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Claim: Boiling the same water twice will make your water dangerous to drink. 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: > It’s something they never taught us in chemistry class, but really > should have. When we boil water, the 

chemistry of it changes, which > is usually a good thing as it boils out volatile compounds and > dissolves [sic] gases. This is why boiling water mostly ensures that > it’s safe to

 drink. >  > If water is left boiled too long or is reboiled, the chemical > compounds change for the worse. By leaving your water to boil down, > you’re actually concentrating 

many harmful chemicals instead of > getting rid of them … The same thing happens when you reboil > water, as the compounds concentrate and increase the risk of > ingesting certain 

chemicals.’ >  > These chemicals could include arsenic, nitrates, and fluoride. Even > the minerals that are healthy for us can become dangerous when > concentrated, such as 

calcium salt which can lead to kidney stones > and gallstones when taken excessively. 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: > In 2010, the Joint FAO/WHO Expert Committee on Food Additives > (JECFA) re-evaluated the effects of arsenic on human health, taking > new data into account. JECFA concluded 

that for certain regions of > the world where concentrations of inorganic arsenic in > drinking-water exceed 50–100 μg/litre, there is some evidence of > adverse effects. 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: > When nitrates are exposed to high temperatures, they convert to > nitrosamines, which are carcinogenic.

>  > Nitrates have been linked to many diseases such as leukemia, > non-Hodkin [sic] lymphoma, and different cancers, including colon, > bladder, ovarian, stomach, pancreatic, 

and esophageal cancer. 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: > Mechanistic studies and results from occurrence studies suggest that > the most important mechanism for nitrosamine formation during > disinfection is the 

chloramination of amine precursors. 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_: > The total amount of NMOR [a nitrosamine] in boiled water did not > change significantly in 5-min samples, but apparently decreased with > boiling time 

in 10- and 20-min samples, corresponding to 18.7 and > 31.8% decreases, respectively, compared to the initial amount. >  > However, the total amount of NDMA [the most important 

nitrosamine] > did not vary with time. This suggests that NMOR is thermally labile > whereas NDMA is thermally stable. An additional study needs to be > conducted on the by-products

 of the thermal breakdown. 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: > NDMA was detected in 15 of 31 raw water samples collected in the

> summer at concentrations up to 2.6 ng/L, and in 9 of 28 raw water > samples collected in winter at concentrations up to 4.3 ng/L. The > NDMA concentrations were higher in raw 

water samples collected from > treatment plants with catchment areas that have high population > densities. 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: > Many studies have been done over the

 controversial chemical fluoride > and its presence in drinking water. The fact is, it’s there and it > could pose a threat to your health. Despite the proof of the > dangerous 

effects of fluoride, the government insists to keep it in > our water. >  > Harvard University took data from over 27 studies that were carried > out for 22 years and linked 

fluoride exposure to neurological and > cognitive function in children. A more recent study completed in > 2013 links fluoride to lower infertility rates [presumably the > author 

meant _higher_ infertility] in male mice. 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: > Our study showed that fluoride reduced the ratio of chemotactic > sperm [sperm capable of following a chemical gradient to the egg] in > mice 

by administrating with 100 and 150 mg [of sodium fluoride per > liter] in drinking water for 8 weeks. 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 some regions of the United States, public water systems and > private wells contain a natural 

fluoride concentration of more than > 2 mg/L, and at this concentration, children 8 years and younger have > a greater chance for developing dental fluorosis, including the > 

moderate and severe forms. These children should have an alternative > source of drinking water that contains fluoride at the recommended > level. 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.