Micron ratings must be absolute to be meaningful, and precise measurements are essentially impossible to make. Micron ratings pertain only to physical removal or straining of particles, so absolute micron ratings are only one means of evaluation of effectiveness. Some product claims confuse retention ratings by listing the size of microbes killed rather than the actual retentions. For example, .01 microns, the size of some viruses, is stated for some iodine products but actual retention may be 100X bigger at 1.0 microns. Many units, even those with very low micron ratings, have little or no ability to remove anything other than particles .
All products carry an EPA Establishment Registration Number.
Iodine used directly or from iodine resins is not effective against Cryptosporidia and larger pathogens, and most iodine resin based systems require double pass through for effectiveness against viruses , especially at low water temperatures. There is no practical way to know if a water source contains pathogens. This means that for consistent virus and perhaps for bacteria reduction, when using iodine resin systems, all water should be pumped through twice, and then held for 10 additional minutes. It is essential that carbon post filters not be used, as they stop pesticidal action. Otherwise they should not be permitted on the market, but EPA registration does not ensure effectiveness.