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I work in a molecular biology laboratory, where we have a special filtration system for producing ultra-pure water for buffers, reagents, etc. Around the lab, I have heard many times what sounds to me like an urban myth - that drinking water from the ultra-pure system is dangerous because it is extremely hypotonic relative to blood. I certainly know that reducing blood tonicity can be dangerous - my grandmother once had to go to the ER after drinking too much tap water (she drinks Gatorade now).

But could the tonicity of the ultra-pure water really be so low that drinking any at all is dangerous? Is it actually significantly more dangerous than just regular tap water?

This is purely hypothetical - I have no reason or desire to drink the ultra-pure water.

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    Just to be accurate: What exactly is that "ultra-pure" water : is it distilled or deionized...? I mean they are different levels of "pure" water. – Jan Jan 16 at 12:27
  • Both. The building has a centralized system that first distills, then deionizes the water (I believe via some RO system?). That water then is delivered to our lab in a separate DI water pipe. We can draw water directly from this line from taps, and we do for certain applications, notably for cleaning glassware. However, that DI line also feeds into our lab's ultra-pure system, which performs two more passes of deionization. It has a resistivity readout on it, which routinely reads 18.1 megaohm-cm. This water is about as pure as it is possible to make water. – Ian Hamilton Jan 16 at 14:09
  • OK, then we can say it's like distilled water. – Jan Jan 16 at 14:16
  • Distilled water can be dangerous if one has limited access to electrolytes or drinks it to excess. A friend ( PhD , Chem Engr) once drank distilled exclusively and did get to a point where he was dizzy / "light headed". This was years ago And i forget details except he did it to get high without the expense of buying liquor. – blacksmith37 Jan 16 at 16:43
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    I don't have time to write a good answer to this right now so I'm going to park this link here for now. who.int/water_sanitation_health/dwq/nutrientschap12.pdf – Carey Gregory Jan 16 at 18:44
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ACUTE effects of drinking distilled water

The idea behind the myth that distilled water is harmful is that its low osmolality ("tonicity") could dangerously decrease the blood osmolality, which is normally: 285-295 mmol/kg. But distilled water has only slightly lower osmolality (0 mmol/kg) than tap water (~3 mmol/kg) (SGSM.ch, Table 2), so if drinking appropriate amounts of tap water does not significantly lower blood osmolality, distilled water also shouldn't.

What happens when you drink distilled water:

  • It is mixed with gastric acid and fluids in the intestine, so at the time of absorption, it's no longer distilled water. Also, at a certain time before and after drinking distilled water, you probably eat something and thus likely get more minerals with food than with most types of water.
  • When absorbed, it tends to lower the blood osmolality, which is sensed by osmoreceptors in the hypothalamus, which signal the pituitary gland to decrease the secretion of the antidiuretic hormone, which stimulates water excretion in the kidneys until the blood osmolality is corrected (Colostate.edu).

You can find "distilled," "demineralized," "reverse osmosis" and "purified" bottled waters in grocery stores on the shelves together with other beverages, so they are not likely "dangerous."

CHRONIC effects of drinking distilled water

The idea behind the hypothesis that long-term intake of distilled water could be harmful is that it could leach minerals from your body. A 1980 report by World Health Organization says:

Salts are leached from the body under the influence of drinking water with a low TDS. (TDS = total dissolved solids)

The 1980 WHO report has been criticized in a 1993 report by Water Quality Association who have found no reliable evidence about harmful effects of water with a low TDS on health:

It has been concluded that the consumption of low TDS water, naturally occurring or received from a treatment process, does not result in harmful effects to the human body.

Another argument against distilled water is that it doesn't contain "healthy" minerals, such as calcium and magnesium. But a list of tap waters from 100 US cities shows that average tap water contains less than 50 mg Ca and less than 10 mg Mg per liter. So, by drinking 2 liters of tap water per day you could expect to get less than 100 mg Ca and less than 20 mg Mg, which contributes only little to the Recommended Dietary Allowance for adults (Ca = 800-1,000 mg/day; Mg = 350 mg/day).

About water intoxication

Any water, distilled or tap, and other beverages that are low in sodium can cause water intoxication (dilutional hyponatremia) if you drink them a lot in a short time, for example, more than 1.5 liters per hour for several hours in a row (Research Gate), because your kidneys may not be able to excrete more than 1 liter of water per hour. Again, distilled water is not significantly worse than tap water in this regard.

In conclusion:

Distilled water is not acutely dangerous and there is no reliable evidence of its eventual chronic harms. Anyway, demineralized/distilled water may not be the optimal beverage because it has a flat taste and does not contribute to calcium and magnesium intake.

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  • So pretty much as I thought, the notion that the ultra-pure water is especially dangerous appears to be lab folklore. Good, another thing to poke fun at my labmates about. Thanks to Jan and Carey for posting the WHO report - real good, well-sourced information in there. – Ian Hamilton Jan 17 at 4:01

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