Used to be 3 microns was adequate, then 2, then 1. The most recent literature I've seen claims that a filter needs to be .1 microns to be effective...though I've never seen a filter better than 1 micron.
My Katadyn Pocket filter is effective to .2 microns. However, this won't filter out viruses so I added a Sawyer SP135 filter to the outlet which filters out everything down to .02 microns. Past that I have a Katadyn Activated Carbon Cartridge for Chemicals. Hopefully that covers just about everything.
And Yes, it does look kind of ridiculous.
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