People cannot smell, taste, or feel carbon monoxide. Therefore, its presence may not be evident until symptoms develop. Symptoms of carbon monoxide poisoning include:

Fatigue

Dizziness

Headache

Rapid heart rate

Irregular breathing

Nausea and/or vomiting

Thought impairment or disorientation

Wheezing/bronchial constriction/pesistent cough

Chest pain in persons with coronary artery disease

Death can occur in the absence of some or all of these symptoms. In this case the victim simply "falls asleep" and never regains consciousness.

Source: Pennsylvania Department of Health