Incident due to wrong catalyst added

At approximately 4:00 p.m. on February 22, 1998, Employee #3, a chemical operator, added the wrong catalyst for a reaction, causing overpressurization of the vessel. The safety relief valve opened, releasing vapors and liquid that settled to the ground near a roll-up door. Employees #4 through #12, contractors doing pipe fitting work on an adjacent reactor, were exposed to the vapors as they left the area. Employee #1 was exposed during clean-up and Employee #2 was performing housekeeping duties near the roll-up doors. Employees #1 through #12 were transported to the hospital complaining of nausea, dizziness, and chest tightness--all symptoms of acute chemical exposure.
SOURCE:OSHA.GOV