A boil water advisory (BWA) is a public health warning issued when water is known or suspected to be contaminated with harmful microorganisms. Advisories are typically issued due to breaches in the water system, such as a water main break, loss of pressure, or failure at a treatment plant. The primary concern is the potential presence of pathogens like bacteria (E. coli), viruses, and protozoa (Giardia, Cryptosporidium). Standard household filtration is not a substitute for boiling the water during a BWA.
Why Standard Filters Fail Against Boil Order Contaminants
Common household filters, such as those found in refrigerator dispensers or pitcher systems, primarily rely on granulated or carbon block activated carbon (GAC). Activated carbon works through adsorption, which attracts and holds chemical contaminants responsible for poor taste and odor, but it does not physically block microscopic organisms. The pore structure of these filters, often in the range of 0.5 to 10 micrometers, is simply too large to reliably capture all biological threats.
Many bacteria, such as E. coli, are generally within the size range of 0.2 to 10 micrometers, meaning they could potentially pass through looser carbon filters. Protozoan cysts like Giardia and Cryptosporidium are larger, making them easier for many filters to trap, but they are not the only threat. The greatest challenge comes from viruses, which are the smallest pathogens, typically ranging from 20 to 300 nanometers (0.02 to 0.3 micrometers).
Viruses can easily flow through the relatively large pores of a standard filter, delivering contamination directly into a drinking glass. Running microbially unsafe water through a filter can contaminate the cartridge itself. This contamination can then leach into subsequent water, making the filtered water more of a risk than the untreated tap water. The physical removal mechanism of these common filters is not fine enough to guarantee safety against the full spectrum of microorganisms during a boil water situation.
Specialized Filtration and Disinfection Methods
Some advanced home systems do possess the physical capability to remove biological contaminants, but their reliance during a BWA remains cautious. Reverse Osmosis (RO) systems use an extremely fine semi-permeable membrane with pore sizes as small as 0.0001 micrometers. This size exclusion mechanism makes RO highly effective at blocking bacteria, viruses, and protozoa, provided the system is in perfect working order. However, if the membrane is damaged or the system’s storage tank was contaminated before the advisory, the water may still be unsafe.
Ultrafiltration (UF) and Nanofiltration (NF) systems also use membranes with progressively smaller pore sizes, often in the range of 0.001 to 0.05 micrometers, which can effectively remove bacteria and many viruses. For any membrane system, the pressure must be consistent, and the system must be regularly maintained, making them a calculated risk during a public health emergency.
For guaranteed microbial inactivation, some systems incorporate Ultraviolet (UV) disinfection, which uses UV-C light to scramble the DNA of pathogens, preventing them from reproducing. This method is highly effective but requires the water to be clear and the UV bulb to be working at full capacity.
For those without advanced filtration, chemical disinfection using unscented household bleach is an emergency option when boiling is not possible. The general guidance is to add eight drops of 5% to 9% concentration bleach to one gallon of water, mixing it thoroughly, and allowing it to sit for at least 30 minutes before consumption. This method will inactivate most microbes, but it is less reliable than boiling and may not be effective against chemical-resistant protozoa like Cryptosporidium.
Safety Protocol for Using Water During a Boil Order
The most reliable method for treating water during an advisory is to follow the official boiling protocol. Water should be brought to a full rolling boil for at least one full minute to ensure all pathogens are killed. At elevations above 6,500 feet, this boiling time should be extended to three minutes due to the lower boiling point of water.
This treated water should be used for all consumption, including drinking, preparing baby formula, making ice, and preparing food. Untreated tap water should never be used for brushing teeth, washing fruits and vegetables, or mixing with powdered drinks. For dishwashing, use a dishwasher with a sanitizing cycle. Alternatively, hand-wash dishes with a final rinse solution containing one teaspoon of unscented bleach per gallon of water, allowing them to air dry completely.
Once the boil order is officially lifted, the plumbing system needs to be flushed to remove any untreated water still sitting in the pipes. This involves running all cold-water faucets for several minutes and flushing appliances like water heaters, refrigerators, and dishwashers. Any contaminated filters, including refrigerator and pitcher cartridges, should be discarded and replaced with new ones to prevent residual contamination.