Maximum Permissible Exposure (MPE) and Nominal Hazard Zone (NHZ)

An MPE is the level of laser exposure to which the eye or (less limiting) the skin, may be exposed without adverse affects. NHZ is the space within which the level of direct, reflected or scattered radiation during operation exceeds the applicable MPE.

When any class IIIb or IV laser is used at levels at or above IIIb in an open beam mode (unenclosed) the MPE will be assumed to be exceeded in that room or area and appropriate precautions shall be taken. In other words, the NHZ (nominal hazard zone) will comprise the enclosure (room or area to which the beam is restricted to by virtue of walls, curtains or other barriers) in which the laser is operating if operated at or above IIIb levels. This is done to account for intentional or unintentional scattered or reflected beam. If the Laboratory LSO believes the NHZ does not apply to the whole area he may justify a more limited NHZ in the SOP by using information supplied by the laser manufacturer, by measurement, or by using the appropriate laser range equations or other equivalent assessments.