The 4-log inactivation requirement is for which group?

• A. Cryptosporidium
• B. Giardia
• C. Viruses
• D. Bacteria

Answer: (C)

Log inactivation measures how effectively disinfection reduces pathogens in drinking water. A 4-log inactivation means a 99.99% reduction—reducing the virus count by a factor of 10,000. Viruses are particularly concerning in drinking water because even small numbers can cause illness, and they can be more resistant to disinfection than some other microbes. To protect public health, regulatory standards specify a four-log reduction for viruses, ensuring a high level of inactivation before water enters distribution. While Giardia and Cryptosporidium are often addressed through physical removal (filtration) or different log targets, the focus for viruses is on achieving this substantial inactivation through disinfection.