If you have perfectly pure distilled deionized water, nothing will happen if you reboil it. However, ordinary water contains dissolved gases and minerals.
The chemistry of the water changes when you boil it, because this drives off the volatile compounds and dissolved gases. There are many cases where this is desirable. However, if you boil the water too long or reboil it, you risk concentrating certain undesirable chemicals that may be in your water. Examples of chemicals that become more concentrated include nitrates, arsenic and fluoride. It’s better to have a hot water dispenser installed in your home as this takes away the tempting option of reboiling old water.
As a chef, it’s part of our training and practices to not allow foods or beverages to go below certain temperatures due to the increased risk of bacteria growth. This is the same for water enclosed in a kettle, which is sometimes a dark, moist area and an ideal breeding containment for harmful bacteria.
Does Reboiled Water Cause Cancer?
There is a concern that reboiled water may lead a person to develop cancer. This concern is not unfounded. While the boiled water is fine, increasing the concentration of toxic substances may put you are risk for certain illnesses, including cancer.
For example, excessive intake of nitrates has been linked to methemoglobinemia and certain types of cancer. Arsenic exposure may produce symptoms of arsenic toxicity, plus it has been associated with some forms of cancer. Even “healthy” minerals may become concentrated to dangerous levels. For example, excessive intake of calcium salt, commonly found in drinking water and mineral water, can lead to kidney stones, hardening of the arteries, arthritis and gallstones.
The Bottom Line
Generally, boiling water, allowing it to cool and then reboiling it does not present much of a health risk. For example, if you keep water in a tea kettle, boil it and add water when the level gets low, you aren’t likely to endanger your health. It’s best if you don’t let water boil down, which concentrates minerals and contaminants and if you reboil water, it’s better to do it once or twice, rather than make it your standard practice. Pregnant women and persons at risk for certain illnesses may wish to avoid reboiling water rather than risk concentrating hazardous chemicals in the water.
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