO poisoning is to remove the victim from the source of carbon monoxide gas and into fresh air. If the victim is not breathing and has no pulse, cardiopulmonary resuscitation (CPR) should be started. Depending on the severity of the poisoning, 100 percent oxygen may be given with a tight fitting mask as soon as it is available.

Taken with other symptoms of CO poisoning, COHb levels of over 25 percent in healthy individuals, over 15 percent in people with a history of heart or lung disease, and over 10 percent in pregnant women usually indicate the need for hospitalization . In the hospital, fluids and electrolytes are given to correct imbalances that have arisen from the breakdown of cellular metabolism.

In severe cases of CO poisoning, individuals are given hyperbaric oxygen therapy. This treatment involves placing the person in a chamber in which the person breathes 100 percent oxygen at a pressure of more than one atmosphere (the normal pressure the atmosphere exerts at sea level). The increased pressure forces more oxygen into the blood. Hyperbaric facilities are specialized and are usually available only at larger hospitals.


The speed and degree of recovery from CO poisoning depends on the length of exposure to the gas and the concentration of carbon monoxide. The half-life of CO in normal room air is four to five hours, which means that in four to five hours half of the CO bound to hemoglobin will be replaced with oxygen. At normal atmospheric pressures, but breathing 100 percent oxygen, the half-life for the elimination of CO from the body is 50 to 70 minutes. In hyperbaric therapy at three atmospheres of pressure, the half-life is reduced to between 20 and 25 minutes.

Although the symptoms of CO poisoning may subside in a few hours, some affected persons show memory problems, fatigue, confusion, and mood changes for two to four weeks after their exposure to the gas.


Carbon monoxide poisoning is preventable. Particular care should be paid to situations where fuel is burned in a confined area. Portable and permanently installed carbon monoxide detectors that sound a warning similar to smoke detectors are available for under $50. Specific actions that prevent CO poisoning include the following:

  • Stop smoking . Smokers have less tolerance to environmental CO.
  • Have heating systems and appliances installed by a qualified contractor to assure that they are properly vented and meet local building codes.
  • Inspect and properly maintain heating systems, chimneys, and appliances.
  • Do not use a gas oven or stove to heat the home.
  • Do not burn charcoal indoors.
  • Make sure there is good ventilation if using a kerosene heater indoors.
  • Do not leave cars or trucks running inside the garage.
  • Keep car windows rolled up when stuck in heavy traffic, especially if inside a tunnel.

Parental concerns

Parents should not allow children to play in areas heated by kerosene space heaters or to use charcoal grills of any kind indoors.


Carboxyhemoglobin —Hemoglobin that is bound to carbon monoxide instead of oxygen.

Hemoglobin —An iron-containing pigment of red blood cells composed of four amino acid chains (alpha, beta, gamma, delta) that delivers oxygen from the lungs to the cells of the body and carries carbon dioxide from the cells to the lungs.

Hypothermia —A serious condition in which body temperature falls below 95°F (35 °C). It is usually caused by prolonged exposure to the cold. pH —A measurement of the acidity or alkalinity of a solution. Based on a scale of 14, a pH of 7.0 is neutral. A pH below 7.0 is an acid; the lower the number, the stronger the acid. A pH above 7.0 is a base; the higher the number, the stronger the base. Blood pH is slightly alkaline (basic) with a normal range of 7.36–7.44.



Braunwald, Eugene, et al, eds. "Hypoxia and Cyanosis." In Harrison's Principles of Internal Medicine , 15th ed. New York: McGraw Hill, 2001, pp. 214–6.

Carbon Monoxide Poisoning: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References. San Diego, CA: Icon Health Publications, 2004.

Penney, David G. Carbon Monoxide Toxicity. Lakeland, FL: CRC Press, 2000.

Robertson, William O. "Chronic Poisoning: Trace Metals and Others." In Cecil Textbook of Medicine , 22nd ed. Edited by Lee Goldman, et al. Philadelphia: Saunders, 2003, pp. 91–9.


Harper A., and J. Croft-Baker. "Carbon monoxide poisoning: undetected by both patients and their doctors." Age and Ageing 33, no. 2 (2004): 105–9.

Huffman, S. M. "Exposure to carbon monoxide from material handling equipment." Journal of Occupational and Environmental Hygiene 1, no. 5 (2004): D54–6.

Piantadosi C. A. "Carbon monoxide poisoning." Undersea and Hyperbaric Medicine 31, no. 1 (2004): 167–77.

Thomassen, O., G. Brattebo, M. Rostrup. "Carbon monoxide poisoning while using a small cooking stove in a tent." American Journal of Emergency Medicine 22, no. 3 (2004): 204–6.

Vacchiano, G., and R. Torino. "Carbon-monoxide poisoning, behavioral changes and suicide: an unusual industrial accident." Journal of Clinical Forensic Medicine 8, no. 2 (2004): 86–92.


American Academy of Clinical Toxicology. 777 East Park Drive, PO Box 8820, Harrisburg, PA 17105–8820. Web site:

American Academy of Emergency Medicine. 611 East Wells Street, Milwaukee, WI 53202. Web site:

American Academy of Family Physicians. 11400 Tomahawk Creek Parkway, Leawood, KS 66211–2672. Web site:

American Academy of Pediatrics. 141 Northwest Point Boulevard, Elk Grove Village, IL 60007–1098. Web site:

American Association of Poison Control Centers. 3201 New Mexico Avenue NW, Washington, DC 20016. Web site:

American College of Emergency Physicians. PO Box 619911, Dallas, TX 75261–9911. Web site:

American College of Hyperbaric Medicine. PO Box 25914–130, Houston, Texas 77265. Web site:

American College of Occupational and Environmental Medicine. 55 West Seegers Road, Arlington Heights, IL 60005. Web site:

American College of Osteopathic Emergency Physicians. 142 E. Ontario Street, Suite 550, Chicago, IL 60611. Web site:

International Congress on Hyperbaric Medicine. 1592 Union Street, San Francisco, CA 94123. Web site:

Undersea and Hyperbaric Medical Society. 10531 Metropolitan Ave, Kensington, MD 20895. Web site:


"A Guide to Prevent Carbon Monoxide Poisoning." Industry Trade Group. Available online at (accessed November 2, 2004).

"Carbon Monoxide Poisoning." Centers for Disease Control and Prevention. Available online at (accessed November 2, 2004).

"Carbon Monoxide Poisoning." National Library of Medicine. Available online at (accessed November 2, 2004).


Carbon Monoxide Headquarters. Wayne State University School of Medicine. (accessed November 2, 2004).

L. Fleming Fallon Jr., MD, DrPH

User Contributions:

Comment about this article, ask questions, or add new information about this topic: