At 11:00 a.m. on May 23, 2019, an employee removed a regulator from a chlorine gas cylinder to perform yearly maintenance and repacking. When the employee attempted the close the shutoff valve, the valve malfunctioned and stopped turning while still slightly open without the employee’s knowledge. When the employee removed the regulator, chlorine gas escaped. The employee removed himself from the area and donned a full-face respirator with total gas cartridges without inspecting the seals or valves. He re-entered the area to close the chlorine cylinder shutoff valve fully when he became overwhelmed by the gas.