Carbon monoxide, also known as CO, is called the “Invisible Killer” because it's a colorless, odorless, poisonous gas. More than 150 people in the Unites States die every year from accidental non-fire-related CO poisoning associated with consumer products, including generators. Other products include faulty, improperly-used or incorrectly-vented fuel-burning appliances such as furnaces, stoves, water heaters and fireplaces.
Because CO is odorless, colorless, and otherwise undetectable to the human senses, people may not know that they are being exposed. The initial symptoms of low to moderate CO poisoning are similar to the flu (but without the fever). They include: