Explainer

Clear, cold, fast-moving water from a high basin still carries pathogens, and the treatment methods available do not all address the same ones.

What is in it

Protozoa — giardia and cryptosporidium. Large, common, and cryptosporidium in particular is highly resistant to chlorine.

Bacteria — E. coli, campylobacter, salmonella. Smaller than protozoa.

Viruses — smaller again. A minor concern in remote North American backcountry, a real one anywhere with upstream human density and abroad.

Chemical contamination — mine drainage, agricultural runoff. No portable method addresses this, which is why source selection still matters.

Filters

A hollow-fibre filter rated at 0.2 microns removes protozoa and bacteria reliably. It does not remove viruses, which pass through.

Filters clog with sediment, and they fail if frozen — ice ruptures the fibres and the damage is invisible. A filter that spent a night below freezing should not be trusted.

Chemicals

Chlorine dioxide handles bacteria, viruses and — given enough contact time — cryptosporidium, but that contact time is long, often four hours, and cold water lengthens it further.

Iodine does not reliably kill cryptosporidium at practical doses, and is not appropriate for long-term use or for people with thyroid conditions.

Reading the contact time on the packaging, and actually waiting it out, is the part that gets skipped.

UV

Effective against all three classes, fast, and dependent on batteries and on the water being clear. Turbid water shields organisms from the light.

Boiling

Reaching a rolling boil is sufficient. Extended boiling times at altitude are not required — the temperature reached on the way to boiling has already done the work.

It costs fuel, which is why it is a fallback rather than a primary method for most trips.

This article was researched and drafted with the assistance of AI tools and reviewed, fact-checked and edited by Leonidas Clark before publication. Rules vary by agency and unit; check the managing office before you go.