They’re not equivalent. Radiation is likely to mutate DNA, but it’s also likely that the body self-repairs such mutations. Lead, however, has no self-defense mechanism. It mimics chemistry in the synapses between neurons causing havoc with nervous communication. It does a host of other horrible things across the body depending on where it ends up.

There is no safe amount. Clearly it exists in nature, as does arsinic and uranium, so we will be exposed to toxins unavoidably. That doesn’t mean it’s safe or good, especially to be consciously added to your daily coffee.