One of the most important conventions in the filtration industry: NOT ALL FILTERS ARE CREATED EQUAL. The pore size of filter media is identified by the diameter of the particle that it can be expected to retain with a defined, high degree of efficiency. Pore sizes are usually stated in micrometer or microns for short (µm), which equals one millionth of a meter. Pore size ratings refer to the size of a specific particle or organism retained by the filter media to a specific degree of efficiency. Ratings can be stated as either nominal or absolute pore size.

An absolute pore size rating specifies the pore size at which a challenge organism of a particular size will be retained with 100% efficiency under strictly defined test conditions. Among the conditions that must be specified are: test organism (or particle size), challenge pressure, concentration and detection method used to identify the contaminant. E.g. live bacteria test

Absolute micro-filters are used for critical applications such as sterilizing and final filtration.

A nominal pore size rating describes the ability of the filter media to retain the majority of particulate at (60 - 98%) the rated pore size. Process conditions such as operating pressure, concentration of contaminant, etc., have a significant effect on the retention efficiency of the filters.

POU nominally rated carbon filters are used on city water only (disinfected water) for general filtration such as particulate, chlorine and it's byproducts, chemical, taste and odors.

Note: Rating parameters vary widely among manufacturers and filtration industries.

A leading POU/POE (domestic devices) water industry association defines "absolute" to mean 85% rejection at the stated micron ratings and at the recommended flow rate. Some POU filter manufacturers (aggressive marketers) using extruded and powdered activated carbon (CB and PAC) claim 0.5 µm "absolute" ratings under this "standard".

In industrial filtration absolute rating provides a much stricter efficiency standard for the filter, typically 98-99% percent rejection rate at the stated micron. Some industrial filter manufacturers use multi-pass standard (BETA ratio) efficiency method (the dirtier the more efficient) which we are not going to get into it.

The high-purity water industry (pharmaceutical, pre-treated RO feed water etc.) even defines absolute as a 99.99% efficiency (4-log retaining efficiency) single pass rejection rate or greater as with Doulton ceramic candles and cartridges (100% at 0.9 µm absolute). However, independent validation might be in order for filter manufacturers making this claim.

In layman's language terms what exactly all these micron rating "standards" mean?

Let assume that one litre of untreated water contains 50.000 particles the size of 3 µm. (the size of cryptosporidium cyst). With a single pass @60/4 psi/bars pressure through the filter 1 L. of filtered water can contain:

NOTE:In ultra-pure and industrial process water, absolute rated organic media micro-filters are designed to operate at low water pressures (about 25/1.7 psi/bars) rather than typical domestic pressure (60/4 psi/bars).

Filters mechanically retain particles through debth or surface filtration principles. Doulton uses both principles.

How can a carbon block filter ("purifier") manufacturer claims to remove 99.99% virus and bacteria whilst only 99.9% cyst retention.

a) They are not removing viruses nor bacteria, they are "poisoning" them with "food grade" pesticides or iodine or it's byproducts incorporated into the filter media or, rely on some magnetic "electro-attractive forces" retention.
b) The "poison" don't work well on cysts. Keep in mind that viruses and bacteria are much smaller than cysts therefore easier to "poison".