Carbon monoxide results from the incomplete burning of carbon-based fuel such as gasoline, wood, or paper. Early symptoms of poisoning include headaches, nausea, and vomiting that get better when you leave the area. Advanced symptoms include loss of concentration, cognition, and memory, soon falling into a coma, and dying. Pets in the house may fall ill and die sooner than adult humans, so if your pet shows any of these symptoms, or you come home to find a pet dead, suspect carbon monoxide.
Any area that contains a car, barbecue, lawn mower, gas stove, hot water heater, furnace, fireplace, or snow blower is capable of containing deadly carbon monoxide fumes not only in the garage, room, or basement where they are located, but in any attached living quarters. You can even inhale too much carbon monoxide outside if you are too near an exhaust.
Never use a gas or charcoal grill in an enclosed space. Regularly service your furnace. Don't idle your car or lawn mower in an attached garage, even with the garage door open.
Have at least one carbon monoxide monitor in your home. If the alarm goes off, throw open all the windows and doors immediately and get everyone out of the house. Then call 911. If you live in an attached apartment or duplex, the fumes may be coming from a common vent. Have the emergency responders check on your neighbors as well.
-- Source material, Consumer Reports,The Best of Health 2011