Lead is a heavy metal that is denser compared to other materials. This is why it was used in a lot of water storage facilities in the past, with lead lining the pipes and vats. Further studies, however, show that lead can be incredibly harmful, especially when ingested.
While lead is naturally present in the environment, most of us would only ever become exposed to it through interactions with manmade structures and facilities. It is particularly harmful to children because their brain and nervous system are more vulnerable to the bad effects of lead exposure which can include anemia, low IQ, and delays in growth.