I amjust now reading a source, the 2010 edition of Medicine for Mountaineering, which states, "The First Need filter has an absolute pore size of 0.4 microns; the Sawyer Viral Purifier has an absolute pore size of 0.2 microns. Both have been demonstrated to remove viruses as well as bacteria and parasites in studies conducted for the EPA." (p.66)
That's very interesting, did it by any chance cite a specific EPA study that could be looked up online for more information?
I ask because it is presenting different & conflicting information than I've seen before. For example, the CDC's guide for
backcountry water treatment says "Filtration is not effective in removing viruses." And in it's guide on
household water treatment, it looks at different classes of filters and rates their effectiveness. It says:
- microfiltration (.1 micron): not effective removing viruses
- ultrafiltration (.01 micron): moderate effectiveness removing viruses
- nanofiltration (.001 micron): very high effectiveness removing viruses
However, as far as I've seen (including the models quoted above from Medicine for Mountaineering) backcountry filters all would fall into that microfiltration category. This CDC information is consistent with the information I've seen presented by several different sources.
As a side note, we mentioned chemical contamination earlier and the CDC only rates nanofiltration as moderately effective at filtering chemical contaminants. So, based on that, it seems likely that backcountry filters wouldn't do anything for water with chemical contamination.
At least in the USA, I understand virus removal is not an issue in the backcountry.
That's my understanding as well. I understand you are much more likely to run into water with potentially harmful bacteria or protozoa than you are to run into viral contamination. That said, viral contamination is possible mainly due to contamination from human and animal waste.
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