Mutagens are agent (physical) or chemical substance that can produce mutations.
Mutagen is classified by IARC depending on their risk of cancer from class 1 to class 4. the class 1 are most likely lead to the patient's tumors.
Examples of molecules classed in class 1 by IARC: Benzene (solvent), asbestos found in the old buildings, chlorambucil (chemotherapy), Cadmium (heavy metal), and o-toluidine (reagent for biochemical analysis).