On June 22, 1982, the main power transformer at a local high school (St. Paul, Minnesota) overheated, causing the pressure relief valve to operate and release smoke and mist throughout the building. The transformer contained thermal-dielectric fluid with the tradename “Pyranol,” consisting of polychlorinated biphenyls (PCB's) in the form of “Aroclor” and chlorinated benzenes. The transformer did not explode or flame. The emmission characterized by a “white mist” occurred over an approximately 4-hour period with resultant contamination of basement and first floor areas. The temperature of the emission was estimated to be approximately 250-300°F.

“Pyranol” contains PCB aroclor 1260 (45%) and chlorinated benzenes (40% trichloro and 15% tetrachlorobenzenes). Commercial PCB preparations manufactured in the United States have been marketed under the trade name “Aroclor.” Several grades of Aroclor have been designated by numbers such as 1260. The first two digits represent the type of molecule (12 = chlorinated biphenyl). The last wo digits give the weight percent of chlorine.

The fire was discovered by the school janitor and the firefighters arrived at 5:40 a.m. As the firefighters walked near the transformer, several men began complaining of nausea, sore throat and burning of exposed skin. Firefighters having symptoms were asked to go out in the open and were immediately hosed down with water. Initially, 14 firefighters were taken to the emergency room of St. Paul-Ramsey Medical Center, where they were evaluated, treated and released. When inquiry was begun to find out what kind of substance was burning, it was discovered that the transformer which contained “Pyranol” (installed in the school in 1958) had overheated.