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The Occupational Safety and Health Administration (OSHA) issued a final rule designed to protect employees who work in confined spaces. The rule went into effect April 15, 1993. The standard gives requirements for entry into confined spaces, designated as permit-required spaces, that the agency believes pose special dangers for entrants because their configurations impede efforts to protect employees from hazards such as toxic, explosive, or asphyxiating atmospheres. Under the final rule, OSHA defines a confined space as a space that: (1) has adequate size and configuration for employee entry, (2) has limited means of access or egress and (3) is not designed for continuous employee occupancy. Areas that OSHA considers confined spaces include central mixer drums, truck mixer drums, underground or aboveground tanks, cement or fly ash silos, aggregate storage bins, weigh or charge hoppers, and tunnels. However, a mixer drum can be reclassified as a non-permit confined space when the engine of the truck is locked out and the mixer drum can be safely entered for maintenance, assuming there are no other hazards inside the drum.