Carbon monoxide is a poisonous gas that cannot be seen, smelled, or tasted but can be detected by its flu-like symptoms in adults such as breathing difficulty, dizziness, headaches, and nausea. It is dangerous because it reacts with hemoglobin in the blood to form carboxyhemoglobin, reducing the blood's ability to carry oxygen throughout the body and potentially causing heart attacks with long exposure or high levels of poisoning. The document provides information on the symptoms of carbon monoxide poisoning in adults and a link to learn about its different effects on children.