Prior to 1984, much effort in toxicology focused on limiting occupational exposures to hazardous materials. These early efforts produced well-known safety limits such as the permissible exposure limit (PEL), the recommended exposure limit (REL), and the threshold limit value (TLV). These limits, conservative by design, were established to protect workers from short-and long-term exposures over the course of their working lifetimes (OSHA, 2006; ACGIH, 2012; NIOSH, 2012).