Abstract: At 12:45 p.m. on April 5, 2021, Employee #1, a regional facilities director at a YMCA in San Diego, CA, was approximately 4 feet away from a reaction involving muriatic acid and sodium hypochlorite. The reaction occurred when Coworker #1, a driver, and Coworker #2, a helper, mistakenly filled a sodium hypochlorite tank with muriatic acid. The chemical reaction produced chlorine gas, creating a greenish cloud of fumes and a fizzing noise. Employee #1 inhaled the chlorine gas and experienced difficulty breathing, chest pain, difficulty speaking, coughing, and a burning sensation in the throat. Employee #1 was treated at Pomerado Hospital's emergency room for approximately 7 hours but was not admitted. The employee was diagnosed with chlorine gas pneumonitis. Coworker #1, who was upwind from the fumes, was not injured. The incident was reported to DOSH by both the San Diego Fire Department and YMCA.