Boiling tap water is a common practice used globally, often recommended during public health advisories or for making questionable water sources drinkable. The simple application of heat is a powerful method of disinfection, but it is not a comprehensive water purification tool. While boiling effectively eliminates one major category of contamination, it has virtually no effect on another. The process merely sets the stage for safer consumption by addressing immediate biological threats, but it does not act as a filtration system to remove dissolved solids or chemicals.
Primary Function: Eliminating Waterborne Pathogens
The primary function of boiling tap water is the elimination of waterborne microbiological contaminants. Bringing water to a boil is a form of pasteurization that successfully kills or inactivates disease-causing organisms like bacteria, viruses, and protozoa. This is why public health officials issue “boil water” advisories when biological contamination is suspected.
The high heat damages the structural components of these microorganisms, such as their outer cell walls and essential proteins, rendering them non-infectious. This process effectively neutralizes common threats like E. coli, Salmonella, and heat-resistant protozoan cysts such as Giardia lamblia and Cryptosporidium parvum. The Centers for Disease Control and Prevention (CDC) recommends that water be brought to a full, rolling boil and maintained for at least one minute at sea level to achieve disinfection.
At higher elevations, the boiling point of water is lower, so the recommended boiling time is extended to three minutes to ensure adequate pathogen inactivation. The heat is sufficient to kill the most prevalent waterborne pathogens. The heat treatment is effective against nearly all organisms that cause waterborne illness, providing a reliable measure of safety against acute biological threats.
Contaminants That Remain: Chemical and Inorganic Substances
While high heat is lethal to living pathogens, it is ineffective against most chemical and inorganic substances present in tap water. Contaminants such as heavy metals, nitrates, and synthetic organic chemicals are not removed by boiling. This is a limitation of using boiling as a general purification method, as these non-biological contaminants can pose long-term health risks.
Heavy metals like lead, arsenic, and mercury remain in the water because they have boiling points significantly higher than that of water. Inorganic compounds such as nitrates, which can enter water supplies through agricultural runoff, are heat-stable and are not broken down or evaporated by boiling. Synthetic chemicals, including pesticides, pharmaceuticals, and PFAS, are also resistant to heat and persist in the boiled water.
These substances remain in the liquid because they are non-volatile and cannot easily transition into a gaseous state along with the water vapor. Consequently, boiling merely disinfects the water without addressing the underlying chemical makeup. Therefore, if a water source is contaminated with chemical pollutants, boiling it will not make it safe for consumption.
The Concentration Consequence of Boiling
A significant consequence of boiling tap water is the physical process of evaporation, which can increase the concentration of non-volatile contaminants. As water reaches its boiling point and converts into steam, only the pure water molecules escape into the air. This reduction in the total volume of water leaves behind all the dissolved solids and non-volatile contaminants.
The mass of the remaining contaminants, such as heavy metals and nitrates, stays the same, but they are now dissolved in a smaller volume of liquid. This mathematically results in a higher concentration of these substances in the final product than was present in the original tap water. For instance, studies have shown that the concentration of lead can increase significantly in the remaining water after boiling.
If the tap water already contains elevated levels of a non-volatile toxin like lead or nitrate, boiling the water could potentially make it more dangerous to drink. This concentration effect is a particular concern in situations where a “boil water” advisory is issued concurrently with a known chemical contamination event. The principle highlights that boiling is a disinfection tool, not a purification method, and it may exacerbate the risk from chemical pollution.