Lead is a soft, gray metal that humans have used for 4,000 years because it is easy to mine and easy to shape into useful objects. Unfortunately, lead is also toxic. Examination of the remains of ancient Romans reveals damaging levels of lead in their bones. In high enough concentrations, lead causes brain seizures and death, but such high exposures are rare. Lower, more insidious exposures are very common.
Children are particularly susceptible to damage from lead.
Lead offers no dietary benefits to humans or animals. Therefore, the American Academy of Pediatrics recommends that the only desirable amount of lead in humans is zero. This is hard to achieve in a society that injects 44,000 tons of lead into the atmosphere each year. Young children regularly put their hands in their mouths and whatever's on their hands ends up in their stomachs. If there's lead in soil anywhere near where children play, outdoor dust and indoor house dust become contaminated with lead and the children end up ingesting lead from their hands.