People arrive at this question from two directions: someone with a deionizer for car washing or an aquarium wondering whether the water is drinkable, and someone who has read that very pure water is better to drink. The answers differ, and in both cases the important issue is not the missing minerals that the debate usually focuses on.
The safety gap is organisms, not minerals
Deionization exchanges dissolved ions on resin, removing calcium, magnesium, sodium, chloride, sulfate and nitrate. It does nothing to anything uncharged: bacteria, viruses, particles and many organic molecules pass straight through because there is nothing for the resin to exchange. A very low dissolved solids reading therefore says nothing about whether water is safe. That is the whole of the practical answer, and it is why the source of the water matters far more than its purity figure.
Technical deionizers are not drinking water systems
Resin sits wet, warm and full of surface area, which is a comfortable environment for bacteria, and a unit used intermittently in a garage is the worst case. Laboratory systems manage this with recirculation, ultraviolet and attention. A car washing or window cleaning deionizer receives none of that, and nothing in it was designed to make water safe. Water from such a unit may be excellent for its purpose and entirely unsuitable to drink, regardless of what a meter says about it.
The mineral argument, in proportion
For someone eating normally, food supplies the great majority of calcium and magnesium and water a small share, so drinking demineralized water is not going to cause a deficiency. The USGS describes hardness as dissolved calcium and magnesium, and deionization removes both entirely, which is why the water tastes flat. That flat taste is the thing most people actually notice and dislike, and it is the reason remineralization stages exist on reverse osmosis systems.
The better household option
If the aim is water with very little dissolved content for drinking, reverse osmosis with a remineralization cartridge is the sensible route: it removes a broad range of contaminants including organisms too large to pass the membrane, it is designed as a drinking water system, and the final stage restores enough mineral for the water to taste like water. Deionization is a technical tool that happens to produce low mineral water. Our drinking water filter guide covers the options by format.
Questions people ask about drinking deionized water
Is drinking deionized water dangerous?
Not inherently. The real concern is that it is not disinfected or filtered for organisms, rather than the missing minerals.
Can I drink from a car washing deionizer?
No. Nothing in that system is designed to produce safe drinking water, whatever its dissolved solids reading shows.
Will it cause a mineral deficiency?
Not for someone eating normally, since food supplies the great majority of calcium and magnesium intake.
What should I use instead?
Reverse osmosis with a remineralization cartridge, which is designed as a drinking water system and restores enough mineral for taste.