OSHA’s definition of Hazards Not Otherwise Classified (HNOCs) states:
Hazard not otherwise classified (HNOC) means an adverse physical or health effect identified through evaluation of scientific evidence during the classification process that does not meet the specified criteria for the physical and health hazard classes addressed in this section. [1910.1200(c)]
If the manufacturer determines that a chemical presents an HNOC, that hazard must be addressed in Section 2, Hazard identification, of the Safety Data Sheet (SDS) shipped with the chemical (and made otherwise available). Including information about the HNOC on the chemical label is not mandatory, but manufacturers can choose to do so. In those cases, the information should be put in the Supplemental Information section of the label. [29 CFR 1910.1200(f)(1)]
OSHA has not yet identified any specific HNOCs. However, if a company finds that one of its products meets the definition of an HNOC, then they must communicate that hazard to their employees. This will be done through additional training for employees.