Sources of lead in blood. Identification by stable isotopes.

The isotope ratios of lead in the blood of ten subjects resident in Dallas, Texas, were measured from April 1974 to June 1975. During the same period, the ratios in the city's air changed linearly by 6%. One subject gave erratic results, but the remainder yielded ratios that changed systematically--some linearly, others cyclically with turning points occurring in August to October and February to April. Two South African subjects were 6 months out of phase with their U.S.-born wives and the other U.S.-born subjects. Since the South Africans are presumed to have in their skeletons lead that is isotopically distinct from that in the U.S. environment, the cycles are attributed to the mixing of skeletal and dietary lead. The dietary component is inferred to be greatest in August to October, which correlates with the time at which 25-hydroxyvitamin D is reported to have maximum concentration in blood. On the assumption that the isotope ratios of dietary leads remained constant, the proportion of airborne lead in the blood of those subjects whose ratios changed linearly could be calculated, and values between 7 +/- 3% and 41 +/- 3% were obtained.