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Claims that boiling water multiple times comes with health risks have been around in various forms for years. A spate of posts on a variety of alternative health websites in 2015 added to the hysteria, with claims of all sorts of damaging health effects from reboiled water. A post from Healthy Holistic Living is representative of the claims made in many locations: The only scientific principles at play here are the concepts of evaporation and concentration, which more than likely are a component of any chemistry class. Boiling water causes it to evaporate but leave behind heavier impurities that are likely dissolved in the water, increasing their concentration. Aside from portraying middle-school science as some sort of secret health scare, these posts also, as is so often the case, ignore any discussion of scale. The implication that the water itself somehow changes into new toxic chemicals in the absence of any other factor, merely through the process of boiling, is not factual. Discussion of increasing concentrations of arsenic, nitrates, and fluorides, therefore, requires the presence of some amount of them in your water to begin with. Arsenic Arsenic is an undeniably toxic chemical that can, indeed, be found in both public water supplies and in groundwater. Arsenic poisoning is a major health problem in places like Bangladesh, where high levels of naturally occurring arsenic occur due to the local geology. Studying areas like these, the World Health Organization has concluded that chronic health problems may occur when concentrations reach around 50 ug/L: Assuming the water you are boiling contains the maximum allowable level considered by the EPA of 10 ug/L (which is the same as the limit recommended by the WHO), you would need to boil 80% of your water away to reach this concentration. While it is not inconceivable, especially if you are a somewhat scatterbrained individual, to boil away this amount of water from time to time, these numbers apply to chronic exposure, and as such would require all of the water you consume for every use to be boiled down by 80% — even your regular drinking water. This requirement is made even more unlikely by the fact that boiling 80% of water away in a teakettle may not leave enough for even a single cup of coffee or tea. The numbers get even more absurd when scaled up to the amount that would cause acute arsenic toxicity (i.e the amount you would have to consume all at once to see fairly immediate and serious negative health consequences). Scientists have estimated that acute toxicity occurs when an individual consumes 0.6 mg of arsenic per kg of bodyweight per day day. Using the average American female body weight of 166 lbs (75 kg), one would have to boil over 2600 gallons (nearly 10,000 L) of water down an amount that could fit in a regular coffee cup. Nitrates Nitrates, a broad group of naturally occurring chemicals containing a nitrate ion (NO3-), can indeed be found in water (and pretty much everything else one runs into on a daily basis) The concern raised in viral anti water reboiling posts is the conversion of organic nitrates to a group of carcinogens called nitrosamines: While the risks posed by nitrosamines, a broad group of chemicals, are factual, the claim that nitrates (in general) in water will be converted to nitrosamines solely through the action of boiling is false, as their formation in drinking water requires specific precursor molecules and environmental conditions to form. The most important source of these chemicals, of which a compound named NDMA is most common, is wastewater (which generally contains high levels of organic nitrogen) that has been disinfected with chloramine, a process called chloraminiation: This means that the formation of these dangerous chemicals has everything to do with what is in the water and, outside of increasing concentration through water loss, nothing to do with boiling it. A 2011 study investigated the effect of boiling water with two different nitrosamines showed that the amount was either unchanged after 20 minutes of boiling, or reduced: Now that we have cleared up the fact that the only way boiling affects nitrosamine in water is that it removes water (thereby mathematically increasing its concentration), we are returned once more to similar scale issues found in the above discussion of arsenic. Nitrosamines in water are a relatively new concern, and as such there is not much information about their variability from different water sources. A 2009 survey of drinking water in Japan found that: A 2011 study of water collected from treatment plants and taps in Korea found a range of NDMA concentrations ranged between 26.1 and 112.0 ng/L. In terms of the United States, multiple wells in California in 2000 were shut down when concentrations ranging from 32 to 91 ng/L were found in water sourced near treatment plants. The takeaway point here is that there is considerable variability of nitrosamines in drinking water caused by local land use factors and water treatment methods. The effect of that original variability is far more significant than any changes that could arise from increasing the concentration of those chemicals by boiling away a fraction of the amount of water contained in a tea kettle. Fluoride As the Healthy Holistic Living page states, Fluoride is indeed added to water in many municipalities around the world in an effort to reduce cavities — a source of wild conspiracy theories for some and of health claims of varying merits from others: Oddly enough, the post makes two faulty claims of health problems while ignoring the more legitimate concern. The science linking IQ and water fluoridation, based on the study mentioned above, is deeply flawed and relies on research that was inadequate to test such a link, as we discussed in a post in which we rated that connection as false. The laboratory study with mice mentioned above, in addition to the usual problems of scaling animal studies up to human studies, found problems with fertility only at extremely high concentrations, per the study: Currently, the EPA’s maximum allowable limit for fluoride in drinking water is 4 mg/L with a voluntary recommendation of 2 mg/L. When fluoride is added to a public water supply, the target concentration is 0.7 mg/L — both multiple orders of magnitude less what was used in the mouse study, without even taking into account the relative size difference between humans and mice. The most significant and demonstrable health effects from fluoride in drinking water is a condition called dental fluorosis, a staining or pitting of teeth caused by too much fluoride intake. The risk of this condition is greater for children, and could occur at levels found in drinking water, according to the Centers for Disease Control and Prevention: In this case, fluoridated water could pose a risk to children regardless of its being boiled with a resulting minor increase fluoride concentration, making the argument that boiling water poses a fluoridation risk somewhat moot. Even if it weren’t, the issue of scale as it applied to the other chemicals of concern, still needs to be considered here. The most aggressive estimate for the concentration at which acute fluoride toxicity may occur (promoted in an expressly anti-fluoridation journal of questionable scientific merits) is 0.8 mg of fluoride per kg of body weight. While this number is an order of magnitude less than almost every governmental health organization on the planet, we can still run the same experiment we did with arsenic using the questionable figure. Assuming the water you are boiling contains the maximum allowable level of flouride considered by the EPA (4 mg/L) and you have an average American female body weight of 166 lbs (75 kg), you would need to concentrate 15 liters of water into a 12 oz cup of tea of coffee. Bottom Line The risks posed by reboiling water are minimal, due to the scale of concentration needed to bring any dissolved component of water to a relevant or harmful concentration. If the change of concentration caused by boiling off some water a couple of times results in dangerous water, that water was almost certainly dangerous to begin with.
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