Abstract: At approximately 3:30 p.m. on June 14, 1990, Employees #3 and #4 were operating an Ansco Cryotherm ethylene oxide (EtO) medical instrument sterilizer in the surgical central supply area adjacent to, but separate from, the surgical suites at a full service hospital. Employees #1 through #5 suffered skin and respiratory exposure to ethylene oxide. Employee #2 responded to the audible alarm and donned an unapproved respirator to shut down the system. All of the employees were treated by a physician on site; none were hospitalized. The ethylene oxide sterilizer mechanical room and the mechanical ventilation system did not meet ANSI design and operational standards because of an inefficient and ineffective gas exhaust and extraction system. The inefficacy of the system was compounded by the temporary malfunction of the general HVAC in the surgical suites, which was down for repair, and the lack of a dedicated make-up air ventilation system. By depending on the general HVAC to make up for the poor system in the sterilizer room, an excessive concentration of off-gassed EtO collected and was dispersed into the ambient atmosphere.