All nursing and nonprofessional personnel, whose duties primarily require their physical presence on duty in the Tuberculosis or Neuro-Psychiatric units of government hospitals, shall be paid a differential of fifteen percent of the pay to which they would otherwise be normally entitled. This differential shall be payable for any shift of eight hours or more during which the employee was assigned to such unit. (Such differential shall also be applicable to the aforementioned employees during periods of annual or sick leave absences, provided that they were assigned to such hospital unit at the time the leave of absence began and further that they returned to the unit following the absence.)