Carbon Monoxide

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Carbon Monoxide (CO)

Carbon monoxide is a colorless, odorless, tasteless and toxic gas produced as a by-product of combustion. Any fuel burning appliance, vehicle, tool or other device has the potential to produce dangerous levels of carbon monoxide gas. Examples of carbon monoxide producing devices commonly in use around the home include:

  • Fuel fired furnaces (non-electric)
  • Gas water heaters
  • Fireplaces and woodstoves
  • Gas stoves
  • Gas dryers
  • Charcoal grills
  • Lawnmowers, snowblowers and other yard equipment
  • Automobiles

Medical Effects of Carbon Monoxide

Carbon monoxide inhibits the blood's ability to carry oxygen to body tissues including vital organs such as the heart and brain. When CO is inhaled, it combines with the oxygen carrying hemoglobin of the blood to form carboxyhemoglobin (COHb). Once combined with the hemoglobin, that hemoglobin is no longer available for transporting oxygen.

How quickly the carboxyhemoglobin builds up is a factor of the concentration of the gas being inhaled (measured in parts per million or PPM) and the duration of the exposure. Compounding the effects of the exposure is the long half-life of carboxyhemoglobin in the blood. Half-life is a measure of how quickly levels return to normal. The half-life of carboxyhemoglobin is approximately 5 hours. This means that for a given exposure level, it will take about 5 hours for the level of carboxyhemoglobin in the blood to drop to half its current level after the exposure is terminated.

Symptoms Associated With a Given Concentration of COHb

  • 10% COHb - No symptoms. Heavy smokers can have as much as 9% COHb.
  • 15% COHb - Mild headache.
  • 25% COHb - Nausea and serious headache. Fairly quick recovery after treatment with oxygen and/or fresh air.
  • 30% COHb - Symptoms intensify. Potential for long term effects especially in the case of infants, children, the elderly, victims of heart disease and pregnant women.
  • 45% COHb - Unconsciousness
  • 50+% COHb - Death

Since one can't easily measure COHb levels outside of a medical environment, CO toxicity levels are usually expressed in airborne concentration levels (PPM) and duration of exposure. Expressed in this way, symptoms of exposure can be stated as in the Symptoms Associated With a Given Concentration of CO Over Time table below.

As can be seen from the table, the symptoms vary widely based on exposure level, duration and the general health and age on an individual. Also note the one recurrent theme that is most significant in the recognition of carbon monoxide poisoning - headache, dizziness and nausea. These 'flu like' symptoms are often mistaken for a real case of the flu and can result in delayed or misdiagnosed treatment. When experienced in conjunction with the sounding of a carbon monoxide detector, these symptoms are the best indicator that a potentially serious buildup of carbon monoxide exists.

Symptoms Associated With a Given Concentration of CO Over Time

PPM CO Time Symptoms
35 8 hours Maximum exposure allowed by OSHA in the workplace over an eight hour period.
200 2-3 hours Mild headache, fatigue, nausea and dizziness.
400 1-2 hours Serious headache-other symptoms intensify. Life threatening after 3 hours.
800 45 minutes Dizziness, nausea and convulsions. Unconscious within 2 hours. Death within 2-3 hours.
1600 20 minutes Headache, dizziness and nausea. Death within 1 hour.
3200 5-10 minutes Headache, dizziness and nausea. Death within 1 hour.
6400 1-2 minutes Headache, dizziness and nausea. Death within 25-30 minutes.
12,800 1-3 minutes Death

Source: Copyright 1995, H. Brandon Guest and Hamel Volunteer Fire Department
Rights to reproduce granted provided copyright information and this statement included in their entirety. This document provided for informational purposes only. No warranty with respect to suitability for use expressed or implied.

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Bailey, Regina. "Carbon Monoxide." ThoughtCo, Aug. 25, 2020, Bailey, Regina. (2020, August 25). Carbon Monoxide. Retrieved from Bailey, Regina. "Carbon Monoxide." ThoughtCo. (accessed June 10, 2023).