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.
A doctor can measure COHb in the blood but cannot m easure CO in the body directly. The more CO in the body, the higher the COHb, and the sicker the person will be. 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. Deaths from unintentional CO poisoning have dropped sharply in recent years, thanks to lower CO emissions from automobiles and safer heating and cooking appliances. The latest year for which statistics are available at this level of detail.