English: Thallium is a metal which is very similar to lead, but is much more toxic and rare than this. Therefore it is used scarcely and only in amounts as small as possible. One application is for special glass. In nature, its toxicity normally is irrelevant, because it naturally only occurs in very low concentrations. But when it is deployed intensively, like it is done where thallium sulfate is used as a rat poison, it causes big environmental damage.