Boiling tap water is a practice deeply ingrained in public health advice, especially when water quality is compromised. This action is highly effective at eliminating biological contamination, but it is completely ineffective against others. Tap water, whether sourced from a municipal system or a private well, contains a range of substances, and boiling selectively addresses only microbial safety. Understanding this limitation is important, as relying solely on boiling may not resolve issues related to chemical or mineral contamination.
How Heat Eliminates Microorganisms
The primary benefit of boiling water is its ability to neutralize waterborne pathogens through the application of moist heat. When water reaches its boiling point, 212°F (100°C) at sea level, this temperature is sufficient to cause structural damage to living organisms. The heat energy irreversibly denatures the proteins and enzymes within microbial cells, causing them to lose their functional shape and coagulate. This process effectively kills the organism and prevents it from causing illness.
This thermal inactivation is highly effective against the three main classes of biological contaminants found in water. Bacteria, such as E. coli and Salmonella, and viruses, including Rotavirus, are rapidly destroyed within seconds of reaching the boiling point. Protozoa and parasites, including resistant organisms like Cryptosporidium and Giardia, are also readily eliminated by the high temperature.
Contaminants That Remain After Boiling
While heat is lethal to living organisms, it has virtually no impact on most chemical and mineral contaminants, which are non-volatile and heat-stable. Heavy metals, including lead, arsenic, and mercury, possess boiling points significantly higher than water and remain fully dissolved even after prolonged boiling. Nitrates, often entering water sources through agricultural runoff, are highly soluble and chemically stable, meaning they are unaffected by the heat.
Other chemical pollutants, such as various pesticides, pharmaceuticals, and industrial compounds like per- and polyfluoroalkyl substances (PFAS), also survive the boiling process because they are heat-resistant. A more concerning effect occurs due to the simple physics of evaporation: as the water converts to steam, the volume of the liquid decreases. Since the non-volatile contaminants do not evaporate, the remaining water contains a higher mass-per-volume concentration of these harmful chemicals. This means that boiling water that already contains high levels of heavy metals or nitrates can inadvertently make the water more dangerous to consume.
Municipal water treatment often uses chlorine or chloramine as a disinfectant. While chlorine is volatile and can be reduced, this requires boiling for an extended period, often 15 to 20 minutes. Less volatile compounds like chloramine are more stable and largely remain in the water. Boiling water is a method of disinfection, not purification, and it fails to remove the wide array of inorganic and chemical pollutants that define overall water quality.
The Correct Procedure for Boiling Water
Following a precise procedure is necessary to ensure the water is effectively disinfected, especially when responding to a “Boil Water Advisory” or emergency. If the source water is cloudy or contains visible sediment, it should first be strained through a clean cloth, paper towel, or coffee filter to remove the larger particulate matter. Removing this debris allows the heat to act directly on the microorganisms and prevents the particulates from shielding pathogens during the boiling process.
The water must be brought to a full, rolling boil, which is when large, continuous bubbles are rapidly breaking on the surface. At elevations near sea level, maintaining this continuous, rolling boil for at least one full minute is sufficient to eliminate biological threats. However, at higher altitudes, the boiling point of water decreases because of the lower atmospheric pressure. For elevations above 5,000 to 6,500 feet, the boiling time must be extended to three minutes to compensate for the lower maximum temperature and ensure full disinfection.
Once the required boiling time has been completed, the water must be allowed to cool naturally before consumption. It is important to store the treated water in a clean, sanitized container with a tight-fitting lid to prevent recontamination from the air or surrounding surfaces. Storing the water properly ensures that the effort to eliminate pathogens is not undone before the water is used for drinking, cooking, or hygiene.