Carbon Monoxide Poisoning


Carbon monoxide poisoning occurs when carbon monoxide (CO) gas is inhaled. CO, which consists of one carbon (C) atom and one oxygen (O) atom, is a colorless, odorless, highly toxic gas that is produced by incomplete combustion. CO interferes with the ability of the blood to carry oxygen. Effects of CO are directly related to the concentration in parts per million (ppm) in the air. As a result, exposure to small amounts of CO mimics flu-like symptoms such as headaches, dizziness, fatigue or nausea. As the level of CO increases, so does the likelihood of severe effects, which can result in coma, seizures, and finally death by asphyxiation.


Carbon monoxide, sometimes called coal gas or carbonous oxide, has been known as a toxic substance since the third century BCE. It was used for executions and to commit suicide in ancient Rome. The chemical discovery of carbon monoxide, between 1772 and 1799, is generally credited to the English chemist Joseph Priestley (1733–1804). The first scientific study of the poisonous nature of carbon monoxide was conducted by French physiologist Claude Bernard (1813–1878). During the middle of the nineteenth century, Bernard concentrated on the physiological characteristics of poisons, most notably carbon monoxide.

Carbon monoxide is found in automobile exhaust fumes, faulty gas stoves and furnaces, fires, and cigarette smoke. Other sources are wood-burning stoves, kerosene heaters, improperly ventilated gas water heaters, and blocked or poorly maintained chimney flues. Throughout human history, people have died from carbon monoxide poisoning, primarily because the gas cannot be seen, smelled, or tasted, and because it is a component of controlled fires used for cooking and staying warm in the winter. In the twenty-first century, a common source of carbon monoxide poisoning is improperly ventilated home gasoline generators used to supply electricity during power outages. Carbon monoxide is considered one of the most dangerous poisons, due to the number of people killed by it each year.

Carbon monoxide causes inadequate oxygen to reach the body's tissues, a condition called anoxia or hypoxia. Normally, oxygen binds with an iron atom in hemoglobin, a molecule found in blood that is responsible for carrying oxygen to cells throughout the body. At the appropriate time, the oxygen is released from hemoglobin so that it can be used by the cells. However, when carbon monoxide is inhaled, it has 230–270 times more affinity for hemoglobin than oxygen, so CO outcompetes oxygen and replaces it in the oxyhemoglobin complex. When this happens, the blood carries insufficient amounts of oxygen to the cells. Deprived of oxygen, the cells die, eventually causing the individual to die.

Risk factors

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

Exposure to carbon monoxide is especially dangerous to developing fetuses because fetal hemoglobin attaches to carbon monoxide more easily than does adult hemoglobin. Because they breathe more frequently and thus are exposed more quickly to carbon monoxide, young children are more susceptible than older children and young adults. When exposed to carbon monoxide, older adults are more likely to develop brain damage than younger adults and children.

Carbon monoxide poisoning results in more than 15,000 emergency room visits each year in the United States, with the death rate ranging from 50–500 people annually. More than three-fourths of exposures occur in residences. About 12% occur in workplaces. CO poisoning tends to increase after severe winter storms when electrical power has been knocked out. Because people are often restricted to their homes after major storms, many people are exposed to high amounts of carbon monoxide from temporary heaters, such as those powered with kerosene, gasoline, and propane. Other exposures can arise from the use of gasoline-powered pressure washers within enclosed spaces or cooking with charcoal indoors. For these reasons, deaths from carbon monoxide poisoning increase during times of natural disasters, such as earthquakes, snowstorms, and floods.

As people have learned more about the dangers of carbon monoxide poisoning, they have developed techniques and devices to prevent such poisoning. Carbon monoxide detectors, like smoke detectors, are commonly required by building codes to be installed in homes and offices within the United States and other developed countries. Safer motor vehicles, furnaces, stoves, and other machines are better at containing and detecting carbon monoxide.


According to the Centers for Disease Control and Prevention (CDC), CO poisoning is a leading cause of unintentional poisoning deaths in the United States. Between 1999 and 2012, unintentional, non-fire-related CO exposure caused on average 15,000 emergency room visits and 438 deaths annually. Subsequently, these numbers decreased slightly. In 2016, unintentional non-fire-related CO exposures resulted in 12,239 emergency room visits with 48 deaths and another 190 serious to permanent disabilities. Males experience CO exposure three times more often than females. Whites outnumber all other racial and ethnic groups in using CO poisoning as a means of suicide.

Causes and symptoms

When a person breathes fresh air, the oxygen in the air binds with a molecule called hemoglobin (Hb) in red blood cells. Oxygen then is carried by the blood from the lungs to every part of the body. When the oxygen/hemoglobin complex reaches a site where it is needed, the oxygen is released. Because the oxygen binding process is reversible, hemoglobin can be used over and over 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 that is not readily released. 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 most common cause of CO poisoning is exposure to automobile exhaust fumes. Other frequent causes are heavy tobacco smoking, fires, and other inhalations caused by combustion. People who are anemic are at higher risk of CO poisonings than are other people because they have fewer red blood cells.

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 at which a person breathes. Heavy smokers can start with up to 9% of their hemoglobin already bound to CO, which they regularly inhale in cigarette smoke. Therefore, they are much more susceptible to the effects of environmental CO. The U.S. 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 headaches, fatigue, nausea, and dizziness. These symptoms correspond to 15%–25% COHb in the blood. A 20% saturation of CO in hemoglobin is likely to induce symptoms such as confusion, dizziness, fatigue, headaches, quickened pulse, nausea, increased respiratory rate, and vomiting. When the concentration of COHb reaches 50% or more, coma and death result 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 are:

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 or 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.


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 as possible after suspected exposure to CO.

Other useful tests for determining the extent of CO poisoning are 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.

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, this is an unreliable symptom for diagnosis.


Immediate treatment for CO poisoning entails removing victims from the source of carbon monoxide gas and getting them into fresh air. If they are not breathing and have no pulse, cardiopulmonary resuscitation (CPR) should be started. Depending on the severity of the poisoning, 100% 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% in healthy individuals, over 15% in patients with a history of heart or lung disease, and over 10% 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, patients are given hyperbaric oxygen therapy. This treatment involves placing the patient in a chamber containing 100% oxygen at a pressure of more than one atmosphere, which means at a pressure higher than 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.

Public health role and response

Many U.S. agencies work together to prevent accidental poisonings from carbon monoxide. These agencies include the National Institute for Occupational Safety and Health (NIOSH), the Consumer Product Safety Commission (CPSC), the Occupational Safety and Health Administration (OSHA), and the Environmental Protection Agency (EPA). The CDC lists the following statistics for limits of carbon monoxide exposure:

Carboxyhemoglobin (COHb)—
Hemoglobin that is bound to carbon monoxide instead of oxygen.
Hemoglobin (Hb)—
A molecule that normally binds to oxygen in order to carry it to the body's cells, where it is required for life.
Development of a subnormal body temperature.
A measurement of the acidity or alkalinity of a fluid. A neutral fluid, neither acid nor alkali, has a pH of 7.




Carbon monoxide poisoning is preventable. 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 at minimal cost.

Specific actions that can prevent CO poisoning include:

See also Smoking .



DiLoreto, Dante, and Idina Corcoran, editors. Carbon Monoxide: Sources, Uses, and Hazards. New York: Nova Science, 2012.


Centers for Disease Control and Prevention. “Carbon Monoxide Poisoning.” (accessed March 21, 2018).

Consumer Products Safety Commission. “Carbon Monoxide Information Center.” (accessed March 21, 2018).

National Library of Medicine. “Carbon Monoxide Poisoning.” (accessed March 21, 2018).

Shochat, Guy A. “Carbon Monoxide Toxicity.” (accessed March 21, 2018).


American Lung Association, 55 W. Wacker Dr., Ste. 1150, Chicago, IL, 60610, (800) 548-8252,, .

Consumer Product Safety Commission, 4330 East West Hwy., Bethesda, MD, 20814, (301) 504-7923; TTY 301-595-7054, (800) 638-2772, .

Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC, 20460, (202) 272-0167; TTY 202-272-0165, .

United States Department of Labor Occupational Safety & Health Administration, 200 Constitution Ave., Washington, DC, 20210, (800) 321-6742; TTY: 877-889-5627, .

Tish Davidson, AM

  This information is not a tool for self-diagnosis or a substitute for professional care.