A spokesman for the Anne Arundel County Fire Department said the fumes were detected after a paramedic treated an employee of The Westin who reported feeling faint, the Baltimore Sun reported Monday.
Lt. Russ Davis said all of the hotel's 64 occupied rooms were evacuated Sunday afternoon.
He said "high levels" of carbon monoxide were detected in all seven stories of the building.
Fire officials said 20 people suffered possible carbon monoxide poisoning. Four employees who worked in the laundry room were transported to a trauma center. Four other employees and a guest were taken to a local hospital.
Guests were not allowed to return Sunday night.
The source of the leak has not been identified.