Carbon Monoxide or, CO is an odorless, colorless gas that can cause sudden illness and death. It's found in combustion fumes, such as those produced by vehicles, stoves, and heating systems. When it accumulates in enclosed spaces, people and animals can be poisoned by breathing it. The most common symptoms of CO poisoning are: headache, dizziness, weakness, nausea, vomiting, chest pain, and confusion. Unless suspected, CO poisoning can be difficult to recognize because the symptoms mimic other illnesses.