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Swimming Pool Chlorine Incident

A large gymnasium was evacuated when the occupants smelled what was believed to be chlorine gas. The gym had a good-sized indoor swimming pool and it was believed that the smell was coming from that general area of the gym.

First Responders arrived quickly but when they entered the building they could not detect any chlorine gas using detector tubes. From the symptoms reported by the occupants and the lack of any residual chlorine gas when the First Responders arrived indicated that the level of chlorine gas had been low, on the order of 1-3 ppm. According to the Chlorine Institute, the odor threshold for chlorine gas is reported to be on the order 0.2-0.4 ppm.

One elderly gentleman in his early 80's was part of a water exercise class at the time of the incident. He was equipped with his own breathing oxygen supply due to a history of a heart condition. Following the incident, the First Responders recommended that he go to the hospital for observation as a precaution. About 24 hours later he died of a heart attack. Initially it was not believed that the incident at gym had anything to do with the subsequent heart attack. Nevertheless, a lawsuit followed alleging that the chlorine incident at the gym was responsible for the heart attack.

Chemaxx was hired to investigate the incident to determine how the chlorine got into the air. There were two opposing hypotheses. One was the accidental mixing of liquid sodium hypochlorite (NaOCl) into a drum of muriatic acid (HCl) by gym workers in the swimming pool equipment room. The other hypothesis was a malfunction of spa sensors and/or controller thereby pumping excess NaOCl into the spa, which was in the same room as the indoor swimming pool. The spa was heated and constantly circulating, but no one was in the spa that morning.


Video 1- Press play button

Experiments showed that the reaction between NaOCl and HCl is vigorous and instantaneous, as shown in the video above. Using a reverse balloon model, it was shown that the concentration of chlorine gas in the smaller pool equipment room would have been intolerably high, at least 30-90 ppm or greater, if the accidental mixing scenario were true. Furthermore, no one had mixed any chemicals just prior to the incident nor had anyone been seen fleeing the pool equipment room. Additionally, everything was in its proper place within the pool equipment room, which could not have been the case had someone accidentally mixed NaOCl with HCl. In other words, no one could have stayed in the room and put everything in its proper place in an atmosphere of 30-90 ppm chlorine or greater. They would have had to flee the room.

Calculations showed that a malfunctioning spa sensor and controller could easily dump more than enough NaOCl into the spa to account for the chlorine released to the gym air. Furthermore, the spa was warm (104°F) and continuously circulating. Unfortunately, no one measured the spa or swimming pool chemistries on the day of the incident. The nature of the spa sensor and controller is that once the power is shut off (as it was by First Responders) it is possible that a malfunction could be "re-set" and not be present after the power was turned back on. This is like re-booting a computer.

Chemaxx believed that the evidence supported the malfunctioning sensor and/or controller scenario. Furthermore, the accidental mixing scenario could be eliminated by the intolerable chlorine levels that would have had to be present in the pool equipment room plus the lack of anyone known to have been mixing chemicals on the day of the incident plus no one seen fleeing the equipment room. The case settled before trial.

Dr. Fox has his Ph.D. in Physical Chemistry and is a Certified Fire & Explosion Investigator with substantial experience investigating complex industrial chemical accidents, fires and explosions as well as chemical-related consumer product accidents, fires and explosions. He is also a Certified Team Leader in OSHA Process Hazard Analysis.