Radioactivity in Antiques

Your parent didn't say below a measurable threshold, they used a much weaker "you don't notice." Lead has been shown in countless studies to be harmful, and a lot of its effects are subtle like behavior and learning problems, hyperactivity, lower intelligence, men's fertility, fetal health, cardiovascular problems, kidney function... so you might not notice low grade lead poisoning but it can still be slowly wrecking your body.

AFAIU, the claim in the literature is that nobody has found a dosage without a measurable effect, at least at population scale. Now, that may simply be because nobody has setup a study to test a dosage at the limits of what can be measured. But in any event the claim is based on numerous studies that postdate most quantitative lead level thresholds. Those regulatory thresholds haven't been updated because the consensus opinion now, in light of the believed mechanism of action in neurological tissue and epidemiological data, is that any ingestion can potentially have a lasting, detrimental impact, however slight; and the consensus goal, however aspirational, is to eliminate all lead exposure during developmentally critical years.

Punching somebody in the head might not cause any meaningful, discernible damage, but nobody is suggesting it's benign, and nobody is going to invest any effort to determine the minimum impact force or number of impacts below which damage is indiscernible or nonexistent. You just aim to avoid all punches to the head, period. Any quantitative thresholds, such as number of known concussions, are simply designed to help allocate limited resources for aggressive interventions. The same is true for lead--preexisting federal and state blood lead levels are designed (or rather reconstrued, ex post) to be a trigger for intervention, not a threshold below which we presume the absence of damage. It's an important distinction.

Of course, it's understood that lead is not only pervasive in the natural environment, but in our industrial processes and especially in our built environment. That's why Federal law deliberately declassified lead as hazardous waste--to make it cheaper and easier to remove and dispose of post haste. Contrast that with asbestos or other hazardous, sometimes more benign, substances.

I remember when everybody on HN complained about the EU banning leaded solder. I think most of the complaints missed the point--there is no safe level for lead in the environment considering its effect on neurological development. We can't objectively set a specific level that we can honestly, knowingly live with as a society, so the policy is simply to slowly, methodically remove it as best we can.