VIDEO: People had to leave their apartments in a building in Denver Saturday morning because of high levels of carbon monoxide. Denver Fire Department dispatch said it was notified about a CO alarm going off at 969 South Pearl Street at about 9 a.m. When firefighters arrived they detected high levels of carbon monoxide on floors 1-3. They did not believe there were any patients, but an evacuation was ordered. Often called the invisible killer, carbon monoxide is an odorless, colorless gas created when fuels (such as gasoline, wood, coal, natural gas, propane, oil, and methane) burn incompletely, according to the National Fire Protection Association.