The following information on carbon monoxide safety was taken from NSP's EnergyWise Newsletter with their permission.
Carbon monoxide is an invisible gas that can overcome a person without warning.
Recognizing early signs of CO poisoning is difficult because symptoms, including headache, nausea, a flushed face and dizziness, are nonspecific and may be mistaken for flu or other illnesses. At high concentrations, CO produces weakness, confusion and even death. If you suspect carbon monoxide poisoning, open windows and go outside. You may need to call your gas utility to check your appliances. Keep appliances and heating equipment regularly maintained.