How much carbon monoxide is lethal?

50%: This varies, but 50% would be lethal for most people. However, small children, an unborn child, and elderly would be more susceptible to lower doses. Carbon monoxide binds to blood cells causing caboxyhemoglobin which makes the red cell unable to oxygenate. At 10 to 15 % most people will be symptomatic- headache, fatigue, flu like symptoms. As the perecentage increases so do the symptoms.