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

FatigueDizzinessHeadacheRapid heart rateIrregular breathingNausea and/or vomitingThought impairment or disorientationWheezing/bronchial constriction/pesistent coughChest pain in persons with coronary artery diseaseDeath 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