A unit of measurement equal to one-millionth of a meter, or a thousandth of a millimeter. You’ll almost certainly hear the term micron used in relation to backcountry water filtration methods. The standard micron size of most popular water filters is usually around .2 microns, but there are filters available with both smaller and larger pore sizes. What’s best? It depends. Most of the things you can actually see in a water source are quite large and will be caught by almost all filters, but it’s perhaps the things you can’t see that are cause for your choice of a filter pore size. Protozoan cysts, like giardia or cryptosporidium, are usually in the larger micron range – 2 microns or larger. Bacteria are smaller but can be comfortably caught in filters with a .2 micron rating. Viruses are the smallest concern, usually much smaller than .1 micron.

The needed micron rating of your water filter or purifier depends on the location, source, and your personal comfort level.

More on Microns

Typically viruses are more of a frontcountry concern that a backcountry concern in North America (they are also the easiest to eliminate via UV light solutions or chemicals if needed), but this is best evaluated on a case by case basis in regards to destination and your own personal comfort level. While some filters exist that are capable of trapping viruses such as the MSR Guardian purifier, they are usually heavy and / or have a very slow flow rate. Filters with quite large pore sizes exist as well, but are not very useful for providing backcountry drinking water unless they’re used in combination with other methods. Additionally, pay attention to the terms nominal and absolute pore size. An absolute micron rating is desired, as no pores will be larger than the micron rating that’s listed.

You can see a list of currently available water treatment options here at REI Co-op. For more on backcountry water treatment / purification / filtration, you can read our Backpacking and Hiking Water Treatment Guide.

Editor's Note: This Jargon installment originally appeared in Issue 23 of TrailGroove Magazine. You can read the original article here.