Unsafe water kills. Diarrheal diseases caused by contaminated water are responsible for roughly 505,000 deaths annually, according to WHO estimates. During outbreaks and emergencies, municipal water treatment can fail suddenly. Flooding overwhelms treatment plants. Power outages shut down pumping stations. Cholera or typhoid bacteria enter the supply through damaged pipes. When that happens, the water coming from your tap may be actively dangerous.
Knowing how to make water safe to drink is a basic survival skill. Four methods work. Each has trade-offs.
Method 1: Boiling
Boiling is the most reliable water purification method available. A rolling boil for 1 minute kills all pathogenic bacteria, viruses, and parasites, including hard-to-kill organisms like Cryptosporidium and Giardia. At elevations above 2,000 meters (6,500 feet), add 1 additional minute of boiling time per 1,000 meters of elevation because water boils at a lower temperature at altitude.
Advantages. Works against every waterborne pathogen. Requires no special equipment. Doesn't change the safety profile of murky or sediment-heavy water (though pre-filtering through a cloth improves taste). Universally recommended by WHO, CDC, and FEMA as the first-choice method when fuel is available.
Limitations. Requires fuel - gas, electricity, or firewood. During extended power outages or disaster scenarios, fuel becomes a limited resource. Boiling also doesn't remove chemical contaminants like heavy metals or pesticides. And it takes time: heating, boiling, and cooling water before you can drink it adds up when you need large volumes.
When to use it. Whenever you have fuel and a heat source. If you're under a boil-water advisory, this should be your default method.
Method 2: Chemical treatment
Two chemical options are widely available: household bleach and water purification tablets.
Household bleach (sodium hypochlorite). Use regular, unscented liquid bleach with 5-9% sodium hypochlorite. Add 8 drops (about 0.5 mL) per gallon of clear water, or 16 drops per gallon of cloudy water. Stir and let stand for 30 minutes. The water should have a slight chlorine smell after treatment. If it doesn't, repeat the dose and wait another 15 minutes.
Iodine tablets. Follow package directions, typically 1-2 tablets per liter with a 30-minute wait time. Iodine leaves a noticeable taste that vitamin C tablets can neutralize after the treatment period.
Advantages. Lightweight, portable, cheap, and shelf-stable. A $4 bottle of bleach can treat hundreds of gallons. Tablets fit in a pocket.
Limitations. Chemical treatment kills bacteria and viruses effectively but does not reliably kill Cryptosporidium, a chlorine-resistant parasite that causes severe diarrhea. If Cryptosporidium is a concern (common during waterborne disease outbreaks involving agricultural runoff), chemical treatment alone is not sufficient. Iodine should not be used by pregnant women or people with thyroid conditions. Both methods require 30 minutes of wait time.
When to use it. As a backup when boiling isn't possible. Good for treating large volumes. Ideal for emergency stockpiles because of long shelf life.
Method 3: UV purification
Handheld UV-C devices (SteriPEN and similar products) use ultraviolet light to destroy the DNA of pathogens, preventing them from reproducing. Treatment time is 60-90 seconds per liter.
Advantages. Fast. Effective against bacteria, viruses, protozoa, and even Cryptosporidium. No chemical taste. Small and portable.
Limitations. UV light cannot penetrate murky water. If the water has visible sediment or cloudiness, you must pre-filter it first or UV treatment will miss organisms hiding behind particles. Devices require batteries or USB charging, making them dependent on a power source. They also treat only 1 liter at a time, which is slow for family-scale needs. If the bulb breaks, you have no backup.
When to use it. Good for travel and personal use when water sources are relatively clear. Less practical for household-scale purification during extended emergencies.
Method 4: Portable filters
Portable water filters use ceramic elements, hollow fiber membranes, or activated carbon to physically remove pathogens from water. Common brands include Sawyer, LifeStraw, Katadyn, and MSR.
Standard filters with 0.2-micron pore size remove bacteria (like E. coli and Salmonella) and protozoa (like Giardia and Cryptosporidium) but do NOT remove viruses, which are far smaller. This is fine for most wilderness water in North America and Europe, where viral contamination is uncommon, but not sufficient during cholera or hepatitis A outbreaks where viruses are the primary threat.
Some newer filters, notably Sawyer's 0.02-micron hollow fiber products, do filter viruses. Check the specifications before relying on a filter during an outbreak.
Advantages. No fuel, no chemicals, no batteries. Many filters process thousands of liters before needing replacement. Some gravity-fed systems can filter water passively.
Limitations. Standard models don't remove viruses. Filters clog over time and need cleaning or replacement. They don't remove chemical contaminants.
When to use it. Best combined with another method. Filter first to remove sediment, bacteria, and parasites, then treat with bleach or UV to catch viruses. This two-stage approach is the most complete field purification method available.
How much water to store
FEMA recommends a minimum of 1 gallon (3.8 liters) per person per day for drinking and basic sanitation. For a family of four, a 2-week supply means 56 gallons. That's a significant volume.
Store water in food-grade containers, not recycled milk jugs (which are difficult to clean thoroughly). Commercially bottled water in sealed containers has an indefinite shelf life if stored away from sunlight and chemicals. Tap water stored in clean containers should be rotated every 6 months, or treated with 8 drops of bleach per gallon for longer storage.
Keep water storage away from gasoline, pesticides, or other chemicals. Plastic containers can absorb fumes from nearby chemicals over time.
When systems fail
During Hurricane Maria in 2017, Puerto Rico lost clean water access for weeks. The resulting waterborne illness surge contributed to an estimated 2,975 excess deaths. During Cyclone Idai in Mozambique in 2019, cholera cases appeared within days of the storm. Every natural disaster that disrupts water infrastructure produces a predictable spike in diarrheal disease.
Having purification supplies on hand before an emergency is the point. You can't buy bleach or a filter when supply chains are disrupted. Include at least two different purification methods in your family outbreak preparedness kit - boiling capability plus either chemical treatment or a filter. Redundancy matters when the first method fails.
For a broader look at what to keep stocked, see our outbreak preparedness supplies guide.