Abstract: At 4:00 a.m. on June 2, 2021, Employee #1 was offloading bleach from a tanker to a storage well at a California Water Services site. The tanker, which held both bleach and hydrochloric acid (HCL), was mistakenly labeled as containing only bleach. After pumping bleach for several minutes, the employee noticed a change in the color of the gas vapor and began inhaling an unfamiliar and irritating vapor. Despite donning a respirator, the employee was overcome by the vapors, resulting in irritation of the eyes and lungs. The employee realized the tanker contained hydrochloric acid, which reacted with the bleach, creating the hazardous vapors. The employee was hospitalized for treatment.