Stop: is it germs or chemicals?
Do not try to treat water that may contain fuel, toxic chemicals, or radioactive material. Boiling and disinfecting cannot make that water safe. Use bottled water or a different source. Contact your health department for advice.
Boiling does not remove chemicals. Disinfection also does not remove heavy metals, salts, or most other chemicals. These methods are for germs.
Quick answers for germs in clear water
- How long do I boil water?
- A rolling boil for 1 minute; 3 minutes above 5,000 feet. Boiling details
- How much bleach per gallon?
- 8 drops of unscented 5%–9% bleach per US gallon of clear water, then wait 30 minutes. Double it for cloudy water. Full dose table
- Will a filter make water safe?
- A 0.3 micron absolute filter removes parasites and bacteria but not viruses. Boil or disinfect after filtering. Filter guide
- What about chemicals or fuel?
- Nothing on this page works. Use bottled water or another source.
The comparison assumes the correct dose, contact time and equipment instructions. Filters remove germs; heat and disinfectants inactivate them. No general rating establishes safety for every contaminant.
| Method | Bacteria | Viruses | Giardia | Cryptosporidium | Chemicals and metals |
|---|---|---|---|---|---|
| Boiling | Yes | Yes | Yes | Yes | No |
| Household bleach | Yes | Yes | Not reliable; filter or boil | No | No |
| Iodine, with the health limits above | Yes | Yes | Not reliable; filter or boil | No | No |
| Chlorine dioxide tablets, per label | Yes | Yes | Yes | Only with a product labeled for this organism and its full contact time; filter first. | No |
| Filter, 1 micron absolute or smaller | Not all | No | Yes | Yes | Not stated by this rating |
| Filter, 0.3 micron absolute or smaller | Yes | No | Yes | Yes | Not stated by this rating |
| Reverse osmosis | Yes | Yes | Yes | Yes | Some chemicals and salt |
| UV unit, in clear water | Yes | Yes | Yes | Yes | No |
| Sun treatment, in clear water | Improves microbiological quality | No | |||
| Pitcher or fridge carbon filter | Most are not designed for germs. During a boil-water advisory, boil filtered tap water too. | Only with a specific reduction claim | |||
Boiling, the default
Boiling is the surest way to kill disease-causing viruses, bacteria, and parasites. Use it when you can.
- Bring clear water to a rolling boil.
- Keep it boiling for 1 minute.
- Above 5,000 feet, keep it boiling for 3 minutes.
- Let it cool. Store it in clean, sanitized containers with tight covers.
Why this page uses 5,000 feet. EPA starts the 3 minute boil above 5,000 feet. CDC starts it above 6,500 feet. Our conservative reconciliation is to use 3 minutes above 5,000 feet. This is our analysis, not a new instruction from either agency.
The extra time gives a wide safety margin. CDC says illness-causing organisms die within seconds at boiling temperature. It recommends a full minute to allow for user differences and add safety.
Boiled water may taste flat. CDC suggests pouring it between clean containers, then letting it stand for a few hours. Another option is one pinch of salt per quart or liter.
Cloudy water first
Cloudy or colored water makes disinfection work less well. Its particles can neutralize chemical disinfectants. Clear the water before using any treatment.
- Filter it through a clean cloth, paper towel, or coffee filter.
- Or let the water settle, then draw off the clear water.
- Treat the clearer water with one method on this page.
Clearing water can reduce germs, but it does not make the water safe to drink by itself.
If you cannot boil: bleach
Use only regular, unscented chlorine bleach marked for disinfection and sanitizing. Do not use scented, color-safe bleach or bleach with added cleaners. EPA says to use bleach stored at room temperature for less than one year.
First follow drinking-water directions on the bleach label. If there are none, find the sodium hypochlorite strength on the label. CDC says it is often 5% to 9% in the United States. CDC also suggests keeping unscented 5% to 9% bleach with emergency water supplies.
| Water amount | 1% sodium hypochlorite | 5% to 9% sodium hypochlorite |
|---|---|---|
| 1 quart or 1 liter | 10 drops | 2 drops |
| 1 US gallon | 40 drops | 8 drops |
| 5 US gallons | 200 drops | 40 drops |
Use these CDC instructions only when the bleach label has no drinking-water directions. Check the strength; do not use this table for another concentration or infer a dose for an unlisted amount. Use a clean dropper.
Clear cloudy water first as described above. If water is cloudy, murky, colored, or very cold, CDC says to double the listed dose. Stir well, then wait at least 30 minutes before drinking. Store the treated water in a clean, sanitized container with a tight cover.
Agency protocols can differ. This table follows CDC's complete dose and waiting-time instructions; do not combine it with drop-to-teaspoon conversions or repeat-dose instructions from another protocol.
Follow your health department's instructions for your specific water advisory. Bleach does not reliably treat every parasite and cannot make chemically contaminated water safe.
Tablets: iodine and chlorine dioxide
Product strengths differ. Follow the label for each tablet or liquid. Chemical treatments kill bacteria and viruses, but they are weaker against some parasites.
Iodine
Iodine can kill bacteria and viruses when used correctly. Do not rely on it for Giardia. It does not kill Cryptosporidium at practical doses. The health limits near the top of this page still apply.
For 2% tincture of iodine, EPA gives one dose per quart or liter. Add 5 drops to clear water, or 10 drops to cloudy or colored water. Stir, then wait at least 30 minutes.
Chlorine dioxide
Chlorine dioxide can treat bacteria, viruses and parasites, but performance and contact time depend on the product and water conditions. For Cryptosporidium, use a product specifically labeled for that organism and allow its full contact time, which can take several hours. CDC advises filtering first for the most reliable protection. Do not substitute a shorter bacteria-treatment time for a parasite-treatment time.
Filters: what they do and do not do
A pitcher or fridge filter does not replace boiling. Most are not made to remove germs. During a boil-water advisory, CDC says to boil tap water even after a home filter or pitcher.
Portable filters do not remove every kind of germ. A 1 micron absolute filter removes Giardia and Cryptosporidium. It does not remove viruses or all bacteria. A 0.3 micron absolute filter removes bacteria and those parasites, but not viruses.
Look for the word absolute. A nominal or mean pore size is only an average, so larger holes may let larger particles pass. After filtering, CDC says to add iodine, chlorine, or chlorine dioxide to kill viruses and bacteria.
Read the specific certified reduction claim, not just a standard number. For Giardia and Cryptosporidium, CDC names filters certified to NSF/ANSI 53 or 58 specifically for cyst reduction. A general 53 or 58 mark alone does not establish that claim. Standard 42 concerns aesthetic effects such as taste and odor; 58 covers reverse-osmosis systems and 62 distillation. Check the actual product certification for the contaminant you need to remove.
A properly functioning reverse-osmosis system can remove parasites, bacteria, viruses and salt. Its chemical-reduction claims are product-specific. Maintain it as instructed and follow local advisories; do not assume it makes an unknown chemically contaminated source safe.
A carbon filter can trap microorganisms without killing them. It is not usually rated for germ removal. A product called a water purifier must show its own test results for bacteria, viruses, and parasites. EPA does not independently test those claims.
UV and sun
A portable UV unit can kill bacteria, viruses, Giardia, and Cryptosporidium in small amounts of clear water. It does not work well in cloudy water. Particles can block germs from the light. Follow the unit's instructions for water volume, clarity, dose and exposure time; check its power supply and treatment indicator.
Sun treatment also needs clear water. Fill clean, clear plastic bottles and lay them on their sides. Leave them in the sun for 6 hours on a sunny day, or 2 days when cloudy.
Limits of boiling and disinfection
Boiling and disinfection do not remove heavy metals, salts, or most chemicals. They cannot make water with fuel, toxic chemicals, or radioactive material safe.
If you know or suspect those risks, use bottled water or another source. Contact your health department for advice.
Follow recommendations from your health department for boiling or treating your water.
Plan a stored drinking-water reserve alongside treatment options. Match any treatment method to the actual contamination problem.
Calculate your household reserve Photo: charlesdeluvio / UnsplashStore safe water before you need it
Reduce the need for emergency treatment by calculating your household water reserve and storing water in clean, food-grade containers. Add that supply to your family emergency checklist.
Sources
Guidance and source links checked September 7, 2026.
- EPA, "Emergency Disinfection of Drinking Water" (last updated February 24, 2026). epa.gov
- CDC, "How to Make Water Safe in an Emergency" (page dated September 19, 2024). cdc.gov
- CDC, "About Water Treatment Options When Hiking, Camping, or Traveling" (page dated January 30, 2025). cdc.gov
- CDC Yellow Book 2026, "Water Disinfection for Travelers" (page dated April 23, 2025). cdc.gov
- CDC, "About Choosing Home Water Filters" (page dated April 10, 2024). cdc.gov
- CDC, "Drinking Water Advisories: An Overview" (page dated March 4, 2024). cdc.gov
- CDC, "Boil Water Advisory" PSA toolkit (page dated February 7, 2024). cdc.gov
- CDC, "How to Create an Emergency Water Supply" (page dated June 27, 2025). cdc.gov