# How to purify water in an emergency: boil or disinfect Canonical: https://romeoapps.app/en/surv/how-to-purify-water-in-an-emergency/ Author: Roméo Gambino · Published: 2026-09-10 · Updated: 2026-09-10 · App: Surv (https://romeoapps.app/en/surv/) The EPA’s exact steps for boiling water and disinfecting it with unscented bleach, what these methods cannot remove, and how to store treated water. ## Local instructions come first When regular water service is interrupted by a flood, a storm or a broken main, the US Environmental Protection Agency notes that local authorities may recommend using only bottled water, boiled water or disinfected water until service is restored. Those instructions outrank any general guide, including this one, because the local water authority knows what is in the water and when it is safe again. Keep a battery radio or alert subscription so you hear them, and treat the steps below as what to do when you have been told to treat water, or have no safe supply. ## What treatment does, and what it does not The EPA is precise about the limit. Boiling and disinfection kill most disease-causing microorganisms that may be present in the water, but they will not destroy other contaminants, such as heavy metals, salts and most other chemicals. That single sentence decides a lot. Water that may contain fuel, industrial chemicals or floodwater runoff cannot be made safe by boiling it or adding bleach. In that case the right answer is a different source, not a longer boil. ## What treated water is for Treated water is not only for drinking. The EPA lists drinking, cooking, making any prepared drink, washing dishes and brushing teeth as uses that need properly disinfected water. Ready.gov adds washing and preparing food and making ice. It is easy to boil water for the kettle and then rinse vegetables or a toothbrush under the tap out of habit. If the water is suspect, it is suspect for all of these. ## Start with the best source you have Bottled water is the first choice in the EPA guidance. After that, the agency points to water already inside your home: melting ice cubes, or draining the hot water tank or pipes. If you must use river or lake water, it says flowing water is generally better than still, stagnant water, and not to use water with floating material in it, or water that has a dark colour or a questionable odour. Whatever the source, it is treated the same way before use. ## Clear cloudy water first Disinfection does not work as well when water is cloudy or coloured, according to the EPA. If the water is cloudy, let it settle, then filter it through a clean cloth, a paper towel or a coffee filter, and store the settled, filtered water in clean containers with covers before treating it. This step does not make water safe on its own. It removes the particles that would otherwise shelter microorganisms from heat or chlorine. ## Boiling If you do not have bottled water, the EPA says to boil water. Bring it to a rolling boil for at least one minute. At altitudes above 5,000 feet (1,000 metres), boil it for three minutes. Let it cool naturally and store it in clean containers with covers. Boiled water tastes flat; the EPA suggests adding one pinch of salt to each quart or litre, or pouring it from one clean container to another several times. Boiling needs fuel, so plan how you would do it safely during a power cut. ## Which bleach to use When you cannot boil, the EPA describes disinfecting with household bleach. Use only regular, unscented chlorine bleach products that the label says are suitable for disinfection and sanitization. The label may say the active ingredient contains 6 or 8.25 percent sodium hypochlorite. Do not use scented, colour-safe, or bleaches with added cleaners. The EPA also asks for fresh liquid chlorine bleach, or bleach stored at room temperature for less than one year, and a clean dropper from your medicine cabinet or kit. ## How much bleach, exactly The EPA table, for 6 percent and 8.25 percent bleach respectively: 2 drops of either for 1 quart or litre; 8 drops or 6 drops for 1 gallon (about 3.8 litres); 16 drops (1/4 teaspoon) or 12 drops (1/8 teaspoon) for 2 gallons; 1/3 teaspoon or 1/4 teaspoon for 4 gallons; 2/3 teaspoon or 1/2 teaspoon for 8 gallons. Double the amount if the water is cloudy, coloured or very cold. Bleach sold in other countries is often a different strength, and these drop counts only apply to the concentrations stated; if your label differs, follow your local authority. ## Wait, then check the smell Stir and let the water stand for 30 minutes, the EPA says. It should have a slight chlorine odour. If it does not, repeat the dosage and let it stand for another 15 minutes before use. Ready.gov adds the next step: if it still does not smell of chlorine, discard it and find another source of water. If the chlorine taste is too strong, the EPA suggests pouring the water from one clean container to another and letting it stand for a few hours. ## Other disinfectants, and where advice differs The EPA lists alternatives if you have no liquid bleach: five drops of 2 percent tincture of iodine per quart or litre, or ten drops if the water is cloudy or coloured, then at least 30 minutes of standing; or disinfection tablets used exactly as their label says, since strengths vary. It also describes granular calcium hypochlorite, a very powerful oxidant to handle only as its label instructs. Ready.gov is stricter and does not recommend iodine or camping products that do not contain sodium hypochlorite as the only active ingredient. When agencies differ, your local health authority decides. ## Portable filters The EPA page used here does not evaluate portable filters, and neither does this guide. What a given filter removes is stated by its manufacturer, and it varies between products. Read the packaging for which organisms it claims to remove and whether it says anything about chemicals. If its stated performance does not cover what you are worried about, treat the filtered water with one of the EPA methods as well. A filter is a product claim to check, not a guarantee. ## Storing water before and after Ready.gov recommends storing at least one gallon of water per person per day for several days, and keeping commercially bottled water sealed in its original container in a cool, dark place. If you prepare your own containers, it recommends food-grade water storage containers, cleaned with dishwashing soap and sanitised with a diluted bleach solution before filling. Treated water goes into clean containers with covers, as the EPA says, and should be labelled with the date so nobody guesses later. ## Practise before you need it The time to find the dropper, read the concentration on the bleach bottle and check its date is now. Write the EPA dose for your bleach on a piece of paper and tape it to the bottle, and keep a clean cloth or coffee filters with it. Know where your hot water tank drains. None of this takes long, and it replaces a stressful calculation in the dark with a label you only have to read. ## Where Surv fits Surv keeps its reference guides on the iPhone, so steps like these stay readable without a network, and its inventory can track your stored water, your bleach and the dates to replace them. It is not a water test, and it cannot tell you whether a source is contaminated with chemicals. A phone can also be flat when you need it, which is why the dose written on the bottle matters more than any app. ## Questions ### How long should I boil water to make it safe? The EPA says to bring water to a rolling boil for at least one minute, or three minutes at altitudes above 5,000 feet (1,000 metres). Let it cool naturally and store it in clean, covered containers. ### How many drops of bleach per gallon of water? For regular unscented bleach, the EPA gives 8 drops of 6 percent or 6 drops of 8.25 percent bleach per gallon, and 2 drops per quart or litre. Double it for cloudy, coloured or very cold water, then wait 30 minutes. ### Does boiling water remove chemicals? No. The EPA states that boiling and disinfection do not destroy heavy metals, salts or most other chemicals. Water that may be contaminated with fuel or chemicals needs a different source. ### Can I use scented or colour-safe bleach? No. The EPA says to use only regular, unscented chlorine bleach labelled as suitable for disinfection and sanitization, and not scented, colour-safe bleach or bleach with added cleaners. ## Further reading - US EPA: Emergency disinfection of drinking water — https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water - Ready.gov: Water — https://www.ready.gov/water - Ready.gov: Build a kit — https://www.ready.gov/kit