Making Water Safe to Drink: Boil, Disinfect or Filter

By GoldBlock WorkshopPublished Updated

The short answer

Bottled water is the first choice; if you have none, the CDC says “Boiling is the best way to kill germs in water”: bring clear water to a rolling boil for 1 minute. If you cannot boil, disinfect with plain, unscented chlorine bleach using the EPA’s table, and wait 30 minutes. Filter cloudy water first. Boiling and disinfecting cannot make water with fuel, toxic chemicals or radioactive material safe (CDC): find another source.

Key numbers

  • 1 minuterolling boil for clear water (3 minutes above 6,500 feet; EPA says above 5,000 feet)Source: CDC; EPA
  • 8 dropsof 6% unscented bleach per gallon of clear water (6 drops of 8.25% bleach)Source: EPA
  • 30 minuteswait after adding bleach before drinkingSource: EPA; CDC
  • 0.3 micronabsolute pore size or smaller for a filter to remove bacteria; most portable filters do not remove virusesSource: CDC

Which method should I use?

Match the method to the problem: boiling handles germs, chemical disinfectants handle most germs, and filters handle particles and some germs. Chemicals are a separate question with two parts. Water that may hold fuel, a spill, floodwater or anything that smells of chemicals is an unknown that the CDC says cannot be made safe by boiling or disinfecting; find another source. A known, specific contaminant (lead, arsenic, nitrate, PFAS) can be reduced by a product certified for that contaminant, which is a matter of the model and its certification, not of the method in general (see Water Filter Ratings and Certifications). The table summarizes what the CDC and EPA say each method does.

Method Bacteria Viruses Parasites (Giardia, Cryptosporidium) Chemicals, metals, salt
Boiling Yes Yes Yes No (EPA)
Chlorine bleach or chlorine tablets Yes Yes Not reliably; do not kill Cryptosporidium No
Iodine Yes Yes Not reliably; do not kill Cryptosporidium No
Chlorine dioxide tablets Yes Yes Cryptosporidium: the CDC’s emergency page says yes if instructions are followed; its travel page says “somewhat effective” No
Portable filter (0.3 micron absolute or smaller) Yes No Yes (1 micron absolute or smaller, or certified to NSF 53 or 58 for cysts) Not covered by CDC guidance; see the filter’s certification
Reverse osmosis Yes Yes Yes Some chemicals and salt
UV light (clear water only) Yes Yes Yes No
Distillation Yes Yes Yes Many, but not volatile chemicals such as solvents or some pesticides

Sources for the table: CDC: How to Make Water Safe in an Emergency, CDC: Water Treatment When Hiking, Camping or Traveling, CDC: Home Water Treatment Systems, EPA: Emergency Disinfection of Drinking Water.

The safest combination when you have a filter: filter first, then disinfect. The CDC advises: “Filter water before disinfecting it,” because filtering removes parasites that disinfectants may miss, and disinfecting covers the viruses most filters let through.

How do I clear cloudy or muddy water first?

Let it settle and filter it through cloth before treating it. The CDC says “small particles can block germs from the light” (UV and solar methods) and doubles its bleach dose for cloudy water. The CDC says: “If the water is cloudy, first filter it through a clean cloth, paper towel, or coffee filter,” or let it settle and pour off the clear water.

Clearing water makes it look better and helps the next step work; it does not make it safe on its own. The CDC’s list of methods that make water safe (boiling, disinfection, filters with a rated absolute pore size, UV, solar disinfection) does not include homemade sand or charcoal filters.

How do I boil water to make it safe?

Bring clear water to a rolling boil for 1 minute, then let it cool. The CDC: “Boiling is the best way to kill germs in water,” and disinfectants “do not work as well as boiling for killing some germs, including the parasites Cryptosporidium and Giardia.”

  • Altitude. The CDC says to boil for 3 minutes above 6,500 feet; the EPA says 3 minutes above 5,000 feet. Boiling for 3 minutes above 5,000 feet satisfies both.
  • Weakened immune systems. The CDC lists “People with a weakened immune system” among those at higher risk of getting sick from untreated water (CDC hiking and camping page), and says disinfectants “do not work as well as boiling” against Cryptosporidium and Giardia.
  • Cool and store the water in clean, sanitized containers with tight covers (CDC).
  • Taste. The CDC’s tips to “Improve the flat taste of boiled water”: pouring it from one container to another and letting it stand for a few hours, or adding a pinch of salt to each quart or liter.

How do I disinfect water with bleach?

Use plain, unscented household chlorine bleach, follow the EPA’s table, stir, and wait 30 minutes. The EPA says to use “only regular, unscented chlorine bleach products” and not “scented, color safe, or bleaches with added cleaners.” Check the label for the strength: the EPA says it may say 6% or 8.25% sodium hypochlorite, and the CDC says bleach in the United States is typically 5% to 9%.

This is the EPA’s table as published on its page (last updated February 24, 2026). Check the EPA page before you rely on it, in case the guidance has changed.

Volume of water 6% bleach 8.25% bleach
1 quart or liter 2 drops 2 drops
1 gallon 8 drops 6 drops
2 gallons 16 drops (1/4 teaspoon) 12 drops (1/8 teaspoon)
4 gallons 1/3 teaspoon 1/4 teaspoon
8 gallons 2/3 teaspoon 1/2 teaspoon

Then, in the EPA’s words: “Double the amount of bleach if the water is cloudy, colored, or very cold.” “Stir and let stand for 30 minutes.” “The water should have a slight chlorine odor.” If it does not, repeat the dose and let it stand another 15 minutes. The CDC’s own table uses 8 drops per gallon for any bleach between 5% and 9%, and gives a separate row for 1% bleach; both agencies’ doses are on their linked pages.

What about iodine, chlorine tablets and chlorine dioxide?

Tablets and drops work, but each has limits; follow the product label exactly, because strengths differ (EPA).

  • Iodine. The EPA: “Add five drops of 2% tincture of iodine to each quart or liter of water that you are disinfecting,” “If the water is cloudy or colored, add 10 drops of iodine,” and “Stir and let the water stand for at least 30 minutes before use.” The CDC warns that people who are pregnant, have thyroid problems or are sensitive to iodine should avoid iodine-treated water, and that “no one should drink water disinfected with iodine for more than a few weeks at a time.” Iodine does not kill Cryptosporidium (CDC).
  • Chlorine tablets kill most germs but not Cryptosporidium (CDC).
  • Chlorine dioxide tablets. The CDC’s emergency page says they “will kill Cryptosporidium as long as you follow the instructions correctly”; its travel page says they are only “somewhat effective” against it. Follow the product label exactly (EPA).

Comparing specific products is the job of Emergency Water Filters Compared.

Do water filters make water safe?

Some filters remove bacteria and parasites, but most portable filters do not remove viruses. The CDC’s thresholds:

  • Parasites: an absolute pore size of 1 micron or smaller, or a filter certified to NSF standards 53 or 58 for cyst reduction or removal.
  • Bacteria: an absolute pore size of 0.3 micron or smaller.
  • Viruses: in the CDC’s words, a filter with an absolute pore size of 0.3 micron or smaller “will remove bacteria and parasites, but not viruses,” so pair a portable filter with a disinfectant. Reverse osmosis filters remove parasites, bacteria and viruses (CDC).

A filter labeled a “purifier” is claiming to remove viruses too. The CDC notes that such claims rest on the maker’s own testing, which the EPA does not independently verify. How to tell a real certification from marketing is covered in Water Filter Ratings and Certifications.

Does UV light or sunlight work?

Yes, for small amounts of clear water. The CDC says portable UV units “can kill germs in small amounts of clear water,” but “UV light does not work well in cloudy water because small particles can block germs from the light.”

Solar disinfection is also on the CDC’s emergency page: “Fill clean, clear plastic bottles with clear water” and “Lay the bottles down on their side and in the sun for 6 hours (if sunny) or 2 days (if cloudy).” The CDC says the sun’s rays reduce the number of germs, and that it “does not work well in cloudy water.”

Can I distill water?

Yes. The CDC: “Distillation systems boil water and then collect the water as it cools,” which “leaves behind many of the germs and chemicals that were in the water”; distillation “generally uses more energy and takes longer than other water treatment systems.” The CDC lists chemicals distillation removes, including arsenic, lead, nitrate and sodium (salt), and warns it “does not remove” volatile organic compounds, volatile solvents and certain pesticides. Volatile solvents are on the CDC’s does-not-remove side of that list, and the CDC’s separate rule for water containing fuel is that it cannot be made safe by boiling or disinfecting it.

The CDC lists sodium (salt) among the chemicals both distillation and reverse osmosis remove.

What can’t be made safe at home?

Water that may contain fuel, toxic chemicals or radioactive material cannot be made safe by boiling or disinfecting (CDC), and those methods do not remove heavy metals, salts or most other chemicals (EPA). In those cases:

  • Use bottled or stored water, or water from a source you know is safe.
  • Contact your local health department or water utility for advice specific to your area (CDC).
  • For a private well, the owner is responsible for its safety (CDC’s well water safety guidance); the CDC says to test the water “if you ever suspect a problem” and not to drink from a well that “is or may be contaminated with harmful germs or chemicals.”

Common misunderstandings about making water safe

Here is what the CDC and EPA say.

  • “Clear water is safe water.” The CDC’s boiling and disinfection instructions begin with clear water; clearing cloudy water is the step before treatment, not the treatment (CDC).
  • “Boiling removes everything.” Boiling kills germs, but it does not remove heavy metals, salts or most chemicals (EPA).
  • “Bleach kills every germ.” Chlorine tablets do not kill Cryptosporidium, and bleach does not work well on parasites (CDC).
  • “A filter is enough.” Most portable filters do not remove viruses; pair a filter with a disinfectant (CDC).
  • “More bleach is safer.” Use the EPA’s amounts, doubled only for cloudy, colored or very cold water; the slight chlorine smell is the check (EPA).

Common questions

How long do you boil water to make it safe?

A rolling boil for 1 minute. Above 6,500 feet the CDC says 3 minutes, and the EPA says 3 minutes above 5,000 feet.

How much bleach do I add to a gallon of water?

EPA: 8 drops of 6% unscented bleach, or 6 drops of 8.25% bleach, per gallon of clear water. Double it for cloudy, colored or very cold water, stir, and wait 30 minutes. Check the EPA page before relying on it.

Can I use scented or color-safe bleach?

No. The EPA says to use only regular, unscented chlorine bleach, not scented, color-safe or bleach with added cleaners.

Does boiling remove lead or chemicals?

No. The EPA says boiling or disinfection will not destroy heavy metals, salts or most other chemicals.

Is it safe to drink from a creek if I use a filter?

A cyst-rated filter (1 micron absolute, or certified to NSF 53 or 58) removes parasites but not all bacteria; one of 0.3 micron or smaller removes bacteria and parasites. Neither removes viruses, so the CDC advises filtering first, then disinfecting. Boiling handles all three.

Where to go next

Sources

  • CDC: How to Make Water Safe in an Emergency (U.S. Centers for Disease Control and Prevention, last reviewed September 19, 2024; read October 19, 2026). Rolling boil 1 minute, 3 minutes above 6,500 feet; filter cloudy water through cloth first; taste tips; bleach table for 5% to 9% and 1% bleach and a 30-minute wait; chlorine tablets do not kill Cryptosporidium; chlorine dioxide tablets kill it when instructions are followed; iodine cautions; filter pore sizes; portable UV; SODIS; fuel, toxic chemicals and radioactive materials cannot be made safe.
  • EPA: Emergency Disinfection of Drinking Water (U.S. Environmental Protection Agency, last updated February 24, 2026; read October 19, 2026). Regular unscented bleach only; the 6% and 8.25% drop table; double for cloudy, colored or very cold water; 30 minutes and a slight chlorine odor; iodine dose; tablets per label; 3 minutes boiling above 5,000 feet; boiling or disinfection will not destroy heavy metals, salts or most chemicals.
  • CDC: About Water Treatment Options When Hiking, Camping, or Traveling (last reviewed January 30, 2025; read October 19, 2026). Filter pore sizes and NSF 53 or 58 cyst certification; most portable filters do not remove viruses; reverse osmosis removes parasites, bacteria and viruses; chlorine and iodine do not work well on parasites; chlorine dioxide somewhat effective on Cryptosporidium; UV in cloudy water; filter before disinfecting.
  • CDC: About Home Water Treatment Systems (last reviewed April 10, 2024; read October 19, 2026). How distillation works, the chemicals it removes and does not remove, energy use, and bacteria on cooling coils.
  • CDC Yellow Book 2026: Water Disinfection for Travelers (Backer and Hill; read October 19, 2026). Makers “must conduct their own testing”; “The EPA does not independently test the validity of these claims.”
  • CDC: Well Water Safety (last reviewed June 4, 2024; read October 19, 2026). Private well owners are responsible for the safety of their water; “test your water and check your well if you ever suspect a problem”; “If your well is or may be contaminated with harmful germs or chemicals, do not drink the water.”
  • CDC: Avoiding Carbon Monoxide Poisoning (last reviewed August 26, 2026; read October 19, 2026). Never use generators, grills or other gasoline-, propane- or charcoal-burning devices inside a home, garage or carport, or near doors, windows or vents.