Nickel is a contact carcinogen and our exposure to it is pervasive. It is insidious because toxic loads are dumped in the sweat and stools, so it does not show up in hair analysis, often even when we are dangerously toxic. It is orders of magnitude more toxic than Thalium, which is a rat poison. Iron and Manganese are among the easiest of metals to remove from the body since they bind up sulfur chemistry least tightly. That does not mean they should be considered safe, and iron or steel pans should always be well-seasoned to prevent rust, the higher (III) oxidation state that can be quite toxic. Seasoning can probably be accomplished most safely with centrifuged coconut oil or clarified butter, since otherwise the polyunsaturated fat is toxic.