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Normal saturated steam processing produces one  log of reduction (90% "kill") 
for each 1 to 2 minutes at 15 Lbs. psi. and 250F.

Sterility is a measurement and expressed as a probability.
Sterility is considered achieved when it reaches what is called a "log of minus 6" rather than as an absolute and is called a "Sterility Assurance Level" (SAL) which can be quantitatively (in numbers) expressed.  

Let's see what this means.

Imagine a glass full of water. Assume for a moment the water represents the microbes and it equals one million microbes (estimated to be on a product). We would call this "level  6", or a "Log of 6" (this is what a log of 6 will mean for our example: equals 1 million).  

Imagine we now empty 90% of the water that is in the glass which would leave 10% of the water.  
Each "log" represents a 90% reduction of microbes.

Our original one million (1,000,000 microbes - our water) minus 90% leaves 100,000 microbes (subtraction), and since we are reducing we go down, as it were, and now have a "log of 5" (or, now only 100,000 microbes-90% have been killed or inactivated).  

We take what we have left and again empty 90% of that and we get a "log of 4" (there were 100,000 microbes left from the previous step,  minus 90%  [or - 90,000 microbes], and now 10,000 microbes are left in the glass).

When we continue again and do this a total number of 6 times we have a "log of 0", which would be one microbe left.  

If we want to continue and empty 90% of what's left (that is, 90% of 1 microbe- there is only 1 microbe left) we enter into a "probability" of a survivor per item. It is obvious that if we have only one microbe we can't get rid of just 90% of that one microbe.

But we can think of it as a probability by increasing the number of items.

If we had one item we would at this level have one microbe on that item.  Suppose we had ten items we were sterilizing.  We could say we have one microbe among the ten items after the 6 logs of reduction.  This level would be (minus one)  Log of -1,  meaning there is a chance of having one microbe per 10 items. 

Continuing with our 90% reduction to -2 therefore would mean having one microbe per 100 items, Log-3 meaning one per 1,000 etc. Until we reach a log of -6 meaning that only one microbe might survive on a million items presuming that the original microbial load was one million microbes or less. 

This is called a Sterility Assurance Level (SAL) 10-6 and it is a number, though it is relative. This is a level of sterilization considered appropriate for implantable items.

Acceptable sterilization for tattoo and piercing means that after you have completed a cycle in your autoclave there is a probability of one microbe surviving on one million items.