Tap water is generally treated to meet strict safety standards. Boiling is a common method considered for further purification, especially when water quality is uncertain. The safety of regularly drinking boiled tap water depends entirely on the original content of the source water. Boiling is highly effective against biological threats, but it does not address all types of potential contamination. Therefore, whether boiled tap water is safe hinges on the specific context of the water supply.
How Boiling Eliminates Pathogens
Boiling water is a definitive method for achieving microbiological safety by using sustained, high heat to neutralize harmful organisms. The thermal energy from a rolling boil works by denaturing the proteins and disrupting the cellular structures of microorganisms. This process rapidly kills waterborne pathogens, including bacteria, viruses, and parasitic protozoa like Giardia and Cryptosporidium.
A full, rolling boil for a minimum of one minute is the standard recommendation to ensure the water is safe from these microscopic threats. This duration is sufficient because the temperature of boiling water, 212°F (100°C) at sea level, achieves a significant reduction in pathogen viability. However, at elevations above 6,500 feet, water boils at a lower temperature due to reduced atmospheric pressure, requiring the boiling time to be extended to three minutes to guarantee microbiological sterilization. While boiling does not remove the dead organisms, their inactivation prevents them from causing illness.
The Risk of Concentrating Contaminants
The major limitation of boiling water is that the process of evaporation removes only pure water molecules, leaving behind non-volatile substances. This means that any dissolved solids, minerals, or chemical pollutants originally present in the tap water will become more concentrated in the remaining liquid. If the tap water already contains substances near or above safe limits, boiling can make the water actively more dangerous.
Heavy metals, such as lead, arsenic, and mercury, are a primary concern because they have significantly higher boiling points than water and are not neutralized by heat. As the water volume decreases, the concentration of these toxic metals increases, heightening the risk of chronic exposure. Similarly, inorganic chemicals like nitrates are heat-stable and become more concentrated upon boiling. Boiling is also ineffective at removing many pesticides and herbicides, which persist even at high temperatures.
When Boil Water Advisories Are Issued
Boil water advisories are formal public health recommendations issued when there is a known or potential threat of microbiological contamination. These advisories are triggered by specific events that compromise the integrity of the water system, allowing pathogens to enter. Common causes include significant water main breaks, which can lead to a loss of positive water pressure, or flooding that introduces surface runoff and sewage into the distribution lines.
The detection of indicator organisms, such as E. coli, directly confirms contamination and prompts a mandatory advisory. If contamination is suspected but not yet confirmed (e.g., after a power outage at a treatment plant), a precautionary advisory is issued until water sample testing is complete. These notices specifically target the elimination of disease-causing organisms and should not be used as a remedy for chemical contamination, which requires different treatment methods.
Improving the Taste and Storage of Boiled Water
Boiling tap water often results in a flat or unappealing taste because the high heat drives off dissolved gases, particularly oxygen and carbon dioxide, that contribute to its fresh flavor. A simple solution to restore the taste is aeration, which involves pouring the cooled, boiled water back and forth between two clean containers several times. This reintroduces air into the water, which helps to improve the palatability.
For safe storage, the boiled water should be allowed to cool naturally, without adding ice, and then transferred to clean, sanitized containers with tight-fitting covers. Using food-grade plastic or glass containers and keeping them sealed prevents recontamination from airborne microorganisms or contact with unclean surfaces. Storing the water in a cool, dark place ensures its quality is maintained.