The waterborne disease hardest to kill with chlorine is which?

• A. Giardia
• B. Norovirus
• C. Hepatitis
• D. Cryptosporidium

Answer: (C)

Chlorine effectiveness varies by the structural toughness of the organism. Cryptosporidium has a very sturdy oocyst wall that protects the internal material, making it extraordinarily resistant to chlorine. Because of this, achieving inactivation requires much higher chlorine doses or much longer contact times, or the use of alternative disinfection methods such as UV or ozone. Giardia cysts are also resistant, but not as tough as Cryptosporidium. Viruses like Norovirus and Hepatitis are generally more susceptible to chlorine under typical treatment conditions, especially with adequate contact time. So, among these options, Cryptosporidium is the hardest to kill with chlorine.