San Francisco Building Inspection Commission Codes § 1202.5 Add a second paragraph and a third paragraph as follows

In other than high-rise buildings, public corridors, public hallways and other public spaces having openings into adjoining dwelling units, guest rooms, or congregate residences within R-1 and R-2 Occupancies, shall be provided with natural ventilation by means of openable exterior openings with an area of not less than 1/25 of the floor area of such rooms or spaces with a minimum of 4 square feet (0.37 m² ).

In lieu of required exterior openings for natural ventilation, a mechanical ventilating system may be provided. Such system shall be capable of providing two air changes per hour in public corridors, public hallways and other public spaces having openings into adjoining dwelling units, guest rooms, or congregate residences with R-2 occupancies, with a minimum of 7-1/2 cubic feet per minute (3-1/2 L/s) of outside air per occupant during such time as the building is occupied.