If a mechanical ventilating system is used in a public garage, the system shall not be required to operate continuously if the system is equipped with an approved detection device that operates automatically upon detection of a concentration of carbon monoxide of 50 parts per million (ppm). The minimum density of carbon monoxide detectors shall be one detector for every 5,000 square feet of garage floor area. If the general exhaust of a building is discharged into a mechanically-exhausted public garage, the garage exhaust system shall exhaust at least 100 percent of the ordinance required air.