Both sodium selenite and selenate are classified as dangerous and toxic to the environment. They can be carcinogenic and genotoxic, and may contribute to reproductive and developmental problems in animals and humans. Nonetheless, they are the primary forms of the mineral sold on the mass market today.
Most mass-market vitamins actually contain chemicals that the EPA has banned from public drinking water at levels above 50 parts per billion. That's the equivalent of a tablespoon of water in an Olympic-size swimming pool.