In an emergency, you can make water safe to drink by boiling it, treating it with chemical disinfectants, filtering it through a certified device, or using sunlight. The right method depends on what you have available and how contaminated the water source is. Before any emergency hits, the CDC recommends storing at least one gallon of water per person per day for a minimum of three days.
Start With the Cleanest Source You Can Find
Not all water sources are equal, and the cleaner your starting water, the more effective any treatment method will be. In a home emergency like a boil-water advisory or natural disaster, your best bet is tap water that simply needs disinfection, water from your hot water heater tank (turn off the heating element first and let it cool), or ice from your freezer. Outdoors, prioritize clear, flowing water from streams or springs over stagnant ponds or puddles.
If your only option is murky or silty water, you need to reduce the cloudiness before disinfecting it. Let the water sit in a container for several hours so sediment settles to the bottom, then carefully pour or siphon off the clearer water on top. You can also strain it through layers of clean cloth, a coffee filter, or a cotton bandana. This step matters because particles in the water shield bacteria and parasites from heat, chemicals, and UV light, making every treatment method less effective.
Boiling: The Most Reliable Method
Boiling is the single most dependable way to kill every category of waterborne pathogen: bacteria, viruses, and parasites including the notoriously tough Cryptosporidium. Bring the water to a full rolling boil and keep it there for one minute. That’s it. At boiling temperatures, all common waterborne pathogens are destroyed in under a minute, even at elevations up to 10,000 feet.
If you’re at high altitude, some agencies recommend extending the boil time. The CDC and EPA suggest three minutes above roughly 5,000 to 6,500 feet, because the boiling point drops about 0.5°C for every 500 feet of elevation gain. The World Health Organization, however, considers reaching a rolling boil sufficient at any common terrestrial elevation. Either way, the extra minutes provide a safety margin and cost you very little fuel.
After boiling, let the water cool naturally in a clean, covered container. It will taste flat because dissolved air escapes during heating. You can improve the taste by pouring it back and forth between two clean containers a few times to re-aerate it.
Chemical Disinfection
When you can’t boil water, chemical treatment is the next best option. The two most accessible choices are household bleach and chlorine dioxide tablets.
Household Bleach
Use regular, unscented liquid chlorine bleach with a concentration of 5% to 9% sodium hypochlorite. Add 8 drops (about ⅛ teaspoon) per gallon of clear water, or 16 drops per gallon if the water is cloudy. Stir and let it stand for 30 minutes. You should detect a faint chlorine smell. If you don’t, repeat the dose and wait another 15 minutes. Bleach is effective against bacteria, viruses, and Giardia, but it does not reliably kill Cryptosporidium.
Chlorine Dioxide Tablets
Chlorine dioxide is more potent than regular chlorine and is the only common chemical disinfectant effective against all waterborne pathogens, including Cryptosporidium. The trade-off is time: tablets and liquid drops require several hours of contact time, not just 30 minutes. Follow the instructions on the package precisely, as concentrations vary by brand. These tablets are lightweight and have a long shelf life, making them a smart addition to any emergency kit.
Iodine tablets and tincture of iodine also work against bacteria, viruses, and Giardia, but they are not reliably effective against Cryptosporidium. Iodine also imparts a noticeable taste and should not be used as a long-term solution, especially by pregnant women or people with thyroid conditions.
Portable Filters and Purifiers
Portable water filters designed for emergencies and backcountry use can remove bacteria and parasites, but not all of them handle viruses. The distinction between a “filter” and a “purifier” matters. Devices tested to EPA and NSF emergency standards (specifically protocols P231 and P248) must demonstrate a 99.9999% reduction in bacteria and a 99.99% reduction in viruses. If a product meets these standards, it’s classified as a purifier and handles all three threat categories.
A filter that only uses a physical membrane, like a hollow-fiber or ceramic element, will typically block bacteria and parasites but let viruses pass through because viruses are far smaller. For water sources that may contain human sewage, such as floodwater, you need either a true purifier or a filter combined with chemical treatment. Check the product label for specific claims and the testing standard it was evaluated against.
Pump filters, squeeze filters, and gravity-fed systems all work. In an emergency, a gravity system has the advantage of processing water without any effort once set up, which matters when you’re exhausted or injured. Flow rates vary, but most portable systems produce between one and two liters per minute.
Solar Disinfection (SODIS)
If you have no fuel, no chemicals, and no filter, sunlight itself can disinfect water. Fill a clean, clear plastic bottle (PET plastic, the kind most water and soda bottles are made from) with water that isn’t too cloudy, ideally below 30 NTU, which roughly means you can see through it. Lay the bottle on its side in direct sunlight, on a dark or reflective surface if possible, for at least six hours on a sunny day.
On overcast days, extend the exposure to 48 hours. During continuous heavy rain, SODIS simply doesn’t work. The UV radiation and heat work together to inactivate bacteria, viruses, and parasites. Two-liter bottles are the recommended maximum size because UV light can’t penetrate deeply into larger volumes. This method is slow and weather-dependent, but it requires nothing you can’t scavenge from debris after a disaster.
Improvised Distillation
Distillation, boiling water and collecting the steam as it condenses, removes pathogens, heavy metals, salts, and most chemical contaminants. It’s the only common method that desalinates seawater. You can rig a basic still with a pot, a lid placed upside down, and a collection cup inside the pot. As steam rises and hits the cooler lid, it condenses and drips into the cup.
Outdoor solar stills, where you dig a pit, place a container at the bottom, cover the pit with clear plastic sheeting, and let condensation drip into the container, produce very little water. Engineered solar stills average only 2 to 4 liters per square meter of surface area per day. A small pit still might yield less than a liter. It’s a survival technique of last resort, not a practical daily supply.
Storing Water Before an Emergency
The simplest way to have potable water in an emergency is to store it in advance. One gallon per person per day covers drinking, cooking, and basic hygiene like brushing teeth. A three-day supply is the minimum recommendation, but a two-week supply is far more practical for events like hurricanes or earthquakes where infrastructure recovery takes time. A family of four needs at minimum 12 gallons for three days, or 56 gallons for two weeks.
Commercially bottled water has no expiration date from a safety standpoint. The FDA, which regulates bottled water as a packaged food, has determined there is no limit to its shelf life. Dates printed on bottles are for quality, not safety. Store bottles at room temperature or cooler, out of direct sunlight, and away from chemicals like gasoline, paint thinner, or pesticides, because vapors can permeate plastic over time. If algae or mold develops from heat or sun exposure, discard the water.
If you’re filling your own containers, use food-grade plastic or glass. Sanitize containers with a teaspoon of unscented bleach per quart of water, shake thoroughly, and rinse. Fill them with treated tap water and label with the date. Replace self-stored tap water every six months to maintain freshness.
Combining Methods for Maximum Safety
No single method is perfect in every situation. Boiling kills everything biological but won’t remove chemical pollutants or heavy metals. Filters remove particulates and most organisms but may miss viruses. Chemical treatments take time, and some miss Cryptosporidium. The safest approach, especially with questionable water sources like floodwater or standing pools, is to combine methods: pre-filter or settle the water to remove sediment, then boil or chemically treat it.
If you suspect chemical contamination from agricultural runoff, industrial sites, or algal blooms, filtration through activated carbon can help, but improvised methods won’t reliably remove these threats. In those cases, distillation is your best field option, or finding an alternative water source entirely.