Carbon Monoxide (CO) is odorless. For homes, the danger level is 35 parts per million, breathed for 8 or more hours. CO occurs when gas equipment is not kept in tune, and when equipment is not installed with proper venting. It causes nausea, light-headedness, dizziness as it deprives the body of oxygen, and finally, death.
Keep your appliances in tune, and install a wired-in or plug-in CO detector in your home. If the detector sounds leave your home immediately and call 911.