Industrial hygienists attempt to protect workers from occupational health stresses using three well-known methods: recognition, evaluation, and control. If an occupational health hazard has already been identified (eg. air sampling for a given chemical) and quantified, the industrial hygienist's attention necessarily shifts to controlling the worker's exposure to that hazard. Methods of controlling worker exposure to physical or chemical health hazards include engineering and administrative controls, and personal protective equipment. Engineering controls remove the responsibility for protection against health hazards from the worker by physically changing the process which generates the hazard. Examples of such types of control include isolation of the process, substitution of hazardous process chemicals, automation (eg. robotics), ventilation, or elimination of the process entirely. Engineering controls are mandated by law and must be used when feasible.Administrative controls, on the other hand, attempt to limit the worker's exposure to a given health hazard by decreasing the time of exposure (eg. rotating shifts) or changing work practices. Education, training and supervision can also be thought of as types of administrative controls. Finally, personal protective equipment (ppe) is the least desirable method of controlling worker exposures. If ppe is chosen as a method of control, the worker assumes most of the responsibility for his/her own protection (eg. proper wear of a respirator or hearing protection).