Carbon Monoxide - CO Standards Summary


This guideline is intended as a source of information for employees, employers, physicians, industrial hygienists, and other occupational health professionals who may have a need for such information. It does not attempt to present all data rather, it presents pertinent information and data in summary form.


Formula: CO
Synonyms: Monoxide
Appearance and odor: Colorless, odorless gas.


The current OSHA standard for carbon monoxide is 50 parts of carbon monoxide per million parts of air (ppm) averaged over an eight-hour work shift. This may also be expressed as 55 milligrams of carbon monoxide per cubic meter of air (mg/m3). NIOSH has recommended that the permissible exposure limit be reduced to 35 ppm averaged over a work shift of up to 10 hours per day, 40 hours per week with a ceiling of 200 ppm. The NIOSH Criteria Document for Carbon Monoxide should be consulted for more detailed information.


Routes of exposure

Carbon monoxide can affect the body if it is inhaled or if liquid carbon monoxide comes in contact with the eyes or skin.

Effects of overexposure

Exposure to carbon monoxide decreases the ability of the blood to carry oxygen to the tissues. Inhalation of carbon monoxide may cause headache, nausea dizziness, weariness, rapid breathing, unconsciousness, and death. High concentrations may be rapidly fatal without producing significant warning symptoms. Exposure to this gas may aggravate heart disease and artery disease and may cause chest pain in those with pre-exicting heart disease. Pregnant women are more susceptible to the effects of carbon monoxide exposure. The effects are also more severe in people who are working hard and in people who are working in places where the temperature is high or at altitudes above 2000 feet. Skin exposure to liquid carbon monoxide may cause frostbite-type burns.

Reporting signs and symptoms

A physician should be contacted if anyone develops any signs or symptoms and suspects that they are caused by exposure to carbon monoxide.

Recommended medial surveillance

The following medical procedures should be made available to each employee who is exposed to carbon monoxide at potentially hazardous levels:

1. Initial Medical Examination:

-A complete history and physical examination: The purpose is to detect preexisting conditions that might place the exposed employee at increased risks, and to establish a baseline for future health monitoring. Persons with a history of coronary heart disease, anemia, pulmonary heart disease, cardiovascular disease, thyrotoxicosis, and smokers would be expected to be at increased risk from exposure. Pregnant women have an increased sensitivity to the effects of carbon monoxide. Examination of the cardiovascular system, the pulmonary system, the blood, and the central nervous system should be stressed.

-A complete blood count: Carbon monoxide affects the ability of the blood to carry oxygen. A complete blood count should be performed including a red cell count, a white cell count, a differential count of a stained smear, as well as hemoglobin and hematocrit.

2. Periodic Medical Examination: The aforementioned medical examinations should be repeated on an annual basis, with the exception that a carboxyhemoglobin determination should be performed at any time overexposure is suspected or signs or symptoms of toxicity occur.

Summary of toxicology

Carbon monoxide (CO) gas causes tissue hypoxia by preventing the blood from carrying sufficient oxygen. CO combines reversibly with the oxygen-carrying sites on the hemoglobin molecule with an affinity ranging from 210 to 240 times greater than that of oxygen; the carboxyhemoglobin thus formed is unavailable to carry oxygen. In addition, carboxyhemoglobin interferes with the release of oxygen carried by unaltered hemoglobin. With exposure to high concentrations such as 4000 ppm and above, transient weakness and dizziness may be the only premonitory warnings before coma supervenes; the most common early aftermath of severe intoxication is cerebral edema. Exposure to concentrations of 500 to 1000 ppm causes the development of headache, tachypnea (rapid breathing), nausea, weakness, dizziness, mental confusion, and in some instances, hallucinations, and may result in brain damage. The affected person is commonly cyanotic. Concentrations as low as 50 ppm result in blood COHb levels up to 10% in an 8-hour day. This greatly increases the risk of angina pectoris and coronary infarctions by decreasing the oxygen supply in the blood and also in the myoglobin of the heart muscle. These effects are aggravated by heavy work, high ambient temperatures, and high altitudes. Pregnant women are especially susceptible to the effects of increased CO levels. Smoking also increases the risk: cigarette smoke contains 4% CO, which results in 5.9% COHb if a pack a day is smoked. The blood of persons not exposed to CO contains about 1% CO, probably as a result of normal heme metabolism. The diagnosis of CO intoxication depends primarily on the demonstration of significantly increased carboxyhemoglobin in the blood. Levels over 60% are usually fatal; 40% is associated with collapse and syncope; above 25% there may be electrocardiographic evidence of a depression of the S-T segment; between 15% and 25% there may be headache and nausea. The reaction to a given blood level of carboxyhemoglobin is extremely variable: some persons may be in coma with a carboxyhemoglobin level of 38% while others may maintain an apparently clear sensorium with levels as high as 55%. The blood of cigarette smokers contains 3 to 10% carboxyhemoglobin, and non exposed persons have an average level of 1%, probably as a result of normal heme metabolism. Several investigators have suggested that the results of behavioral tests such as time discrimination, visual vigilance, choice response tests, visual evoked responses, and visual discrimination thresholds may be altered at levels of carboxyhemoglobin below 5%.


Physical data:

1. Molecular weight: 28
2. Boiling point (760 mm Hg): - 191.5 C (-313 F)
3. Specific gravity (water = 1): 0.79 (liquid at boiling point)
4. Vapor density (air = 1 at boiling point of carbon monoxide): 0.97
5. Melting point: - 199 C (-326 F)
6. Vapor pressure at 20 C (68 F): Greater than 1 atmosphere
7. Solubility in water, g/l00 g water at 20 C (68 F): 0.004
8. Evaporation rate (butyl acetate = 1): Not applicable


1. Conditions contributing to instability: Elevated temperatures may cause cylinders to explode.
2. Incompatibilities: Contact with strong oxidizers may cause fires and explosions.
3. Hazardous decomposition products: None
4. Special precautions: None


1. Flash point: Not applicable
2. Auto ignition temperature: 609 C (1128 F)
3. Flammable limits in air, % by Volume: Lower:12.5; Upper:74
4. Extinguishant: Dry chemical. If flow of gas cannot be stopped, let fire burn.

Warning properties

1. Odor Threshold: The AlHA Hygienic Guide points out that carbon monoxide is odorless.
2. Eye Irritation Level: Grant states that carbon monoxide is a non-irritating gas.
3. Evaluation of Warning Properties : Carbon monoxide is an odorless, non-irratating gas. It has no warning properties.

(Material contain on this web page excerpted from the US Dept. of Health and Human Services / US Department of Labor "Occupational Heath Guideline for Carbon Monoxide" printed September 1978 as a general summary of available CO information. Refer to latest official printed materials for current information sufficient to achieve compliance with all requirements of OSHA regulations.) - Advanced training and support for novice teen drivers.
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Expert Advice
Carbon Monoxide Detectors w/ PPM Displays and Battery Backup

Although all home carbon monoxide detectors use an audible alarm signal as the primary indicator, some versions also offer a digital readout of the CO concentration, in parts per million.

Typically, they can display both the current reading and a peak reading from memory of the highest level measured over a period of time.

The digital models offer the advantage of being able to observe levels that are below the alarm threshold, learn about levels that may have occurred during an absence, and assess the degree of hazard if the alarm sounds.

They may also aid emergency responders in evaluating the level of past or ongoing exposure or danger.