Dosage is the poison. Oxygen can become toxic quite easily.
I doubt a single atom of lead will be unsafe. Or two. There is definitely a level at which the intake of lead will not lead to any measurable effects during the lifetime of a human.
However, the question is about how much it matters compared to global intake.
We get lead from many sources, some of them hard to avoid while still living a normal life. "Safe" doses are doses that represent only a small fraction of that unavoidable part. The standards may change as the unavoidable become avoidable.
For radiation, the baseline is clear, that's background level, I don't know the equivalent for lead but it's clear that we can't keep away every single lead atom.
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.
