So, is chlorine enough to keep our swimming pools safe? There’s no doubt it’s a necessity when it comes to keeping bacteria and viruses at bay. Indeed, outbreaks of water-borne disease in recreational swimming pools due to facally derived viruses and bacteria have generally been associated with inadequately chlorinated or non-chlorinated water. In addition to chemical means, such as adding chlorine- or bromine-based disinfectants, pathogenic microorganisms can also be inactivated by physical methods, such as treatment with UV radiation.

Many people think that chlorine kills germs instantly. In fact, this isn’t the case. It actually takes different times for the same amount of chlorine to work, depending on the microorganism. Chlorine will kill bacteria such as E. coli in under a minute, but around 16 minutes to kill the hepatitis A virus. It’ll need 45 minutes to kill Giardia, and (wait for it) up to 10 days to kill Cryptosporidium.

Australian Academy of Science