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. 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.

The following table describes the symptoms associated with a given concentration of COHb:

% COHb Symptoms and Medical Consequences

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

PPM CO Time Symptoms

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

As can be seen from the above information, 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 a the sounding of a carbon monoxide these symptoms are the best indicators.