I was under the impression that inhalation played a large role; suggested by elevated crime rates in urban areas where there was an abundance of car fumes but next to zero farming.

I may be wrong about that; I'll have to read more about this topic.

What I remember reading is that kids ingest a finite amount of soil in the course of simply being outdoors. In fact lead forms some compounds that are actually tasty, and I think it was in the time of the ancient Romans that they boiled wine in lead pots to produce lead acetate, an artificial sweetener.

But inhalation would make sense too.