Abstract: On March 23, 2009, a batch preparation by lesser-run process of a chemical intermediate for pharmaceutical research use was being carried out in the P-10 pilot plant. During a step of reagent addition to the reactor, involving triphosgene, over-pressure of the batch prompted operator shut-down and the cooling of process equipment and an area evacuation. Venting via rupture disc to the atmosphere outside the building (roof) involved the release of an unspecified small amount of phosgene gas. The release cloud immediately impacted employees in the outside area through the acute inhalation exposure of workers. Of the 16 affected employees, 13 were taken to two area hospitals and admitted for observation lasting one or two days.