bone lead measurements can assess long - term lead dosimetry because the residence time of lead in bone is long . 
 bone lead measurements thus complement blood and plasma lead measurements , which reflect more short - term exposure . 
 although the noninvasive , in vivo measurement of lead in bone by x - ray fluorescence ( xrf ) has been under development since the 1970s , its use is still largely confined to research institutions . 
 there are three principal methods used that vary both in the how lead x - rays are fluoresced and in which lead x - rays are fluoresced . 
 several groups have reported the independent development of in vivo measurement systems , the majority adopting the 109cd k xrf method because of its advantages : a robust measurement , a lower detection limit ( compared to 57co k xrf ) , and a lower effective ( radiation ) dose ( compared to l xrf ) when calculated according to the most recent guidelines . 
 these advantages , and the subsequent widespread adoption of the 109cd method , are primarily consequences of the physics principles of the technique . 
 this paper presents an explanation of the principles of xrf , a description of the practical measurement systems , a review of the human bone lead studies performed to date ; and a discussion of some issues surrounding future application of the methods.imagesp172-a