Distribution of 210Pb at endosteal surfaces of bone from Canadian Arctic caribou

Alpha particle energy spectra were measured at femoral endosteal surfaces of Canadian Arctic caribou (Rangifer tarandus) to assess the profile of concentration with depth of 210Po supported by 210Pb. Femur samples from five caribou all showed a pronounced superficial concentration of 210Po, in a layer 1.9 - 6.4 microns thick. Within this layer 210Po was concentrated 1.5 - 10 times with respect to diffuse volume-distributed 210Po. This result is consistent with an earlier study of 210Po at human cranial bone surfaces, which showed 210Po to be concentrated about four times in a surface layer <3 microns thick. However, the present results have higher precision than the human bone data due to the much greater concentration of 210Pb and 210Po in caribou bone. The validity of using 210Po as a marker of 210Pb, and the in vivo 210Po/210Pb ratio are discussed. As a result of the measured endosteal superficial concentration of 210Po in caribou, the alpha particle dose was calculated to be enhanced by a factor of 1.06 - 1.96 (mean 1.48) for bone lining cells, and of 1.08 - 2.39 (mean 1.69) for soft tissue above the bone surface, assuming equilibrium between 210Pb and 210Po. It is suggested that an additional longer-lived compartment for bone surface lead could be incorporated into bio-kinetic models for lead.