The main thing we know about non-catastrophic lead in the United States is that the biggest problem is inner-city soil contaminated by decades-old gasoline. Gas went unleaded in the mid-1970s, but all the old lead burned in the past was dumped into the air and then fell back to earth. The tiny lead particles don’t biodegrade. They mix in with the soil, get tracked into houses, and, most of all, end up on the hands and toys of little kids, who have a marked tendency to stick anything and everything into their mouths, leading to the ingestion of lead.
This lead is everywhere, but it’s most heavily concentrated in places that were close to a lot of vehicle traffic during the leaded gasoline days — in other words, the centers of big cities. (Link)