Boiling tap water is a common practice used to make water safer for consumption. However, the term “purify” implies removing all harmful substances, while boiling only addresses biological contamination. Boiling is highly effective at eliminating biological threats, making water safe from disease-causing organisms. It does not remove chemical contaminants, heavy metals, or other inorganic compounds, meaning the water is disinfected but not completely purified. This distinction is important for understanding the limitations of boiling as a standalone water treatment method.
Eliminating Pathogens: The Primary Benefit
The advantage of boiling water lies in its ability to destroy nearly all waterborne biological contaminants. High heat works by denaturing the proteins and structural components of microorganisms, effectively killing or inactivating them. This method is considered a form of pasteurization, making the water safe from the common causes of waterborne illness.
Boiling is effective against bacteria such as E. coli, Salmonella, and Vibrio cholerae, which cause diarrheal diseases. Viruses, including Rotavirus and Hepatitis A, are also inactivated by the sustained high temperature. Boiling successfully destroys protozoan cysts, such as Giardia lamblia and Cryptosporidium, which are known for their resistance to chemical disinfectants like chlorine.
Waterborne pathogens are inactivated at temperatures below the boiling point of 212°F (100°C) at sea level. Bringing the water to a full, rolling boil maximizes the safety margin, neutralizing the risk of infection from biological threats. This makes boiling a reliable, low-cost intervention during “boil water” advisories or in emergency situations where microbial contamination is suspected.
Contaminants That Remain: Chemical Concerns
Despite its success against pathogens, boiling water cannot achieve complete purification because it fails to remove non-volatile chemical contaminants. When water is boiled, water molecules vaporize and escape as steam, but substances with much higher boiling points are left behind. This evaporation reduces the volume of water while concentrating dissolved solids and chemical pollutants in the remaining liquid.
Heavy metals, such as lead and arsenic, are a concern because they do not evaporate and become more concentrated in the boiled water. Boiling water contaminated with lead can increase the concentration of this neurotoxin, making the resulting water more dangerous than the tap water was initially. Similarly, inorganic compounds like nitrates, which can harm infants by causing methemoglobinemia (blue baby syndrome), are also concentrated during boiling.
Total Dissolved Solids (TDS), including minerals like calcium and magnesium, also increase in concentration, often leading to a noticeable change in taste and promoting scale buildup. While chlorine, a volatile compound often used for disinfection, may partially dissipate, some of its byproducts, like Trihalomethanes, can persist or become slightly more concentrated. Relying on boiling for water with suspected chemical contamination is ineffective and potentially counterproductive.
Effective Boiling Techniques
To ensure effective disinfection, the water should be brought to a vigorous, rolling boil where large bubbles continuously break the surface. At elevations up to 6,500 feet (2,000 meters), maintaining a rolling boil for one minute is sufficient to inactivate all waterborne pathogens. This duration accounts for the time needed to fully heat the water volume and provide a safety margin.
For locations higher than 6,500 feet, the recommended duration is extended to three minutes. This adjustment is necessary because lower atmospheric pressure at high altitudes causes water to boil at a reduced temperature. Once boiling is complete, the water must be allowed to cool naturally before consumption. To prevent recontamination, the disinfected water should be stored in a clean, covered container.
Situational Use: When Boiling Is and Isn’t Enough
Boiling is the most accessible and reliable response when biological contamination is the primary risk. This includes following a municipal “Boil Water Advisory” issued after a water main break or flooding event. It is also the standard protocol for treating water sourced from rivers, lakes, or streams during camping and emergency survival scenarios, provided the water is first filtered to remove particulate matter.
Boiling is insufficient when the source water contains significant levels of non-biological threats. Water from old plumbing systems, which may leach lead, or from wells high in nitrates should not be treated by boiling alone. In these cases, supplementary purification methods are necessary to safely remove chemical pollutants.
Techniques such as activated carbon filtration can adsorb organic chemicals, pesticides, and some chlorine byproducts. For the removal of non-volatile inorganic contaminants like heavy metals and nitrates, advanced systems such as reverse osmosis or specialized ion-exchange filters are required. Combining boiling with a filtration system offers comprehensive protection, addressing both microbial and chemical threats.