Abstract: On August 14, 1990, Employee #1, working for an oilfield service contractor, was sent with two coworkers and a supervisor to the Calaveras Avenue Unocal gas refinery to service a hydrogen sulfide strip. The process called for the removal of a catalyst from three small, 5 ft tall by 4 ft in diameter, vessels. The Unocal crew removed the vessel lids and tested the atmosphere for hydrogen sulfide and oxygen. Less than 1 percent hydrogen sulfide was detected, and the oxygen level was 21 percent. At this point, Unocal suggested to Employee #1 and coworkers that they needed only dust masks for respiratory protection, and not the Scott Air Paks that they had brought to the site. The workers had not been fit tested nor trained in the use of dust masks because no monitoring was conducted for vanadium pentoxide dust. They completed the work by leaning over the edges of the vessels and removing the pelletized catalyst with coffee cans. It took two days, working 4 to 5 hours per day, to recharge the catalyst. On the morning of August 16, Employee #1 reported that he felt ill. He was admitted to Avenal Hospital, reporting flu-like symptoms and a headache. He was treated for headache and high blood pressure; the treating physician's diagnosis was that the symptoms were caused by exposure to vanadium pentoxide. Employee #1 had worn a dust mask for 30 minutes on August 14, then used his Scott Air Pak for the rest of the August 14 and all day on August 15. The two coworkers engaged in exactly the same task and wore the dust masks the entire time on August 14 and 15. Neither of them reported feeling ill. A supervisor later stated that he believed Employee #1's illness was due to alcohol because Employee #1 had told him that he had drunk heavily on the night of August 15.