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Carbon monoxide poisoning

Medically reviewed by Last updated on Mar 22, 2023.

What is Carbon monoxide poisoning?

Harvard Health Publishing

Carbon monoxide is a tasteless, colorless, odorless gas found in the fumes of fuels that contain carbon, such as wood, coal and gasoline. Carbon monoxide poisoning is a potentially fatal illness that occurs when people breathe in carbon monoxide. 

All sorts of sources can release carbon monoxide, including cars, trucks, small gasoline engines (like lawnmowers), stoves, lanterns, furnaces, grills, gas ranges, water heaters and clothes dryers. The risk of poisoning is especially high when equipment is used in an enclosed place and ventilation is poor. Carbon monoxide poisoning can occur in victims of smoke inhalation during a fire. More than one-third of carbon monoxide-related deaths occur when the victim is asleep. 

Once inhaled, carbon monoxide passes from your lungs into your bloodstream, where it attaches to the hemoglobin molecules that normally carry oxygen. Oxygen can't travel on a hemoglobin molecule that already has carbon monoxide attached to it. As exposure continues, the gas hijacks more and more hemoglobin molecules, and the blood gradually loses its ability to carry enough oxygen to meet your body's needs. Without enough oxygen, individual cells suffocate and die, especially in vital organs such as the brain and heart. Carbon monoxide also can act directly as a poison, interfering with cells' internal chemical reactions.


Symptoms vary depending on the concentration of carbon monoxide in the environment, the length of time you are exposed, and your health. If you are exposed to very high levels of carbon monoxide gas in a poorly ventilated room, you can develop: 

  • Headache 
  • Shortness of breath 
  • Personality changes 
  • Unusually emotional behavior or extreme swings in emotions 
  • Fatigue 
  • Malaise (a generally sick feeling) 
  • Dizziness 
  • Clumsiness or difficulty walking 
  • Vision problems 
  • Confusion and impaired judgment 
  • Nausea and vomiting 
  • Rapid breathing 
  • Chest pain 
  • A rapid or irregular heartbeat 

Without immediate treatment, you can lose consciousness, have a seizure, enter a coma, and potentially die. Death can result from only a few minutes of exposure to higher concentrations or from an hour of exposure to lower levels. 

If you are exposed to very low levels of carbon monoxide over a longer period (weeks or months), your symptoms can appear like the flu, with headache, fatigue, malaise (a general sick feeling) and sometimes nausea and vomiting. People with long-term exposure to low levels of carbon monoxide also can have numbness, unexplained vision problems, sleep disturbances, and impaired memory and concentration. 


If you are unconscious, your doctor's first priority is to stabilize your condition, providing emergency treatment such as oxygen, fluid and treatment for seizures. Information about the exposure will be collected from the emergency personnel, your relatives, or both. This is especially important if you are a victim of smoke inhalation during a fire, because you could have inhaled other toxic gases besides carbon monoxide. 

After a poisoning that occurs indoors, your doctor will ask about the condition of fuel-burning appliances and equipment in your home and at work and about the quality of ventilation in these areas. Your doctor will want to know how long you were exposed, whether your symptoms improve when you leave the area, and whether any of your family members or co-workers complain of symptoms similar to yours. 

If you are pregnant, tell your doctor immediately. Carbon monoxide attaches to fetal hemoglobin at a level 10% to 15% higher than in the mother, placing the fetus at special risk. 

When your doctor examines you, he or she will pay particular attention to your nervous (neurological) system. To confirm the diagnosis, your doctor will draw blood to determine your levels of oxygen and carboxyhemoglobin (carbon monoxide attached to hemoglobin). Additional tests may be needed depending on your specific symptoms. You may need an electrocardiogram (EKG) to evaluate symptoms of chest pain or an irregular heartbeat. In people with neurological symptoms, a magnetic resonance imaging (MRI) or computed tomography (CT) scan of the brain may be needed. In a pregnant woman, the fetus may have to be monitored. An external monitor will be placed on the woman's belly to measure the fetus' heart rate over time to look for signs of distress suggesting low oxygen levels.

Expected duration

Carbon monoxide gas leaves the body the same way it got in, through the lungs. In fresh air, it takes four to six hours for a victim of carbon monoxide poisoning to exhale about half of the inhaled carbon monoxide in their blood. This "clearing" time can be reduced if the person is given 100% oxygen or is placed in a hyperbaric oxygen (high-pressure oxygen) chamber, which creates a higher oxygen pressure than the normal outside pressure. 

Because carbon monoxide poisoning can kill body cells, especially in the brain, there is a risk of long-term neurological problems in people who have had severe poisoning.


Carbon monoxide can kill without warning because it has no color, odor or taste. Here are some suggestions to reduce your risk: 

  • Install carbon monoxide detectors in your home in hallways near bedrooms and in garaged attached to living areas. 
  • Open the flue when you use a fireplace. 
  • Never use charcoal grills or hibachis indoors. 
  • Buy appliances that vent to the outside. Choose brands tested and certified as safe by the Underwriters Laboratories (UL), the American Gas Association (AGA), or other recognized certifying organizations. 
  • Have fuel-burning appliances professionally installed. After installation, periodically check the vents for blockages or cracks. 
  • Before turning on your heater for the winter season, have your heating system, flues and chimneys professionally inspected. If necessary, have chimneys and flues professionally cleaned. 
  • Never heat your home by using an oven, stovetop or clothes dryer. 
  • Never operate gas-powered tools or engines inside, even if windows are open and ventilation seems good. 
  • Never leave your car running inside an attached garage. Also, when your car idles outdoors, keep one or two windows slightly open. 


You should immediately get out of the environment with high carbon monoxide level. You should receive oxygen as quickly as possible, preferably high flow oxygen through a non-rebreather mask. This prevents you from inhaling the gas that you just exhaled.  

You will be taken to an emergency department. If your symptoms disappear after oxygen treatment for several hours and your physical exam and blood tests are normal, you may be able to go home. You may need to schedule a follow-up visit with your doctor to check for complications to your nerves and brain. 

If you have severe carbon monoxide poisoning and are unconscious, you will be connected to a respirator (a machine that breathes for you) and you will get 100% oxygen. Patients with very severe poisoning may be treated in a hyperbaric oxygen chamber.

When to call a professional

You are at risk of carbon monoxide poisoning whenever you are near a fuel-burning appliance or tool. At the first hint of symptoms, move quickly to fresh air. Don't wait for more severe symptoms to start. 

If you are concerned about the condition of a fuel-burning appliance in your home and you notice headaches, flu-like symptoms or sleep problems, call your doctor. 

If you suspect that someone else is suffering from severe carbon monoxide poisoning, call for emergency help immediately. If possible, move an unconscious victim to an area with fresh air. Be cautious. Remember that high levels of carbon monoxide can make you sick, too, even before you can bring the victim to safety.


The prognosis depends on the severity of carbon monoxide poisoning. Among people with severe symptoms, as many as two out of three people may have long-term complications, especially neurological problems. In people with mild to moderate symptoms, as many as one in five can develop lasting neurological problems. Neurological problems range from mild personality changes to severe intellectual impairment, blindness and deafness. In pregnant women, poisoning can cause fetal death or cerebral palsy in the child.

Additional info

U.S. Environmental Protection Agency (EPA)

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.