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 nonfire-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. Source: Consumer Product Safety Commission

Know the symptoms of CO poisoning
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:
Headache
Fatigue
Shortness of breath
Nausea
Dizziness

High level CO poisoning results in progressively more severe symptoms, including:
Mental confusion
Vomiting
Loss of muscular coordination
Loss of consciousness
Ultimately death