The Occupational Safety and Health Administration issued the United States Postal Service a citation alleging a repeat violation of the Occupational Safety and Health Act’s general duty clause, 29 U.S.C. § 654(a)(1), for exposing employees of a postal station in Des Moines, Iowa to an “excessive heat” hazard. In the summer of 2016, two of the station’s letter carriers began feeling ill while delivering mail and were treated at a hospital or urgent care clinic. The Secretary alleges that both carriers became ill due to excessive heat. Administrative Law Judge Sharon D. Calhoun vacated the citation. For the reasons discussed below, we set aside her decision and remand for further proceedings.