Alpha radiation from polymetallic nodules and potential health risks from deep sea mining

Polymetallic nodules at the abyssal seafloor are targeted for future mining operations. Nodules efficiently scavenge and retain several dissolved particlereactive uraniumseries radioisotopes, which predominantly emit alpha radiation during decay. Here, we present new data on the activity concentrations of the naturally occurring radioisotopes thorium-230, radium-226, and protactinium231, as well as on the release of radon-222 in and from nodules from the NE Pacific Ocean. Together with abundantly published data from historic studies, it becomes apparent that the radioisotopes in the nodules mostly exceed exempt activity levels. These exempt levels are in place for naturally occurring radioactive materials (NORM) such as uranium ore, where, due to the enhanced radiation exposure, handling these materials is regulated to protect the public and ensure occupational health. In comparison with these exempt levels, activity concentrations in the nodules are up to three orders of magnitude higher. Seen in this light, inappropriate handling and treatment of polymetallic nodules may pose health risks, particularly if large quantities are stored in enclosed spaces such as during mining operations at sea, and if nodule fines are inhaled during processing.