A good water filter stops bacteria and protozoa, and does not stop viruses or chemicals — that one sentence is most of what the marketing won't tell you plainly. The skill isn't owning a filter; it's knowing which problem you're pointing it at.
What the micron number means
Filters strain water through pores measured in microns; anything bigger than the pore stays behind. As our Sawyer Squeeze review puts it: the 0.1-micron filter physically strains out bacteria and protozoa, like giardia and cryptosporidium, but viruses are small enough to slip through. That's not a flaw of one brand — it's physics that applies to every hollow-fiber filter in this class.
The reassuring context, from the same review: in most North American tap water and backcountry streams, viruses aren't the concern — bacteria and protozoa are. So for the realistic household cases — a boil advisory, a questionable stream, murky stored water — a quality 0.1-micron filter is genuinely the right tool.
The honest capability table
| Threat | 0.1-micron filter | Boiling | EPA bleach dose |
|---|---|---|---|
| Bacteria | Yes | Yes | Yes |
| Protozoa (giardia, crypto) | Yes | Yes | Not reliably |
| Viruses | No | Yes | Yes |
| Chemicals, fuel, runoff | No | No | No |
Read the bottom row twice: nothing on this table fixes chemical contamination. If officials say fuel or industrial runoff is in the water, stored water is the answer — no filter, flame, or drop count changes that.
The order of operations
- Stored water first — it's already safe; that's its whole job.
- Filter for cloudy or bug-suspect water: it handles the realistic North American threats and improves taste while it's at it.
- Boil or bleach after filtering when viruses are plausible — sewage contamination, flood water. The bleach-math post carries the exact EPA drops.
Where this falls short
Filters are consumables with moods: they clog with silty water (pre-strain through a cloth and everything lasts longer), most hollow-fiber cartridges are ruined by freezing — a real consideration in a cold garage — and flow rates slow as they age. Our filter reviews cover which units hold up and what each honestly claims. Buy the filter for the bugs; keep the kettle and the dropper for the rest.