Abstract: At approximately 3:30 p.m. on May 13, 1994, Employees #1, #2, and #3 sustained serious discomfort attributed to ammonia exposure combined with heat stress. At the time of the accident, the employees were in a permit-required confined space, the hot box of a precipitator at a refinery, where the employer was doing contract work. Employee #1, a helper, was standing outside the hot box in a nonpermit-required confined space and wearing a self-contained breathing apparatus escape pack. Employee #2, a structural welder, was welding inside the hot box, along with two other uninjured welders. Employee #3, an ironworker and lead worker, was overseeing the operations inside the hot box. After the accident, the employees were transported to the hospital and released the same day.