When you hit a microbial population with heat, radiation, or a chemical, the survivors don't drop by a fixed number each minute — they drop by a fixed fraction. Slide through exposure time, then flip the y-axis between log and linear to see why that one fact changes everything.
Survivor curve
1,000,000 survivors0 min · 0-log reduction · 0% killed
Exposure time0 min
Survivors1,000,000
% killed0%
Log₁₀ drop0
Why the log-scale line is straight: a constant fraction lost per minute becomes a constant distance dropped on a log₁₀ axis. Flip to Linear scale and the very same deaths look like a curve that “flattens out” — but the microbes are still dying just as fast.
D-value = decimal reduction time. Here one minute kills 90% (one log₁₀), so D = 1 min. Real D-values depend on the organism, temperature, and agent. Because you can only ever cut the count by tenths, sterilization is defined by probability — a Sterility Assurance Level of 10⁻⁶ means under one chance in a million a microbe survives. Cleaning an instrument first lowers the starting bioburden, so the same exposure clears that margin sooner.