Finally, there are seven long-lived fission products. They have much longer half-lives in the range 211,000 years to 15.7 million years. Two of them, technetium-99 and iodine-129, are mobile enough in the environment to be potential dangers, are free (Technetium has no known stable isotopes) or mostly free of mixture with stable isotopes of the same element, and have neutron cross sections that are small but adequate to support transmutation. Additionally, Tc can substitute for uranium-238 in supplying Doppler broadening for negative feedback for reactor stability. Most studies of proposed transmutation schemes have assumed Tc, I, and transuranium elements as the targets for transmutation, with other fission products, activation products, and possibly reprocessed uranium remaining as waste. Technetium-99 is also produced as a waste product in nuclear medicine from Technetium-99m, a nuclear isomer that decays to its ground state which has no further use. Due to the decay product of Tc (the result of Tc capturing a neutron) decaying with a relatively short half life to a stable isotope of ruthenium, a precious metal, there might also be some economic incentive to transmutation, if costs can be brought low enough.