Soybeans and many other vegetables will pull in trace amounts of lead from the soil. There is probably some amount of lead in most/all soy products (as well as many other products).

California’s lead threshold for prop-65 warning is 0.5 µg/day. From what I gather, this is a very very low threshold.

Private citizens in California can sue and collect damages against companies selling products that should have a prop 65 warning but don’t. This has created a bit of an industry of citizens who go out, buy random products, test them for prop 65 chemicals, and if they find a violation get themselves a reward. From what I remember reading, Asian markets/producers are a very popular target.

This leads to a lot of companies putting labels on their product just to cover their asses. With such a low threshold for labeling and the fact that soybeans can contain lead, it seems to me to be smart business to always put that label on soy products because you might get a batch of soybeans that put your product over the threshold and get yourself sued. My hunch is that there is just as likely to be lead in the prop-65 labeled tofu as the non-labeled stuff and the difference comes down the to producers risk tolerance (or awareness).

The California advisory doesn’t mean the product contains lead. It means that somewhere along the production process, some potentially toxic chemicals were used. This could be a chemical that is destroyed in the baking process, or it could be a chemical that is used in the fabrication of the packaging.

For example, if the packaging contains paper, that fabrication process uses some nasty chemicals, none of which should remain by the time the product gets to you.

The advisories are on literally everything and are basically useless since they don’t give you any information about what the relevant chemicals are, how they were used, and what remains of them.