http://www.semic.sanyo.co.jp/eng/product/lead_free/influence.htm said:
Harm to human health due to lead
Lead is a toxic material. Lead ingested and accumulated in the body is thought to cause a wide range of problems, including central nervous system damage, kidney damage, developmental problems in children, and intellectual (IQ) defects.
Mechanisms by which lead damages the environment and health
When soldered products are discarded in landfills, the lead can be dissolved by rainwater (especially acid rain due to air pollution) and contaminate the ground. In the worst case, this contamination can spread to contaminate aquifers used as water supplies and be ingested through drinking water.