Three chemical agents treat backcountry and emergency water: iodine, sodium hypochlorite (household bleach), and chlorine dioxide. Each has a different dosage, contact time, and pathogen coverage. The critical difference: iodine and bleach do not kill Cryptosporidium at field-safe concentrations, but chlorine dioxide does given sufficient contact time. This article is a companion to water purification without boiling: all 7 field methods — it covers only the chemical treatment options in full detail.
Iodine: Dosage, Contact Time, and the Crypto Gap
Iodine tablets — Potable Aqua and Polar Pure are the two common field products — release tetraglycine hydroperiodide into water, killing bacteria and Giardia lamblia at the following doses:
| Water condition | Dose | Contact time (room temp) | Contact time (below 40°F) |
|---|---|---|---|
| Clear water | 2 tablets per liter | 30 minutes | 4 hours |
| Turbid or cold-looking | 4 tablets per liter | 4 hours | 4 hours |
Potable Aqua bottles contain 50 tablets, treating 25 liters of clear water. Weight is approximately 28 grams (1 ounce). A second Potable Aqua product — PA Plus neutralizing tablets — removes the iodine taste after treatment; they do not affect efficacy and are not required for safety.
The gap that matters for field use: a 2016 CDC review confirmed that iodine at standard field concentrations provides no reliable protection against Cryptosporidium parvum oocysts. Crypto oocysts are 4–6 microns in diameter and resistant to halogen disinfection at any concentration that does not cause harm to the person drinking it. In surface water in North America — rivers, lakes, streams — Crypto is endemic. Using iodine as a sole treatment method in Crypto-endemic water is an incomplete strategy.
Iodine degrades over time when exposed to air and humidity. A bottle that has been opened and carried for one season may have reduced efficacy. Replace iodine tablets annually or after any extended exposure. Iodine is contraindicated for pregnant women and individuals with thyroid conditions — this is a real medical consideration, not a disclaimer to ignore.
Household Bleach: Sodium Hypochlorite Dosages by Concentration
Unscented household bleach at 6–8.25% sodium hypochlorite — the concentration found in standard Clorox and store-brand bleach — treats water at these doses per the CDC and EPA:
| Water condition | Dose per gallon | Dose per liter | Contact time |
|---|---|---|---|
| Clear water | 8 drops | 2 drops | 30 minutes |
| Turbid water | 16 drops | 4 drops | 30 minutes |
If using bleach at lower concentration — 1% sodium hypochlorite (some pool shock products) — multiply the dose by 6: 48 drops per gallon of clear water. If no concentration is labeled on the bottle, do not use it — the dose cannot be calibrated safely without knowing the concentration.
Post-treatment test: after 30 minutes, smell the water. A faint chlorine odor confirms active residual. No odor means insufficient bleach was added — repeat the treatment at full dose and wait another 30 minutes. An overwhelming bleach smell means over-treatment; let the water stand uncovered for 30–60 minutes to off-gas, or add a small amount of untreated water to dilute.
Bleach degradation: sodium hypochlorite degrades at approximately 20% per year in standard storage conditions (room temperature, dark). A 2-year-old bottle at 8.25% original concentration is now approximately 5.3% concentration. Adjust dose upward by 25% for bleach over 12 months old. Replace stored bleach annually for emergency use — it is an inexpensive supply.
Bleach shares iodine’s limitation: no reliable kill against Cryptosporidium at safe drinking concentrations. For Crypto coverage with chemical treatment, chlorine dioxide is the only option.
Chlorine Dioxide: The Only Chemical That Covers All Three Pathogen Types
Chlorine dioxide (ClO₂) kills bacteria, Giardia, and Cryptosporidium — the only chemical tablet that covers all three pathogen categories without boiling. MSR Aquatabs and Potable Aqua CIO₂ are the two primary field products; both release chlorine dioxide on contact with water.
| Target pathogen | Dose | Contact time — above 59°F (15°C) | Contact time — below 59°F |
|---|---|---|---|
| Bacteria, Giardia | 1 tablet per liter | 30 minutes | 4 hours |
| Cryptosporidium | 1 tablet per liter | 4 hours | 8–10 hours |
The dose does not increase for Crypto — the contact time increases. One tablet per liter is the correct dose in both cases. The oocyst wall of Cryptosporidium requires extended chlorine dioxide exposure to be breached; reducing concentration does not compensate for reduced time.
MSR Aquatabs cost approximately $0.25 per tablet (treating 1 liter) — the most economical chemical treatment per liter. Potable Aqua CIO₂ runs approximately $0.40–0.50 per treatment. Both products have a shelf life of approximately 4 years in unopened packaging.
Chlorine dioxide does not work in turbid water. Suspended particulates consume the disinfectant through chemical reactions before it reaches pathogens. Pre-filter visibly cloudy water — a bandana, coffee filter, or hollow-fiber filter like the Sawyer Squeeze — to reduce turbidity before chemical treatment. If water passes light but is not clear, the chlorine dioxide contact time should be extended by 50% as a precaution.
Head-to-Head: Iodine vs Bleach vs Chlorine Dioxide
| Factor | Iodine (Potable Aqua) | Bleach (6–8.25%) | Chlorine Dioxide (MSR Aquatabs) |
|---|---|---|---|
| Kills bacteria | Yes | Yes | Yes |
| Kills Giardia | Yes | Yes | Yes |
| Kills Cryptosporidium | No | No | Yes (4+ hrs) |
| Kills viruses | Yes | Yes | Yes |
| Weight (50-treatment supply) | 28g | N/A (bulk product) | ~20g |
| Cost per liter | ~$0.10 | <$0.01 | ~$0.25 |
| Shelf life (sealed) | 4 years | 1 year active | 4 years |
| Taste | Strong iodine | Chlorine | Mild |
| Works in turbid water | Reduced (double dose) | Reduced (double dose) | No — pre-filter required |
| Pregnancy/thyroid safe | No | Yes | Yes |
When to Combine Chemical Treatment With Filtration
Chemical treatment and mechanical filtration address different threats and work better together than either alone. A 0.1-micron hollow-fiber filter (Sawyer Squeeze) removes bacteria and protozoa physically but does not affect viruses. Chemical treatment kills viruses but requires clear water and adequate contact time.
Two scenarios warrant combining both methods:
- International travel or post-flooding with sewage intrusion: Filter first (removes bacteria, protozoa, reduces turbidity) then treat with chlorine dioxide (kills viruses, provides secondary kill on any remaining pathogens). This combination covers all three pathogen categories under any water clarity condition.
- Cold water where Crypto is a concern: Filter with hollow fiber (immediate Crypto removal, no wait time) then treat with chlorine dioxide for virus and secondary bacterial coverage. Eliminates the 8–10 hour cold-water Crypto contact time by handling Crypto mechanically.
In North American backcountry use without flood event contamination, viral contamination of surface water is uncommon. Filtration alone covers the realistic threat profile. Chemical treatment is carried as a backup for filter failure, not as a primary method.
Where to Go Next
For the full comparison of all seven treatment methods including UV sterilization and solar SODIS, see water purification without boiling — the parent article for this series. Filter comparisons — Sawyer Squeeze vs Berkey vs Lifestraw on weight, flow rate, and freeze risk — are in best survival water filter comparison. The solar SODIS method in full detail is at solar SODIS water purification.
