The distinction that gets people sick: most filters stop protozoa and bacteria but not viruses. Filtering and purifying are a pair, not a choice.
“Filtered” and “safe to drink” are not the same thing, and confusing the two is how careful people still end up sick. A filter works by physical pore size; a virus is smaller than the pores of most filters. Know which job each method does.
| Method | What it does |
|---|---|
| A filter removes… | Sediment, protozoa (Giardia, Crypto), most bacteria — by physical size. |
| A filter usually misses… | Viruses — too small for a standard filter unless it’s rated for them. |
| Purifying (boil / chemical / UV)… | Kills viruses too, but doesn’t remove sediment or chemicals. |
| Best practice | Filter to clear it, then purify to kill viruses — the two are a pair. |
Viruses are the gap. In North American backcountry they’re a lower risk; after flooding, sewage contamination, or overseas, treat them as present — filter and purify.
