Distilled Doesn't Mean Pure

Here's a comment a reader posted in response to my article on removing fluoride from water:
"I've been taught that distilled water is the purest that one can drink. On the original article you write that this is not a safe assumption. How so?"
Distillation does purify water, but it can't remove all contaminants. Actually, distilled water can be very impure. Consider how distillation works. First, you're basically boiling water and then letting it cool to collect it again. Ideally contaminants with different boiling points will be removed, if you are careful to collect the distilled liquid at exactly the right temperature and pressure. It's not as easy as it sounds. Plus, there are contaminants that won't separate from the water just from vaporization. Sometimes the distilling process actually adds contaminants that weren't originally present, from the glassware or metal components.
Distillation is great at removing metals with a low vapor pressure and ionic solids. Distilled water typically contains less salt and metal than its source water. Organic compounds or metals with a high vapor pressure (e.g., mercury) will be in the vapor over boiling water and will get condensed back into the distilled water.

For distilled drinking water, keep in mind even if the distillation process is scrupulous, impurities come from the container into which the water is placed. Heavy metals are used to stabilize packaging plastics and can leach into the water over time. For that matter, plastic monomers coat a new container and become a part of bottled water.
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