While the older alarms were often a nuisance, the improved technology means that when a newer alarm sounds it should be investigated.

CO that drifts in and out of a cabin can be dangerous, since the effects of CO are cumulative and can build up gradually in a person's bloodstream over hours or even days before it reaches critical levels.

This is true even if when the person breathes fresh air periodically; the CO remains in the bloodstream. How quickly the CO builds up is a factor of the concentration of the gas being inhaled (measured in parts per million [PPM]) and the duration of the exposure.

The half-life of CO is approximately five hours, which means that it takes five hours for the level of CO in the blood to drop to half its level when exposure was terminated.

There is also some recent research challenges traditional thinking that the effects of CO poisoning are transitory. Studying 96 victims for one year beyond their exposure, researchers found over 25% showed evidence of brain damage 12 months after exposure. These long-term CO injuries can include apathy, memory loss, inattention, and depression.

How Much CO is Too Much?
(In Parts Per Million (PPM)

• 200 PPM   Slight headaches within two to three hours.
• 400 PPM   Frontal headaches within one to two hours.
• 800 PPM   Dizziness, nausea, and convulsions within 45 minutes. Insensible within two hours.
• 1,600 PPM   Headache. Dizziness and nausea within 20 minutes. Death within 30 minutes.
• 3,200 PPM   Headache and dizziness within five minutes. Death within 30 minutes.
• 6,400 PPM   Headache and dizziness within one to two minutes. Death in less than 15 minutes.
• 12,800 PPM   Death in less than three minutes.

CO Concentration at the Source

• Gasoline Engine   10,000 - 100,000 PPM
• Diesel Engine   1,000 PPM