Lead based paint is very common in home and buildings constructed prior to 1978.  Lead based painted was widely used because it provided superior durability, color, and finish.  Because you will find lead based paint on the inside and outside of the building it is possible for the lead based paint dust to accumulate on window sills, floors, and the soil around the exterior of the building.
Lead based paint in good condition is generally not a concern for most.  On the other hand, lead based paint that is cracking, peeling, flaking or has otherwise separated from it substrate (wood, concrete, brick, ect) is a hazard.  As the paint flakes and peels the chips are ground into fine dust particles which are referred to as lead contaminated dust.

Children are the most susceptible to lead poisoning.  There are several reasons children have a greater risk for lead poisoning, however, it is really as simple as they have smaller bodies so the lead accumulate faster and they play in areas where lead dust accumulates (soil around the home and on the floors).  As the dust accumulates, it contacts the child’s hands and toys which ultimately find there way to the the child’s mouth and lead is ingested.