Water Purification Tablets vs Bleach vs Boiling: A Comparison

Quick answer: boiling is the most reliable way to kill germs, and Ready.gov calls it the safest method. Bring clear water to a rolling boil for one minute, or three minutes above 6,500 feet. Unscented household bleach is cheap and effective against most bacteria and viruses when dosed correctly and given 30 minutes. Tablets are light and easy for go-bags, but they vary: chlorine dioxide handles Cryptosporidium better than chlorine or iodine, with longer wait times. None of the three removes chemicals.

When the tap is unsafe or stored water runs out, three treatment methods come up again and again: boiling, household bleach and purification tablets. Each kills germs, but they differ in what they handle best, how long they take, what they need and how long the supplies last.

This comparison sets them side by side, using guidance from the Centers for Disease Control and Prevention, the U.S. Environmental Protection Agency and Ready.gov. It belongs to the water series anchored by our emergency water guide. For a wider list that also covers filters, distillation and solar methods, see our roundup of ways to purify water from any source.

Start With Clear Water

All three methods work best on clear water. Dirt, silt and organic material can shield germs and use up chemical disinfectants before they reach the germs.

EPA guidance on emergency disinfection says to let cloudy water settle, then filter it through a clean cloth, paper towel or coffee filter, and draw off the clear water before treating. A simple setup of two clean buckets and a cloth handles most of this.

Clear water is not the same as safe water. Water that looks clean can still carry germs, so treat it even when it looks fine.

Chemicals are a separate problem. None of the three methods removes fuel, pesticides, industrial chemicals or heavy metals. Water that may be chemically contaminated, including floodwater, should not be used for drinking even after treatment.

Boiling

Boiling uses heat to kill germs. It is the method most utilities name in boil-water notices.

How to do it

CDC and EPA guidance gives the same steps:

1. Start with clear water, filtering or settling it first if it is cloudy.
2. Bring the water to a full rolling boil for one minute.
3. At elevations above 6,500 feet, boil for three minutes.
4. Let the water cool on its own, without adding ice.
5. Store it in clean, sanitized containers with tight lids.

Boiled water can taste flat. Pouring it back and forth between two clean containers adds air and improves the taste, a tip CDC includes in its guidance.

What it kills

Boiling kills bacteria, viruses and protozoa, including Cryptosporidium and Giardia. That broad coverage is why Ready.gov calls boiling the safest method of treating water.

Strengths and drawbacks

• Kills all major groups of germs
• Needs no special chemicals
• Easy to understand and teach
• Requires fuel and a heat source, which may be scarce during an outage
• Takes time to boil and cool
• Hot water and open flames bring burn and fire risks
• Indoor use of grills, camp stoves or generators can cause carbon monoxide poisoning

During a power outage, never run a charcoal grill, camp stove or generator indoors. Our guide to boiling water without electricity covers safe options in more detail.

Household Bleach

Regular unscented liquid chlorine bleach is one of the most practical treatments to keep at home. It is inexpensive, widely sold and effective against most bacteria and viruses when dosed correctly.

Choosing the right bleach

Use only regular unscented bleach. Avoid scented, color-safe and splash-less products, and any bleach with added cleaners. Check the label for the concentration of sodium hypochlorite, which on most household bleach today is about 6 or 8.25 percent.

How much to use

EPA’s emergency disinfection chart gives these amounts for clear water:

• 6 percent bleach: 8 drops per gallon, or about one-eighth of a teaspoon
• 8.25 percent bleach: 6 drops per gallon, or about one-eighth of a teaspoon for 2 gallons

For larger batches, multiply by the number of gallons. A clean medicine dropper makes measuring drops easier.

Contact time

Stir the water and let it stand for 30 minutes. EPA says the treated water should have a slight chlorine smell. If it does not, repeat the dose and wait another 15 minutes. If it still has no chlorine smell after the second dose, discard it and find another source.

What it kills

Chlorine is highly effective against bacteria and viruses. CDC guidance rates it as less effective against Giardia and notes that Cryptosporidium is highly resistant to chlorine. For water that might carry those parasites, such as surface water from streams or ponds, boil it or filter it with a suitable filter before or after chlorinating.

Shelf life

Bleach slowly loses strength in storage, and heat speeds that up. Many households replace the bottle once a year and note the purchase date on the cap. Store it in a cool, dark place, away from children and other chemicals, and never mix it with ammonia or acidic cleaners.

Purification Tablets

Purification tablets are small, light and made for emergency kits and travel. Three main types are sold, and they behave differently.

Chlorine dioxide tablets

Chlorine dioxide is a different chemical from household bleach. CDC guidance rates it as effective against bacteria, viruses and Giardia, and notes that it can also inactivate Cryptosporidium with a long enough contact time. Some products list waits of about 30 minutes for most germs and up to four hours for Cryptosporidium, so read the label for the exact times.

Chlorine tablets

Tablets based on sodium dichloroisocyanurate release chlorine when dissolved. They work much like bleach, with strong action against bacteria and viruses and weaker action against Cryptosporidium.

Iodine tablets

Iodine has long been used for water treatment in the field. CDC guidance says iodine is not recommended for pregnant people, people with thyroid problems or known iodine sensitivity, or for continuous use over long periods. Iodine also leaves a taste many people dislike, and like chlorine, it is not effective against Cryptosporidium.

Strengths and drawbacks of tablets

• Very light and compact for go-bags, cars and travel
• Pre-measured doses reduce guesswork
• Many have long printed shelf lives
• Higher cost per gallon than bleach
• Wait times vary, from minutes to hours depending on the product and the germ
• Effectiveness drops in very cold or cloudy water, so follow the label for those conditions

Choose products registered with the EPA for drinking water treatment where possible, and follow the maker’s directions exactly.

How Temperature and Time Affect Treatment

Chemical disinfectants work more slowly in cold water. Product labels for tablets often list longer wait times for cold water, and some double the dose for cloudy or very cold conditions. Follow those directions exactly rather than shortening the wait.

Boiling is less affected by water temperature, though cold water takes longer to reach a boil and uses more fuel. At high altitude, water boils at a lower temperature, which is why CDC and EPA extend the boiling time to three minutes above 6,500 feet.

Treated water should also be used within a reasonable time. Store it in clean, covered containers, keep it cool and dark, and label it with the date. If treated water sits for a long time or is exposed to dirt, treat it again or replace it.

Side-by-Side Comparison

• Boiling: kills bacteria, viruses and protozoa; needs fuel and a heat source; about one minute of boiling plus cooling time; no chemical taste
• Bleach: kills most bacteria and viruses; weak against Cryptosporidium; 30-minute wait; very low cost; loses strength over time
• Chlorine dioxide tablets: kills bacteria, viruses and Giardia, and Cryptosporidium with a long wait; light; higher cost; follow label times
• Chlorine tablets: similar to bleach; light and pre-measured
• Iodine tablets: works on bacteria and viruses; not for pregnant people or those with thyroid conditions; not effective against Cryptosporidium; taste issues

Combining Methods

No single method covers everything, so many plans combine two. Some practical pairings:

Filter, then chlorinate

Many portable filters remove protozoa and bacteria but not viruses. Adding bleach or chlorine dioxide after filtering covers the viruses. CDC suggests this combination for backcountry and emergency use.

Settle, then boil

For cloudy water, letting it settle and pouring off the clear top layer before boiling saves fuel and improves taste.

Boil, then store in clean containers

Boiled water can pick up germs again if it goes into dirty containers. Sanitize containers first, following the Ready.gov method of one teaspoon of unscented bleach per quart of water, then rinse.

Which Method to Keep for Which Situation

At home during a boil-water notice

Boil as the utility directs. Bleach is a backup if fuel runs low, though the utility’s instructions take priority. Our guide to boil water advisories explains what those notices mean.

During a long outage

Bleach is the most efficient choice for treating larger amounts without fuel. Keep a fresh bottle and a dropper with the water supply.

In a go-bag or car kit

Tablets win on weight and simplicity. Chlorine dioxide tablets are a strong all-round choice.

Water from streams or ponds

Surface water may carry Cryptosporidium. Boil it, or combine a filter rated for protozoa with a chemical disinfectant.

A Treatment Kit to Keep on Hand

A small, dedicated kit means you are not hunting for supplies in the dark. Keep it with your stored water and check it twice a year.

• A fresh bottle of regular unscented bleach, with the purchase date written on the cap
• A clean medicine dropper and a set of measuring spoons used only for water treatment
• A printed copy of the dosing chart from EPA’s emergency disinfection guidance
• Clean cloths or coffee filters for pre-filtering cloudy water
• Two food-grade buckets or jugs, one marked for untreated water and one for treated water
• A pot with a lid for boiling, plus a safe outdoor heat source and fuel
• A pack of chlorine dioxide tablets for the go-bag
• A marker and tape for labeling treated water with the date

Keep the kit away from heat and out of reach of children. Bleach and tablets are chemicals, and the containers should stay sealed and clearly labeled.

Common Mistakes to Avoid

A few errors undo careful planning:

• Using scented or splash-less bleach, which is not meant for drinking water
• Guessing at drops instead of measuring, which can leave water under-treated or unpleasant to drink
• Skipping the wait time, so the chemical has no chance to work
• Treating cloudy water without settling or filtering it first
• Pouring treated water into dirty containers, which can contaminate it again
• Running a camp stove, grill or generator indoors to boil water, which can cause deadly carbon monoxide buildup
• Assuming any method removes chemicals or fuel from floodwater

Taking an extra minute to measure, wait and label keeps the treated supply safe. For more on why untreated water is risky in the first place, see our article on reasons to purify drinking water.

Frequently Asked Questions

?? Which is better, boiling or bleach?

Boiling kills a wider range of germs, including Cryptosporidium, and Ready.gov calls it the safest method. Bleach is easier when fuel is scarce and works well on clear water against bacteria and viruses.

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

EPA guidance calls for 8 drops of 6 percent unscented bleach or 6 drops of 8.25 percent bleach per gallon of clear water. Stir and wait 30 minutes before drinking.

?? How long do I need to boil water to make it safe?

Bring the water to a rolling boil for one minute. At elevations above 6,500 feet, boil for three minutes, according to CDC and EPA guidance.

?? Do purification tablets expire?

Yes. Tablets carry a printed expiration date, and their strength can drop after that date or if the packaging is damaged. Replace them as the maker directs.

?? Can I use iodine tablets if I am pregnant?

CDC guidance does not recommend iodine treatment for pregnant people or people with thyroid problems. Chlorine dioxide tablets, bleach or boiling are alternatives.

?? Does any of these methods remove chemicals from water?

No. Boiling, bleach and tablets kill germs but do not remove chemicals. Water that may be chemically contaminated should not be used for drinking.

Sources: Centers for Disease Control and Prevention guidance on making water safe in an emergency and on drinking water treatment methods; U.S. Environmental Protection Agency, Emergency Disinfection of Drinking Water; Ready.gov (Water). Follow product labels for tablets and bleach, and follow any instructions from your local water utility.