Mild carbon monoxide poisoning causes headache, nausea, vomiting, drowsiness, and poor coordination.
Most people who develop mild carbon monoxide poisoning recover quickly when moved into fresh air.
Moderate or severe carbon monoxide poisoning causes confusion, unconsciousness, chest pain, shortness of breath, and coma. Thus, most victims are not able to move themselves and must be rescued. Severe poisoning is often fatal. Rarely, weeks after apparent recovery from severe carbon monoxide poisoning, symptoms such as memory loss, poor coordination, and uncontrollable loss of urine (which are referred to as delayed neuropsychiatric symptoms) develop.
Carbon monoxide is dangerous because a person may not recognize drowsiness as a symptom of poisoning. Consequently, someone with mild poisoning can go to sleep and continue to breathe the carbon monoxide until severe poisoning or death occurs. Some people with long-standing, mild carbon monoxide poisoning caused by furnaces or heaters may mistake their symptoms for other conditions, such as the flu or other viral infections.