Carbon Monoxide Toxicity

Carbon monoxide, also called CO, is invisible, odorless, tasteless, and dangerous. Carbon monoxide is not commonly found in the atmosphere, but maybe more frequent when encountered in cities or enclosed environments such as houses or cars.

Carbon monoxide poisoning causes up to 5000 to 6000 deaths per year, and causes up to 40,000 emergency room visits.  Many of these cases are intentional. Unintentional poisoning is more common during the winter months, asked the gas results from combustion of hydrocarbons, meaning many of the things that we burn to generate heat.

Common sources include poorly vented fuel-burning devices, such as grills or motor vehicles.  Ingestion of methylene chloride, an industrial solvent, can also lead to carbon monoxide toxicity even though the gas is never inhaled.

Why is Carbon Monoxide Poisonous?

Our bodies need oxygen to survive. We breathe oxygen in, and it crosses the membrane in the lungs, where it then enters the bloodstream. Oxygen molecules are carried through the blood on hemoglobin molecules like tiny taxi passengers.  The oxygen then gets off the hemoglobin to enter tissue where it is needed. 

The trouble is that carbon monoxide is even better at binding to hemoglobin than oxygen is.  Unlike oxygen, carbon monoxide may not release from the hemoglobin as easily.  Carbon monoxide also changes the shape of the hemoglobin molecule so it doesn’t release oxygen as easily.  Imagine someone jumping into a taxi and forcefully locking all the doors, and you kind of get the idea.

  Carbon monoxide also interferes with how oxygen is used in the tissue by targeting cytochrome oxidase, the same enzyme affected by cyanide.  So essentially, even if oxygen can get out of its hemoglobin taxi and get to work, the same chemical bully is waiting in the office to stop work from getting done.

As if that all weren’t enough, CO also damages the nervous system through mechanisms that remain poorly understood.  Neurological damage occurs even after recovery from CO exposure.

What are the Signs of Carbon Monoxide Poisoning?

Signs of CO toxicity are pretty nonspecific, meaning that they could easily be mistaken for something like a cold or flu.  Headache is the most common symptom, as well as nausea or dizziness. If the CO toxicity is severe, other neurological problems such as seizures or coma may result.  Injury to the heart is also common. 

In about 40 percent of patients who have been exposed to CO, neuropsychiatric problems result 3 to 240 days later.  This can manifest as a movement disorder, personality change, thinking problems, or a range of other neurological problems.  Diagnosis of chronic exposure is notoriously challenging. 

How Do Doctors Detect Carbon Monoxide Poisoning?

Because of the high risks involved with missing such a diagnosis, it’s not uncommon for doctors to do a routine CO check on children who come into the ER with headache.

  The levels may be measured in either arterial or venous blood.  Levels in smokers may be as high as 10-15%, but in nonsmokers levels are usually below 3%.  While the levels may signify exposure to CO, they do not correlate well with the severity of symptoms. 

After exposure is confirmed further testing with an electrocardiogram (ECG) can be used to evaluate for heart problems, and an MRI may be useful in detecting changes in the basal ganglia, particularly the globus pallidus, which may be present in delayed neurological symptoms of CO exposure.  

How is CO Exposure Treated?

The immediate treatment of CO exposure is medically administered oxygen.  The idea is to maximize the amount of oxygen that can effectively be administered to tissue, outcompeting the CO.  Such oxygen should be administered as quickly as possible.  While this treatment is useful for helping people survive CO exposure, it is unclear whether the severity or frequency of delayed neurologic symptoms is reduced.

The most important step that can be taken regarding CO toxicity is preventing it from occurring.  Every home should be equipped with a CO monitor.  More information is available at


Peter Clardy Scott Manaker and Holly Perry.  Carbon monoxide poisoning.  Up to Date, last updated Aug 27, 2014, accessed February 20, 2015. 

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