Lead concentration in bone indicates:

The absorbed lead, both inhaled and ingested, is transpoed to the blood stream where a rapid equilibrium is established between plasma and red cells in a propoion of 1:16. From the blood stream, the lead is delivered to all organs, especially bones which may retain around 90% of the body's lead content. Therefore, while the blood lead concentration reflects the recent exposure, bone lead concentration reflects the cumulative exposure. Ref :