I think it's totally rude, ignorant, offensive, & insensitive. But thinking about the possible ways you could react/respond, and thinking around the corner, there's not much you can do right? And stay honorable with any chance of being understood?

Although it might be funny (from my perspective) to write, "Be careful with that stuff, they make it with lead and cobalt for the color. If you bite your nails and ingest it could cause further brain damage!"

But that's terribly passive aggressive, which solves nothing.