Boiling and Chemical Treatment: The Basics
Boiling is the most reliable water purification method for biological contamination (bacteria, protozoa, viruses) requiring no consumables. The CDC recommends bringing water to a rolling boil for 1 minute at sea level; at altitudes above 6,500 feet (2,000 meters), boil for 3 minutes because lower atmospheric pressure reduces the boiling point to below 94°C, reducing thermal inactivation efficiency. Boiling does not remove chemical contamination, heavy metals, or turbidity. After boiling, allow to cool in a covered container and aerate by pouring between containers to improve taste. Chemical treatment with sodium hypochlorite (unscented household bleach, 6–8.25% concentration): 8 drops per gallon for clear water, 16 drops for turbid water; mix and wait 30 minutes. At 5°C (cold water), wait 60 minutes. This kills bacteria and viruses but does NOT reliably kill Cryptosporidium protozoa — for surface water in the backcountry or during floods, combine bleach treatment with filtration or use chlorine dioxide. Chlorine dioxide tablets (Aquatabs, Potable Aqua CIO2) kill all three major pathogen categories (bacteria, viruses, Cryptosporidium) and are the CDC-preferred choice for comprehensive water treatment in emergency kits: 4-hour contact time kills Cryptosporidium, versus 30 minutes for other pathogens. Iodine tablets are a historical alternative but are not recommended for pregnant women, thyroid conditions, or extended use beyond 3 weeks.