Both ^198^Hg and ^197^Au are perfectly stable isotopes and so according to the paper not at all!

However, my layman's understanding is that the breeder is a form of neutron irradiation, and neutron irradiation of ^197^Au yields ^198^Au which is radioactive and produces ~0.411 MeV of radiation and has a half-life of 2.69 days before decaying back to ^198^Hg.

Compare that with ^235^U. In comparison ^198^Au has no decay chain, ~10x less energy than the highest emitters, a much shorter half life and beta- decay instead of alpha decay.

Coverting to something like sieverts gets real tricky, but the beta decay means it's not very penetrative, ~4mm in biological tissue, and the short half-life means it will quickly stabilize itself. Therefore as long as it's not predominantly ^198^Au it's probably fine.