CO is poisonous and can kill cells of the body. CO also replaces oxygen in the bloodstream, leading to suffocation. Mild effects feel like the flu, while severe effects include difficulty breathing and even death.
Higher concentrations are dangerous even for a short time.
Table 1 shows typical symptoms, based on concentration and time of exposure. A doctor can measure COHb in the blood but cannot measure CO in the body directly. The more CO in the body, the higher the COHb, and the sicker the person will be. Table 2 links typical symptoms to the level of COHb saturation. Of all the unintentional gas and vapor poisoning deaths in the U.S., more than one-third involve motor vehicle exhaust gas, and more than one fourth involve heating or cooking equipment.