I mean, the much more obvious sources of lead are paint and pipes, which are still present in many homes. The federal government estimates that about 30% of homes still have lead-based paint, and one nonprofit estimates that about 20% of utilities still have lead pipes somewhere in use. One depressing estimate is that roughly 70% of children in Chicago, the US's third largest city, are in households served by lead pipes.

Gun use may lead to lead exposure, sure, but lead itself was such a commonly used substance that guns aren't the only source of lead.

That may be part of it, a larger part is probably environmental in the sense a lot of worse jobs are going to have more exposures. Then water quality, air quality due to poor filtration (equipment or greenery) and the fact that many poor areas are basically built on old dumping grounds that then leaches into everything.