from EPA
Sodium hypochlorite is used for water disinfection and, due to degradation, perchlorate has been detected in hypochlorite solutions. A 2009 study by the American Water Works Association (AWWA) and Water Research Foundation found that perchlorate can be present in hypochlorite solutions and can continue to form with a rate of formation that depends on storage conditions. The study found that to minimize perchlorate formation, hypochlorite solutions should be stored in dark and cool conditions, diluted if possible and used within a few weeks of manufacture.
In response to concerns raised by stakeholders and pursuant to the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), the EPA published an Interim Decision for Calcium, Sodium and Potassium Hypochlorite in March 2018. The Agency requires that advisory best management practices be added to hypochlorite drinking water disinfection product labels to minimize the potential for chlorate and perchlorate formation during storage. These best management practices can be used individually and in combination, and include limited storage time, adequate solution pH range, sunlight exposure avoidance, controlled storage temperature, and dilution.