Log Reduction = log₁₀(N_before / N_after)
Log reduction measures the order-of-magnitude decrease in microbial population. A 1-log reduction = 90% kill, 2-log = 99%, 3-log = 99.9%, 5-log = 99.999%, 6-log = 99.9999%. Standards: drinking water requires 4-log virus removal, 3-log Giardia. Pasteurization achieves 5-log reduction of pathogens. Hospital-grade disinfectants must achieve 6-log reduction. Autoclave sterilization at 121°C for 15 min achieves >12-log for most organisms. Log reduction is preferred over percentage because it better communicates the residual risk — "99% kill" sounds good but leaves 10,000 survivors from 1 million organisms, while "99.999%" leaves only 10.