Abstract: Employee #1 was purging an ABS plastic from the Sterling extruder before adding new ABS plastic. The barrel was heated to 500 degrees F. New material received from a sister plant was added to the machine. When this material entered the heated barrel chamber, it decomposed instantaneously, releasing an acrid smoke cloud of, most likely, hydrochloric acid. The material was not ABS plastic as labeled, but was, instead, a PVC plastic that decomposes and releases hydrochloric acid at temperatures above 480 degrees F. The PVC was a BF Goodrich product, called Geon 87241 white pellets 1264. This facility receives pelletized plastic from a sister plant and normally uses ABS plastic exclusively. The two plastic products could not be distinguished by sight. The material label contained no hazard warning or manufacturer's information, and was mislabeled as ABS not PVC plastic. Employee #1 was treated for inhalation and released from the hospital.