Unfortunately, a lot of the Prop 65 warning labels don't even say what particular substance they're warning you about. And I've even seen such stickers on pieces of solid wood, warning the consumer because wood dust is a _suspected_ carcinogen when inhaled.
To me, these labels are useless and dangerous for the same reason a lot of anti-drug education/propaganda is bad: it doesn't do anything to inform you about the relative level of danger. e.g. regardless of the affects of the drug itself, smoking a joint is _much_ less dangerous than injecting any substance with needles/syringes/etc which are not sterile.