At least four people were sent to the hospital after elevated carbon monoxide levels were detected in the Arlington Court Suites Hotel at 1200 North Courthouse Road.
Firefighters believe the CO may have wafted into the hotel after a vehicle was left running for an extended period of time in a parking garage.
Several victims complained of headaches as a result of carbon monoxide exposure. One other person was evaluated on the scene and released.
Firefighters have brought in portable fans to ventilate the building.