Boiling tap water (sink water) is a traditional method used when concerns about drinking water quality arise. This practice focuses on making municipal water safe by addressing potential biological contamination. While heating water is highly effective for eliminating disease-causing organisms, it is fundamentally ineffective against the chemical pollutants often found in a public water supply. The safety of drinking boiled sink water is therefore a nuanced matter, depending entirely on the specific contaminants present.
What Boiling Successfully Eliminates
Heating water to its boiling point, 212°F (100°C) at sea level, is a reliable form of disinfection, often described as pasteurization. This process targets waterborne pathogens such as bacteria (E. coli, Salmonella) and viruses (Hepatitis A). The intense heat works by damaging structural components and denaturing proteins within these organisms, effectively rendering them inactive.
Boiling also successfully inactivates hardy protozoan cysts, including Cryptosporidium and Giardia, which are resistant to typical chlorine disinfection methods. The standard recommendation is to bring the water to a full rolling boil, ensuring the temperature threshold for pathogen inactivation is met across the entire volume. This process destroys disease-causing microorganisms, making the water biologically safe to consume.
Contaminants Boiling Cannot Remove
While heat sterilizes water of biological life, it has almost no effect on non-volatile inorganic or organic chemical compounds. Heavy metals, such as lead, arsenic, and mercury, possess boiling points significantly higher than water and remain entirely in the liquid after heating. The inability of boiling to remove these elements is a limitation, especially since lead often leaches from aging pipes and is highly toxic.
The boiling process also leaves behind nitrates, which originate from agricultural runoff. Nitrates are a particular concern for infants due to their interference with oxygen transport in the blood, potentially causing “blue baby syndrome.” Other persistent contaminants like fluoride and certain synthetic chemicals (including pesticides or pharmaceuticals) are also heat-stable and are not removed by boiling. These compounds require specialized filtration methods for effective removal, not simple thermal treatment.
Some volatile compounds, like free chlorine, may partially dissipate into the air during boiling. However, related disinfectants, such as chloramines, are more stable and remain largely present. Boiling water contaminated with certain natural substances, such as cyanotoxins produced by harmful algal blooms, is actively discouraged. These toxins are stable at high temperatures, meaning their concentration increases as the water volume reduces, posing an even greater health risk.
The Risk of Increased Concentration
A concerning outcome of boiling chemically contaminated water is the physical process of concentration. When water boils, pure water molecules convert into steam and evaporate. Substances dissolved in the water, specifically non-volatile compounds, minerals, and heavy metals, do not evaporate with the steam. Instead, these substances are left behind in the reduced volume of remaining water.
If the original water supply contained contaminants like lead or arsenic, boiling increases the ratio of the toxic substance to the water. This heightened concentration means each cup of boiled water contains a greater dose of the toxic substance than it did before heating. Consequently, chemically contaminated water can become substantially more hazardous after boiling, potentially increasing exposure to harmful levels of pollutants. Boiling should never be a substitute for chemical filtration.
Safe Practices for Boiling and Storage
When a public health official issues a “Boil Water Advisory,” the goal is to eliminate potential microbial threats, and a specific procedure should be followed. First, if the water is visibly cloudy, it should be filtered through a clean cloth, paper towel, or coffee filter to remove large particulate matter before heating. This initial step ensures the boiling process can act directly on the microorganisms and is not impaired by high turbidity.
The clear water must then be brought to a full, rolling boil for a minimum of one minute at sea level. For individuals living at elevations above 6,500 feet, the boiling time must be extended to at least three minutes. This longer duration compensates for the lower atmospheric pressure, which causes water to boil at a cooler temperature.
After boiling, the water should be allowed to cool naturally before consumption to prevent accidental burns. The purified water should be stored in clean, sanitized containers with tight-fitting lids to prevent recontamination. If the water source is suspected to have chemical contamination, such as elevated levels of lead or nitrates, boiling should be avoided entirely. In these instances, using certified filtration systems, like reverse osmosis or activated carbon, is the appropriate method to remove chemical pollutants.