A: The thing about aluminum is that it’s highly reactive with oxygen (and a number of other things too.). But the compounds that result from these reactions are very stable. In the case of aluminum, that oxygen reactivity forms, virtually instantly on contact with air, a rather durable and inert film of aluminum oxide (which is essentially sapphire, though not quite the same structure). The result of that is that your skin is not generally in contact with aluminum metal, but rather with the usually inert oxide layer.

But here’s the catch: much of the aluminum we use is anodized. That process makes the oxide layer much thicker, but the layer it produces is initially rather porous. That is what allows it to be dyed. It is then sealed, often with a salt solution, that changes the structure to a mostly non-porous one, sealing in the dye. So contact with anodized aluminum is not just contact with aluminum oxide. It’s also contact with traces of whatever salts may have been used to seal it, as well as perhaps contact to some degree with whatever dye was used. Even with aluminum that is not specifically anodized and dyed, some surface trapping of contaminants may be possible, again meaning that contact with the stuff is not just contact with aluminum oxide. All of that makes possible the instances of dermatitis in some people in contact with some examples of aluminum. It’s not easy to predict which ones will be a problem. If you do your own anodizing, seal with boiling water only, and use dyes you know are not allergenic, then just maybe you can mostly predict its safe. But it’s still not a total guarantee. It’s not commonly a problem for most people, but perhaps that just makes it harder to track down when it is a problem.