Abstract: On the afternoon of April 12, 1991, a truck delivered 2,500 gallons of nitric acid, which was inadvertently pumped into an 8,500 gallon hydrochloric acid (HCl) storage tank that was located on the west side of the fuel manufacturing operation (FMO) building. The acid mist generated from this mixing apparently entered the process sewer after passing through the scrubber unit on the HCl tank and the deionization unit. The acid mist escaped through the sewer drains and entered the FMO building, which was under a slight negative pressure. As a result of this exposure, Employees #1 through #9 were sent to the hospital and released without treatment. The FMO building was evacuated until the acid mist dissipated. Heritage Co. of Charlotte, NC, a licensed hazardous materials transporter, had removed the acid mixture at the time of the investigation. Although there was a written procedure for chemical verification, which was routinely used, Employee #1 failed to follow this procedure.