Smoking areas may be designated by proprietors or other persons in charge of restaurants, except in places in which smoking is prohibited by the Fire Marshal or by other law or regulation. Smoking areas designated by proprietors or other persons in charge of restaurants shall comprise no more than 75% of the seating capacity of the restaurant. Where smoking areas are designated, existing physical barriers and ventilation systems shall be used to minimize the toxic effect of smoke on persons in adjacent no-smoking areas in the case of restaurants consisting of a single room. The provisions of the bylaw shall be considered met if one side of the room is reserved and posted as a no-smoking area, provided that the no-smoking area comprises no less than 25% of the seating capacity of the restaurant.