At 2:30 p.m., Employee #1, Employee #2, and Employee #3 were tasked with dismantling a scaffolding level within a containment dome (identified as PMDI A-3600). During their dismantling work, at approximately 80 feet above the ground, alarms specific to phosgene gas were triggered. The process within this Dome was the manufacturing and using phosgene to facilitate the production of PMDI (an isocyanate). The leak occurred when an employee of another contractor, working below the three scaffolding employees, removed a “slip blind” between a valve and a downstream section of pipe. The valve and subsequent piping were used to transport liquid phosgene. After the bolts connecting the valve and line were broken, the valve leaked phosgene, triggering the alarms.