On what was described as “the first nice day of summer break” in Pleasant Grove, Utah, Veterans Memorial Swimming Pool became a scene of mayhem when suddenly a chlorine gas release sickened 50 people. Reports of dozens rushed to the hospital due to the June 4 tragedy, including the admission of children to pediatric intensive care units, detail the magnitude of the event.

Although relatively rare, incidents of this type turn the tables on our expectations for chlorine at swimming pools. Suddenly, a substance we use to help keep swimming healthy and safe by destroying waterborne pathogens becomes a threat to public safety. How can this be possible?