47 the significant but low concentrations observed in bone FREQUENCY 20h ° | from fetuses and young children. The validity of the confidence limits depends on the statistical distribution of the data. The total data and the subgroupings given in Table 19 were normally distributed as shown by a cumulative probability test. ILowever, since the concentration increases with age, the distribution could be biased by the sample age distribution, that is, by the number of samples from younger subjects relative to those from older ones. This age effect was removed by testing the distribution of the residuals (the deviations of the data points from the least squares line). A histogram for all 105 samples i Lh -50 -40 ix given in Figure 40. A best-fit gaussian curve for these TT -30 T -20 data is shown by the solid line. The distribution of the deviation from the least-squares line appears to be skewed and @ 2-gaussian-fit reduces the variance (P < 0.10). This skewness is attributed, at least in part, to ] -10 T 0 T 10 I 20 T 30 ] 40 RESIDUALS (ug/g ash) Fic, 40—Deviation of lead concentration from least squares fit for all data. Gaussian fit of the distribution is also given. the additional Jead intake by smokers,“ Theeffect of smoking on lead content of bone would be most ap- parent in subjects over 30 years of age. The histogram for these 41 trabecular “normal” cases is given in Figure 41. The solid lines represent two normal curves. These two curves give a significant variance reduction (P < 0.05) over a single normal curve. The smaller curve, centered on + 11.2 yg/g ash represents 29% of the total area. This value, while lower than the 44% of the smokers in the adult population (above 17 years TH “| | > Oo ig 2 ° SINGLE aaGAUSSIAN rc \ “ LEAD IN BONE (ug/g ash) B0 iedian —-0-- SARCOMA 0 -40 -30 -20 -10 GO 10 20 RESIOUALS (g/g ash) 30 40 50 Fic. 41—Deviations of lead concentrations from least squares fit for “normal” cases over 30 years of age. Best ft for 2 gaussians also shown. 704; 60h ° o ° of age), is comparable to the fraction of those smoking more than 11 cigarettes per day.47) Oo If one assumes the exponential model of mineral metabolism given in the ICRP Report,“'? and that lead 50 intake is constant over the lifetime, the body should reach equilibrium within a period of time equal to several half-lives of lead in the skeleton. Thus, from the previously estimated biological half-life of about 15 years,'®? the content should level off at about 50 4 ; ' f ; et -50 —o— NORMAL ‘eletal * teeth le. As level that d low eatly . The with DOUBLE ; 0 ; \ p=" GAUSSIAN years. This value of the half-life is inconsistent with the data. The slopes of the linear regression curves de- 20 30 40 50 60 AGE OF SUBJECT (years) 70 80 90 It. 39——Concentration of lead in human bone ash versus aa of subjects for “normal” and “sarcoma” cases. Lines are ‘ter Teast squares fit to the data. rived for the concentration versus age data for specimens from people above 30 years of age are very similar to those of the whole group, but with larger variances. However, these slopes are still significantly greater than zero (P < 0.01). The half-life of lead in the body can be estimated if