Boiling water is a long-standing, effective method for water purification, primarily because the intense heat successfully neutralizes biological pathogens such as bacteria, viruses, and protozoa that cause waterborne illnesses. Reaching a rolling boil for a specified time, usually one to three minutes depending on altitude, renders the water microbiologically safe to drink. However, the safety of boiled water depends entirely on the initial quality of the source water, as heat alone cannot solve every contamination problem. The physical and chemical effects of boiling can alter the remaining water in ways that concentrate non-biological health hazards. Understanding these changes to the water’s composition is necessary before relying on boiling as the sole purification method.
The Physical Changes: Concentration of Dissolved Solids
The most immediate physical effect of boiling water is the loss of water molecules through evaporation, which fundamentally changes the ratio of water to dissolved substances. This process causes non-volatile compounds, which have boiling points far higher than water, to become increasingly concentrated in the reduced volume of remaining liquid. These non-volatile substances are collectively known as Total Dissolved Solids (TDS).
TDS commonly includes naturally occurring minerals and salts like calcium, magnesium, sodium, and chloride. As the water turns to steam, the quantity of these minerals remains constant while the water volume decreases, resulting in a higher concentration. For source water that is already considered “hard,” this concentration effect is noticeable as scale or white residue left on the boiling vessel.
While the minerals themselves are generally safe, their increased concentration can pose issues for individuals with certain health conditions. For example, excessive intake of concentrated calcium salts can be a concern for people predisposed to kidney stones. The concentration of these solids also affects the taste, often giving the water a flat or slightly metallic flavor due to the removal of dissolved oxygen and the increased mineral content. Boiling is ineffective for improving the quality of water with high TDS levels.
Chemical Contaminants That Remain or React
Boiling is highly effective against biological threats, yet it is powerless against most chemical contaminants, and in some cases, it can exacerbate their danger. Heavy metals, such as lead and arsenic, are non-volatile and cannot be removed by heating. When water evaporates, these heavy metals are left behind, increasing their concentration in the drinking water. This is particularly concerning since there is no safe exposure level for lead.
The presence of nitrates in source water, often from agricultural runoff, presents another issue with boiling. Nitrates themselves are not always highly toxic, but the heat can increase their concentration and facilitate their conversion into more harmful nitrites. In the human body, nitrites interfere with the blood’s ability to carry oxygen. This condition, known as methemoglobinemia or “blue baby syndrome,” is a serious risk for infants.
A chemical reaction occurs when municipal tap water treated with chlorine is heated. The chlorine can react with natural organic matter to form disinfection byproducts (DBPs), including trihalomethanes (THMs). Research indicates that while some THMs may form, their overall concentration often decreases significantly during boiling in an open container. This is because THMs are highly volatile and rapidly escape the water and evaporate into the air as the water boils. Removal rates often exceed 90% after three to five minutes of boiling, meaning boiling is not a simple solution for chemically contaminated water.
Why Re-Boiling Water Requires Caution
The act of repeatedly boiling the same water supply, such as in a kettle, dramatically compounds the physical effects of concentration. Each successive boiling cycle causes a further reduction in water volume through evaporation, leaving behind the non-volatile substances from all previous cycles. This cumulative process leads to an exponential increase in the concentration of minerals, salts, and chemical contaminants like heavy metals and nitrates.
This cumulative concentration is the primary reason for caution regarding re-boiling, as it can push the levels of certain substances into potentially unsafe ranges. For example, if the water contains trace amounts of arsenic, re-boiling it multiple times can significantly increase the total amount consumed. Although the effect of a single re-boil on otherwise clean water is negligible, continued repetition should be avoided, particularly for water used in infant formula or by individuals with compromised health.
The clearest indicator of this concentration effect is the increased scale buildup inside kettles and pots, which is predominantly made up of calcium and magnesium carbonate deposits. Furthermore, repeated boiling drives out nearly all dissolved gases, which results in a noticeably flat or stale taste. To mitigate this risk, it is best practice to use fresh water for each boiling cycle, especially when the source water is not known to be chemically pure.