5 C.F.R. § 550.902
In this subpart: Agency has the meaning given that term in 5 U.S.C. 5102(a)(1) .
Duty involving physical hardship means duty that may not in itself be hazardous, but causes extreme physical discomfort or distress and is not adequately alleviated by protective or mechanical devices, such as duty involving exposure to extreme temperatures for a long period of time, arduous physical exertion, or exposure to fumes, dust, or noise that causes nausea, skin, eye, ear, or nose irritation.
Employee means an employee covered by the General Schedule (i.e., covered by chapter 51 and subchapter III of chapter 53 of title 5, United States Code).
Hazardous duty means duty performed under circumstances in which an accident could result in serious injury or death, such as duty performed on a high structure where protective facilities are not used or on an open structure where adverse conditions such as darkness, lightning, steady rain, or high wind velocity exist.
Hazard pay differential means additional pay for the performance of hazardous duty or duty involving physical hardship.
Head of an agency means the head of an agency or an official who has been delegated the authority to act for the head of the agency in the matter concerned.
5 C.F.R. §550.902