In June 2015, personnel from California's Contra Costa Health Services Environmental Health and Hazardous Materials (hazmat) divisions were alerted to a possible chemical release at a swimming pool in an outdoor municipal water park. Approximately 50 bathers were in the pool when symptoms began; 34 (68%) experienced vomiting, coughing, or eye irritation. Among these persons, 17 (50%) were treated at the scene by Contra Costa's Emergency Medical Services (EMS) and released, and 17 (50%) were transported to local emergency departments; five patients also were evaluated later at an emergency department or by a primary medical provider. Environmental staff members determined that a chemical controller malfunction had allowed sodium hypochlorite and muriatic acid (hydrochloric acid) solutions to be injected into the main pool recirculation line while the recirculation pump was off; when the main recirculation pump was restarted, toxic chlorine gas (generated by the reaction of concentrated sodium hypochlorite and muriatic acid) was released into the pool. A review of 2008-2015 California pesticide exposure records identified eight additional such instances of toxic chlorine gas releases at public aquatic venues caused by equipment failure or human error that sickened 156 persons. Chemical exposures at public aquatic venues can be prevented by proper handling, storage, and monitoring of pool chemicals; appropriate equipment operation and maintenance; training of pool operators and staff members on pool chemical safety; and reporting of chemical exposures.On June 18, 2015, at 2:29 p.m., an initial 9-1-1 call reported 10-12 persons experiencing vomiting or respiratory symptoms at one of five swimming pools at an outdoor municipal water park in Contra Costa County. Contra Costa EMS and fire department personnel were dispatched. At 2:42 p.m., fire personnel requested that hazmat personnel assist in incident response, but at 2:44 p.m., the request was cancelled after fire personnel determined that there was no active chemical leak. At 3:07 p.m., fire personnel again requested hazmat personnel to investigate a possible chemical leak. Hazmat staff members arrived at the water park at 4:14 p.m. and Environmental Health personnel arrived at 4:30 p.m.; both integrated into a fire department-led incident command structure. EMS personnel evaluated and transported patients to local emergency departments. Among the 17 patients transported to an emergency department, 16 (94%) were released the same day; one patient who was experiencing tachycardia and wheezing was admitted for monitoring and breathing treatments and discharged the next day. Hazmat staff members reviewed the pool chemical controller data and performed air monitoring around the perimeter of the water park, within the immediate vicinity of the affected pool, and at the chemical storage building. Environmental Health staff members measured the free chlorine concentrations and pH of each pool and interviewed municipal water park employees and the pool maintenance contrac...