Ok. The main difference between DI water vs regular water is that DI takes out all the ions, like Ca, Mg, etc. Distilling water also does, especially when it is distilled several times, like they usually advertise. Some very high performance RO filters also do.
We need those minerals, so it’s nice that we get some in our water. In the US with modern water supplies the amount of these mineral ions is low, so drinking water is not a significant source of them compared with food intake. Therefore losing them in the water isn’t a significant loss of them overall.
Water with these mineral ions is called hard water, and when they are removed its called softened water. People have been intentionally drinking and using softened water for many years safely.
DI and softened water can leach the minerals out of some metal pipes (like copper and lead) in minuscule quantities, which can cause potentially problems. But we aren’t talking about buying water from Whole Foods and then running it through old metal pipes, and we don’t run our RODI water through metal pipes either, so a non-issue here.
Our skin and GI tract is perfectly fine in contact with pure water with no ions. It doesn’t cause damage in normal situations. The amount of soluble and nonsoluble stuff in your saliva, mucus, digestive juices, etc is enough to very quickly make the pure water less hypotonic anyway. You could probably come up with some extreme scenario that would be a problem, like soaking in running pure water for hours or days where the osmotic gradient would cause some problems. Or drinking many gallons all at once like in fraternity hazing, which throws your osmotic balance off and can be very dangerous. But even these extreme scenarios are nearly as dangerous with normal tap water vs DI water anyway.
The issue of DI or pure water being acidic and therefore dangerous is just wrong. Pure water has essentially no buffering capacity with the ions removed. It inherently has a neutral pH of 7.0 (this is the definition of neutral pH). However, the moment CO2 from the air touches and dissolves into the water it makes it acidic, and since there is almost no buffering capacity it swings to an acidic pH quickly. If you were instead to add a tiny amount of a base, it would swing to a basic pH. The point is since it has no buffering capacity, it takes on the pH of whatever it touches or gets dissolved into it, so it isn’t dangerous at all.
The main reason I’m aware of not to drink our hobby-grade DI water is that the resin isn’t made to drinking standards (unlike drinking-water grade softeners and presumably the resin used in the WF machine) so it could potentially leach out small amounts of unwanted chemicals from the resin. This is probably more theoretical than real, but it’s a reason.
The main reason people don’t actually drink commercial DI water is that it tastes funny to most people. We are used to water with mineral ions in it, so pure water tastes flat or a little off. So just preference really.
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