What percentage of removal/inactivation does 4-log removal represent?

4-log removal represents a level of pathogen removal or inactivation that corresponds to a 99.99% reduction in the number of organisms present. The "log" scale is a logarithmic measure, where each "log" represents a tenfold decrease in organisms. Therefore, a 1-log removal indicates a 90% reduction (1 out of 10 remains), a 2-log removal indicates a 99% reduction (1 out of 100 remains), a 3-log removal indicates a 99.9% reduction (1 out of 1,000 remains), and a 4-log removal signifies that only 1 out of 10,000 remains, which translates to a removal or inactivation percentage of 99.99%. This level of removal is especially significant in water treatment processes that aim to ensure the safety and quality of drinking water by effectively eliminating pathogens.