In the retort area of the cannery, Employee #2 watched as Employee #1 took a 150-pound chlorine cylinder out of service. Employee #1 failed to follow posted instructions to ensure that the cylinder valve was fully closed. When the vacuum regulator was removed, chlorine gas began escaping. Employees #1 and #2 left the area. Employee #1 donned an SCBA and returned to close the valve. It appears that the SCBA was worn in the DEMAND MODE, with employee #1 wearing eyeglasses that interfered with the face seal. Chlorine gas entered the facepiece. Employee #1 tried to hold his breath and close the valve. Minutes later, he was found by Employee #2 lying on the ground outside, gasping for breath. Employee #1 was hospitalized for three (3) days.