This is Part Five of a 13-part series of modules on HAZWOPER designed to provide the annual refresher training as required by OSHA for all employees affected by this regulation. Part Five covers toxicology. Everyone is exposed on and off the job to a variety of chemical substances. Most are not hazardous under ordinary circumstances, but they all have the potential to cause injury at some concentration. How a material is used is the major factor for determining its hazard potential. The module provides an overview of toxicology to help trainees understand the potential effects of the hazardous substances they work with.