Common symptoms of mild carbon monoxide poisoning can feel similar to flu or food poisoning. They include: Headache. Nausea. Dizziness. Vomiting. Tiredness. Confusion. Breathing difficulty. Shortness of breath. Stomach pain. Along with the above symptoms, prolonged exposure to low levels of carbon monoxide can lead to: Erratic behavior or emotional changes, such as depression, anxiety, and irritability. Difficulty focusing or thinking, or mental fatigue. Symptoms of exposure to high levels of carbon monoxide include: Loss of consciousness, which in extreme cases can lead to death within minutes if the amount of gas is exceptionally high. Vertigo. Heart rate greater than 100 beats per minute (aka tachycardia). Muscle spasms and seizures due to increased electrical activity in the brain. Angina or heart attack. Loss of coordination.