Some MTR certify that "the material is free from Mucury, Cadmium, Radium,low melting element and Alpha source contamination".
What is the bad effect in these element in ferrous and nonferrous material.
What is the alpha source.
These are radioactive elements. They are to be controlled to ensure the part is free of any radioactive material. Remember the 9/11 steel scrap which was contaminated and the deaths that took place. A reference link is being provided for you.
I don't think cadmium poses a radiation hazard but it does pose a toxic hazard, and there are restrictions in many places like the EU that prohibit importing products containing cadmium.