On August 19, 2004, an explosion and fire occurred at the Sterigenics International medical sterilization facility in Ontario, California. The incident involved the release of ethylene oxide (EO) — an extremely flammable and toxic gas used to sterilize medical equipment — which ignited and caused structural damage to the facility, injuring several workers.

During a sterilization cycle, a flammable EO mixture was discharged from the sterilization chamber to an abator — a thermal oxidizer designed to destroy residual EO. The abator contained an open flame ignition source as part of its design. The EO-air mixture entering the abator was within the explosive concentration range when it contacted the open flame, causing an explosion that damaged the treatment equipment and surrounding facility.

The CSB found that Sterigenics lacked engineering controls to prevent a flammable EO mixture from reaching the abator's ignition source, and that the process hazards of the EO sterilization cycle were insufficiently evaluated. The absence of interlocks to verify safe EO concentrations before ignition sources were engaged was a critical safeguard gap. EO's exceptionally wide explosive range (3%–100%) makes concentration control a fundamental design requirement.