If myelodysplastic syndrome is mainly caused by exposure to certain elements. how long ago would that exposure have been ?

The exposure: Was likely years and maybe even decades ago. Do you know what the "element" was?

MDS Carcinogens: Alkylating chemotherapies can cause MDS 5-7 years from last dose. Radiation can cause MDS 10-40 years after last dose. Although it's debatable whether modern radiation therapy can cause MDS (see my PubMed article: 24577814). Benzene can cause bone marrow aplasia, MDS or AML 4-14 years after exposure. Tobacco (benzene et al) smoking is significantly associated with MDS with onset 10-20 years.