Chemical water treatment

01 — Household bleach

Using unscented household bleach to treat water

Unscented liquid household bleach containing 5 to 9 percent sodium hypochlorite is the CDC's and EPA's recommended chemical disinfectant for emergency water treatment. It kills bacteria and viruses effectively at the correct dose. Pre-filtering cloudy water before treating is required, as suspended particles can shield pathogens from the chlorine.

The dose depends on the bleach concentration. Most standard household bleach sold in the United States contains 6 percent sodium hypochlorite. Concentrated bleach typically runs at 8.25 percent. Both are usable; the dose differs.

After adding bleach, stir and let the water stand for 30 minutes. At the end of 30 minutes the water should have a faint chlorine odor. If it does not, repeat the same dose and wait another 15 minutes. No odor after the second wait means the bleach has lost potency and a different treatment method is needed.

Bleach degrades over time. The EPA recommends using bleach stored at room temperature for less than one year. Opened bottles lose potency faster than sealed ones. A household that stores bleach specifically for emergency water treatment should note the purchase date and replace it annually.

02 — Iodine limitations

Who should not use iodine, and why it is a secondary option

Iodine tablets and iodine drops can kill bacteria and viruses in water, but the CDC identifies three groups who should avoid drinking iodine-treated water: people who are pregnant, people with thyroid conditions, and people with a known hypersensitivity or allergy to iodine.

Beyond those absolute contraindications, iodine has practical performance limits. The CDC's guidance on water treatment notes that iodine is only moderately effective against Giardia lamblia and is not effective against Cryptosporidium. Chlorine bleach shares the Cryptosporidium limitation, but it performs better against Giardia. Chlorine dioxide tablets are the only widely available chemical option effective against Cryptosporidium at practical doses.

The CDC also advises that no one should drink water treated with iodine for more than a few weeks continuously. This is a meaningful constraint in extended emergencies. Iodine tablets work well as a lightweight option for short-term use, but they are not a substitute for bleach in a household water treatment plan.

03 — What chemical treatment cannot do

The limits shared by bleach and iodine

Chemical disinfectants kill microorganisms. They do not remove chemical contaminants, heavy metals, sediment, or turbidity. The CDC states clearly that boiling and chemical disinfection are both ineffective against water containing fuel, pesticides, or industrial chemicals. If the water source is chemically contaminated, neither bleach nor iodine will make it safe.

Filtration fills part of this gap but not all of it. Gravity filters and portable filters remove parasites and often bacteria, but most do not remove viruses. Reverse osmosis filters remove the broadest range of contaminants including viruses, but require pressure and are not portable. The practical response to unknown contamination is to use the best available source, not to attempt to treat a known chemical source with a biological disinfectant.

Bleach also does not neutralize heavy metals such as lead or arsenic. In post-disaster scenarios where pipe infrastructure has been damaged, lead contamination is a genuine concern. Boiled and bleach-treated water from a damaged distribution system may still contain lead. In those situations, authorities typically issue specific advisories about using only bottled water or a certified filter.

04 — Treatment redundancy

Why two different methods matter more than one perfect method

Every water treatment method has a failure mode. Bleach loses potency over time and requires an accurate dose. Filters can clog, crack, or reach end-of-life without visible indication. Iodine is not usable by everyone and not appropriate for extended use. Boiling requires fuel and heat. No single method covers every scenario.

Having two different methods addresses this. If a bleach supply is depleted or contaminated, a filter provides a backup. If a filter fails mid-emergency, bleach or boiling covers microbial risk. The methods also layer effectively: filtering first removes particles and parasites, then chemical treatment or boiling addresses viral and bacterial risk that passes through many filters.

The CDC's guidance on hiking and travel water treatment specifically notes that filtering before chemical disinfection improves effectiveness, and that layering methods is the most reliable approach when water source quality is uncertain. A household that has one method has a supply; a household that has two has a system.