June 17, 2008 
This entry was posted on Tuesday, June 17th, 2008 at 5:45 pm and is filed under Water Filters. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
A water filter is composed of a screen with many microscopic holes. The smaller the holes, the more contaminants the filter can remove. Filter holes are measured in microns. ( A micron is a very small unit of measurement. How small? The period at the end of this sentence is 500 microns ). When considering filter size, be sure to look for an absolute ( the largest hole ), not a nominal ( the average hole ) rating. For instance, the EPA and CDC recommend an absolute one micron filter ( or one labeled for cyst removal ) to remove Cryptosporidium.
Some of the most common contaminants and their size in microns are:
Giardia Lamblia - 8 - 12 microns
Cryptosporidium Parvum - 4 - 6 microns
Bacteria ( such as E. Coli and Samonella ) - 0.2 - 4 microns
Viruses - 0.004 to 0.1 microns ( Generally, only a few filters, such as ultra-filters and reverse osmosis, have holes small enough to assure removal of all viruses. However, viruses can also be killed using a household disinfectant ).
Filters that attach to a faucet or are installed under the sink for a drinking water third faucet generally use the same technologies as their pour-through pitcher counterparts. Some filters use fabrics, fiber or ceramic screening to physically remove contaminants, although the most common types use a model block of activated carbon. These filters are reasonably effective at improving the taste of tap water, and some will also reduce lead, protozoan cysts, and many other contaminants. Like filter pitchers, shelf lives and specific contaminants removed will vary so read the label and instructions carefully.