Some states allow a residual of bromine or another sanitizer provided it can be shown to keep the microbial risk low. Because bacteria are constantly being added to the water from the environment and from other bathers, a disinfectant must be present at all times. Chlorine gas and the hypochlorites are universally accepted for use in pools and spas because the chlorine supplied by chlorine-based sanitizers is the strongest oxidizer that persists in a pool. Residual chlorine also kills pseudomonads entering from the environment, making dermatitis and ear infections unlikely. Disinfectants must be effective against a wide variety of organisms.
Chlorine deactivates a variety of microorganisms at rates dependent on the species.
Bacteria and viruses are destroyed within seconds, while the slower destruction times of Cryptosporidium and Giardia require special care in avoiding and taking care of fecal accidents. In addition to rapid kill and persistence in water, an effective pool or spa disinfectant should destroy pathogens, not merely injure them, and should not allow resistant bacteria strains to grow up. Chlorine-based sanitizers are effective disinfectants that retain their ability to maintain water purity over weeks, months and years of use, demonstrating that resistant strains of bacteria don't readily develop.