Abstract: At approximately 1:00 p.m. on April 20, 1992, Employees #1 through #4 and seven coworkers exited a 17 ft in diameter tunnel that they had lined with ready-mix concrete. At about 10:00 a.m. the ventilating fan had burned out a bearing and the foreman had been told to remove the crew; however, he chose to wait until the crew finished pouring its supply of concrete. A diesel-fueled locomotive and generator were being used in the tunnel. Some of the employees complained of headaches. All were sent to an industrial clinic, were examined, and released. Later, after 12:00 midnight, Employees #1 through #4 suffered enough discomfort that they independently went to the emergency rooms of local hospitals and were admitted. Preliminary diagnosis was pulmonary edema, with no anticipated long-term effects.