Microbes are small, seriously small.
If you were to drop a single bacterium onto an unglazed terra cotta plate, it would fall through some of the holes and come to rest somewhere under the surface.
Do the same with a virus and it would fall right through.
Viruses really are that small – right down near the size of atoms, about as small as it’s possible to get.
That smallness means microbes are almost impossible to measure.
So sterility levels are all about the probability of something being sterile, not the actual reality.
This is the basic Sterility Assurance Level , achieved by making something clean – washing away surface grime and dirt so an object appears physically clean. Depending on how clean it actually is, this is referred to as a 2-log reduction, eliminating 99% of germs.
Soap and water will achieve this, with about two minutes scrubbing.
Beyond cleaning, this involves the deliberate action of destroying germs – either eliminating them or making them harmless. Actual germ kill can vary from 99% to 99.999% – a Sterility Assurance Level range from 2-5 log germ reduction.
Bleaches and other types of disinfectant are used, ranging through to ozone exposure and radiation by ultra violet light at the top end.
This is the process of completely eliminating microbial viability altogether – killing all non-pathogenic and pathogenic spores, fungi and viruses. Because total annihilation can never be proved, this is defined as a 99.9999% kill – a Sterility Assurance Level that is a 6-log reduction.
Ozone and hydrogen peroxide achieve this – killing all viruses and bacteria down to less than 1 in a million, so low that they can no longer be measured.
The Maths of Sterility Assurance Levels
A 1-log kill reduces a germ colony to 100,000 bacteria after a 90% reduction.
A 2-log kill reduces a germ colony to 10,000 bacteria after a 99% reduction.
A 3-log kill reduces a germ colony to 1,000 bacteria after a 99.9% reduction.
A 4-log kill reduces a germ colony to 100 bacteria after a 99.99% reduction.
A 5-log kill reduces a germ colony to 10 bacteria after a 99.999% reduction.
A 6-log kill reduces a germ colony to 1 bacterium after a 99.9999% reduction.