[isilofi]

"metal" is too unspecific. You usually want stainless steel, preferrably CrNiMo steel (V4A, 1.4401 or similar).

Aluminium bottles are always coated with plastic in the factory, which flakes off and usually isn't really better than plastic-bottle-plastic, sometimes even worse (containing PTFE, BPA, ...). As soon as the coating is gone, aluminium will dissolve in the water, and aluminium salts are known to be "not good", in addition to the influence on taste. Same for copper, in addition to the hassle to keep copper clean. Steel bottles are better because proper kitchen-quality stainless steel (the one with Molybdenum in it, in addition to Chromium and Nickel) doesn't leech out into common drinks (including salty hot broth, which would stain CrNi steel) and doesn't need inner coating.

[bryanlarsen]

I'd be stronger than that. The soft plastic inside an Aluminum bottle is always worse than a plastic bottle. Even if it's not PTFE or BPA, it's less durable than the hard plastic bottles. And the neurological effects of aluminum are well known.

Given that almost all metal bottles are coated aluminum, the vast majority of "metal water bottle only" parents are doing more harm than good IMO.

[a_imho]

Is there anything against glass containers? I'm debating what to use for medium/long term storage (replacing the contents) and indeed I'm wary of metal coatings.

[isilofi]

Glass is usually harmless, if you don't use lead or uranium glass. But breakage will lead to harmful shards, which is why many locations such as some schools or party places ban glass bottles.

For storage, use glass bottles. The material of the seal is usually not a problem if there is no contact to the liquid and the seal doesn't crumble. When reusing bottles, make sure to only use fresh or at least good-looking caps. And sterilize things properly (taking into account what the seal is suitable for), because mold and bacteria are far more harmful than common metal or plastic contamination.

[mc32]

I guess if they want to go whole hog, they can get cork seals.

[isilofi]

Cork is oak, which is carcinogenic (in different amounts in different parts of the plant). Of course for some things one doesn't usually care about that because drinks such as wine are also stored in carcinogenic oak barrels and contain carcinogenic ethanol among other harmful things, so it doesn't really matter.

Also, corks promote microbiological contamination of various kinds, leading to e.g. the common failings in wine (there is actually a multitude of "cork" faults, not just one): https://en.wikipedia.org/wiki/Wine_fault

I'd prefer any metal or plastic to cork actually. Just because it is "natural" or "traditional" doesn't make it any good.

[mc32]

Good points. I think the lesson is it's about trade-offs and very few things are ideal. Glass is pretty close but it's brittle...

[askvictor]

The glass itself is inert. The seal for the lid often has plastic. Usually not too much of a concern unless you're dealing with acidic foods afaik