Because of the presence of cryogens (for the super conducting magnets of the HMS, the Møller polarimeter and for cooling the cryogenic targets) Hall C is listed as an Oxygen Deficiency Hazard (ODH) area of Class 0 (with the exception of the area above the crane railing which is ODH-2). This rating requires that those wishing to have unescorted access to the hall must take the JLab ODH training course once every two calender years. This course is typically taught once monthly by a representative of the EH&S group. Further, those working above the crane railing must have more extensive ODH training as documented in the JLab EH&S manual.
There are ODH sensors in the hall which automatically trigger audible alarms and blue lights. The alarms sound and the lights will flash if an ODH condition is detected.
Aside from the more or less global concerns about rapidly expanding cryogenic gases displacing the air there is another way in which one could encounter an ODH situation in connection with Hall C operations. The problem is that of work in confined spaces where the atmospheric composition may be different from that of the hall at large. Examples of such spaces are the main spectrometer vacuum vessel, which is evacuated during normal use (and is usually brought back to ambient pressure by filling it with dry nitrogen gas), the HMS gas Cerenkov detector which is normally filled with an oxygen-free gas mixture, and the SOS gas Cerenkov detector. Before work can be done in any of the above areas the oxygen content of the atmosphere in the closed vessel should be verified by EH&S personnel.