When lead is ingested, it enters the blood stream and travels throughout the body.  Over the next 30 days or so, the lead is deposited in the bone and soft tissues, including the brain. 

The lead stays in the bones for a lifetime, resurfacing again in the blood whenever the body's natural processes tap the bones for calcium (which lead resembles).

Lead in the body is most easily measured in the blood (although blood lead tests measure only current exposure; adults who were lead poisoned as children cannot be identified through blood tests). 

Blood lead is measured in micrograms per deciliter, expressed as mg/dL.

The Centers for Disease Control have steadily dropped the level that they identify as "of concern" as researchers have learned more about the insidious effects of lead.  They are currently considering lowering it yet again.