Every factory, mercantile establishment, mill or workshop shall be provided with a sufficient number of water closets, and whenever both male and female persons are employed, separate water closets shall be provided for the use of each sex, and plainly marked by which sex they are to be used. No person or persons shall be allowed to use the closets assigned to the opposite sex. Such water closets shall be constructed in an approved manner and properly enclosed, and at all times kept in a clean and sanitary condition. The closets, where practicable, shall be located so that they shall have direct ventilation with the outside air. Where it is impracticable to locate the closets so as to have direct ventilation with the outside air, they shall be placed in an inclosure, and every such closet shall be properly and effectively disinfected and separately ventilated, and shall be properly lighted by artificial light, except when the influx of natural light makes artificial light unnecessary.