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Archives of Environmental Health: An International Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/vzeh20

X-Ray Fluorescence Measurements of Lead Burden in Subjects with Low-Level Community Lead Exposure Howard Hu M.D., Sc.D., M.P.H.

a b

c

, Fredric L. Milder Ph.D. & Douglas E. Burger Ph.D.

c

a

Channing Laboratory Department of Medicine , Brigham and Women's Hospital Harvard Medical School , Boston, Massachusetts, USA b

Departments of Epidemiology and Environmental Health (Occupational Health Program) , Harvard School of Public Health , Boston, Massachusetts, USA c

ABIOMED, Inc. , Danvers, Massachusetts, USA Published online: 03 Aug 2010.

To cite this article: Howard Hu M.D., Sc.D., M.P.H. , Fredric L. Milder Ph.D. & Douglas E. Burger Ph.D. (1990) X-Ray Fluorescence Measurements of Lead Burden in Subjects with Low-Level Community Lead Exposure, Archives of Environmental Health: An International Journal, 45:6, 335-341, DOI: 10.1080/00039896.1990.10118752 To link to this article: http://dx.doi.org/10.1080/00039896.1990.10118752

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X-Ray Fluorescence Measurements of Lead Burden in

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Subjects with Low-Level Community Lead Exposure

HOWARD HU, M.D., Sc.D., M.P.H. Channing Laboratory Department of Medicine Brigham and Women’s Hospital Harvard Medical School and Departments of Epidemiology and Environmental Health (Occupational Health Program) Harvard School of Public Health Boston, Massachusetts FREDRIC 1. MILDER, Ph.D. DOUGLAS E. BURGER, Ph.D. ABIOMED, Inc. Danvers, Massachusetts

ABSTRACT. A k-x-ray fluorescence (K-XRF) instrument that can measure in vivo bone lead at low levels was used on a population of 34 adults with no known history of excessive lead exposure. A questionnaire that gathered information relevant to occupational and environmental lead exposure was administered prior to the measurement. A 30-min measurement that produced an average estimated uncertainty of 6 mcg lead/g bone mineral was taken at the mid-tibia1 diaphysis for each subject. Eighteen subjects had bone lead levels below the measurement uncertainty. The remainder had bone lead levels ranging up to 21 mcg lead/g bone mineral. Bone lead levels were greater among older subjects. Among young adult subjects, bone lead levels greater than the measurement uncertainty were confined entirely to subjects who had grown up in housing that was estimated to have been built prior to 1955. Such a childhood environment i s at high risk of fostering exposure to biologically absorbable lead through ingestion of lead paint-contaminated dust and lead pipecontaminated water. We conclude that the K-XRF technique has the potential to distinguish between low levels of lead burden in epidemiologic studies.

AS THE LEVEL of lead exposure associated with adverse health outcomes continues to decline with advances in research, increased attention is being directed to finding an integrated measure of chronic lead absorption in population studies.’ X-ray fluorescence (XRF) estimations of bone lead content has emerged as a promising method in this iegard.* Because 90-95% of total body lead burden (70-80% in children) is contained in bone, XRF-measured bone lead level is a reasonable proxy for total body lead NovembedDecember1990 [Vol. 45 (No. 6 ) ]

burden. Skeletal lead also has been found to be a more dynamic compartment than previously assumed. Physiologic states such as pregnan~y,~ lactation,6 and postmenopausal osteoporosis7have been shown in population studies to be associated with higher levels of lead in blood, presumably from skeletal release. Although a number of studies have appeared recently in the literature describing XRF studies of lead-toxic workers and children, only a few have re335

ported data in subjects who only have had exposures to lead that would typically occur in the general population of an industrialized country,8-10and none of these have focused on associating results with environmental histories. Nevertheless, the low level of maternal and umbilical blood leads that has been associated with poor fetal neurobehavioral outcomes 11,12 and the low level of blood leads that has been associated with increases in blood p r e s ~ u r e ’ ~suggest .’~ that relatively low lead burdens may play a major role in lead toxicity. In this paper we describe the use of a sensitive state-of-the-art K-XRF instrument to measure lead burdens among a group of subjects without any known history of lead toxicity or occupational lead exposure. Lead burdens were compared to dernographic variables and responses to a medical/environmental history questionnaire.

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Methods Subjects. Eligible subjects consisted of all employees of a biomedical company in Massachusetts that is engaged in the research, development, and production of medical devices. No work processes or maintenance procedures at the company are known to involve exposure to absorbable lead products. Each employee was asked to volunteer after the nature of the procedure had been fully explained. Consenting individuals reported to a room on the premises where the administration of the questionnaire, interview, and K-XRF procedure were conducted. K-XRF measurements. Technical specifications and validation data for the K-XRF instrument used in the present work have been fully described e 1 s e ~ h e r e . l ~ In summary, the instrument uses a ”%d gamma-ray source (which was at 125 mCi strength at the time of this study) and a high-purity germanium detector in a back-scatter geometry. The source-to-skin distance i s nominally 2.5 cm. Gamma and x-ray signals are shaped and digitized and then acquired by a multichannel analyzer board in a personal computer. At the completion of data acquisition, the spectrum data are automatically stored and analyzed, which provides a near instantaneous measure of bone lead content. The lead fluorescence signal i s normalized to the coherent scatter signal, which comes principally from the calcium in bone mineral (calcium hydroxyapatite). This renders the measurement insensitive to variations in bone shape, size, density, histomorphometry, overlying tissue thickness, and move~ n e n t The . ~ unit of measurement so derived i s lead in mcg lead/g bone mineral. Previous authors have outlined the close correspondence between this unit of measurement and mcg lead& bone ash.9 In addition to the analyzed ppm lead, an estimate of the measurement uncertainty is also provided. This parameter is equivalent to the expected standard deviation of multiple measurements at the same site, and it is inversely related t o precision. It i s 336

obtained by summing one standard deviation in the net signal counts (total signal minus background photon counts), i n quadrature, to the previously measured systematic error (1ppm).15 In practice, the parameter is predominantly dependent on the duration of the measurement and the mass of bone being measured; i n contrast, it i s nearly independent of the actual lead concentration. Control of the measurement is exerted using a desktop computer and specialized software that provide a user-friendly, menu-driven interface. The subject to be measured is seated i n a lead-free plastic chair with Velcro straps that are used to gently restrain the target leg in the dependent, relaxed position (Fig. 1). Positioning of the source/detector assembly is accomplished by adjusting the height of the motorized stage supporting the assembly in relation to the subject’s leg. Validation of the system’s accuracy and estimates of measurement uncertainty have been performed on both plaster-of-Paris lead-doped limb phantoms and cadaver limbs (with soft tissue intact).15Because lead concentrations have been shown to vary within otherwise homogeneous sections of bone,lb phantoms provide a more faithful target for accuracy testing at low levels of lead concentration. Eight lowerlimb phantoms with lead concentrations ranging from 0 to 84 mcg lead/g plaster-of-Paris were constructed using molded plaster-of-Paris (calcium sulfate dihydrate) as tibias and water as soft tissue. Lead doping was accomplished by mixing dry lead acetate with plaster-of-Paris powder by weight measures, then hydrating the plaster-of-Paris mixture, and molding into the shape of a tibia. The plaster-of-Paris tibias were then dried thoroughly to remove excess water and sealed i n polyurethane. The tibias are surrounded by water in a polyetheylene container and held in anatomical position relative to the phantom “skin” surface. The distance from the phantom surface to the nearest tibia point was approximately 4 mm. A total of 57 K-XRF measurements were taken o n these phantoms and are included in Figure 2, plotted as K-XRF measurements versus true lead concentration by weight. Just as the calcium coherent scatter signal of hydroxyapatite provides the normalization for the measurement unit of mcg lead/g bone mineral, the calcium coherent scatter signal of calcium sulfate dihydrate provides the normalization for the unit mcg lead& plaster-of-Paris. Each measurement was taken with a 30-min acquisition time at a source strength of 120 mCi. The correlation of K-XRF measurements to atomic absorption spectroscopy (AAS) was investigated by use of cadaver limbs. A total of eight sites on three different intact cadaver limbs were measured with KXRF (four sites, one limb; three sites, one limb; one site, one limb). The sites included the mid-tibia, proximal tibia diaphysis, calcaneus, and patella. Data acquisition times were from 30 to 150 min at a source strength of 120 mCi. Results ranged from 16120 mcg lead/g bone mineral. O n the contralateral limb of each cadaver, flesh was dissected away and Archives of Environmental Health

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Fig. 1. Sub* sitting in lead-free plastic chair undergoing m i d - t i b i a K - X R F. rneasurement.

two full-thickness 2-cm slices were removed from the each region corresponding to K-XRF-measured sites (lateral symmetry of lead concentration has been shown previouslylh). The slices were dried, ashed, and analyzed for lead content using the flameless A A S method of Wittmers et al.” In Figure 2, each K-XRF limb measurement i s thus plotted against two A A S measurements. Analysis of the combined data, with no arbitrary adjustable parameter between the phantom and limb data sets, revealed a correlation coefficient of 0.98, a linear regression slope of 1.02, and an x intercept of -1 mcg lead& bone mineral (Fig. 2). Measurements for the current work. For the present study, a 30-min measurement was taken at the midshaft of the tibia of each subject after washing the region with a 50% solution of isopropyl alcohol. Subjects used the time during the measurement to complete the environmental/medicaI history questionnaire. At least one of the authors was present at all times during the measurement to answer questions concerning the procedure or the questionnaire. An average of three subjects were measured each day, and the system was calibrated each day before the first use. Based on measurements taken on limb phantoms with thermoluminescent dosimeters placed at the skin surface, at the tibia surface, and in the marrow cavity, the on-axis skin exposure for a 30-min measurement at 125 millicuries of lwCd strength i s 1.6 NovembedDecember 1990 [Vol. 45 (No. 6 ) ]

mGy. The on-axis exposure to the marrow cavity is 0.55 mCy. The exposure falls to near zero at 2.5 cm off-axis. The equivalent dose to the total red marrow organ i s 0.45 microSv. Using worst-case conservative assumptions, the extrapolated total body absorbed energy per measurement is less than 1 milliJoule. In comparison, the typical skin exposure for a single dental bite-wing x-ray is 4 mCy with a red bone marrow absorbed dose of 7-10 microSv.’* Medical environmental history questionnaire and interview. The questionnaire gathered basic demographic data and information on the environment in which subjects spent their childhoods. Subjects were asked to recall the town and state of their childhood (before age 8 y); classify the neighborhood as urban, suburban, or rural; and estimate the approximate age of the housing. If a subject had moved during childhood, information was recorded for the oldest and most urban place of residence in which he or she had spent at least a year in residence. A medical history, supplemented by interview, was used to determine whether a subject had a history consistent with childhood or adult lead toxicity. The interviewer reviewed all hobbies and employment, noting instances where potential exposure to significant levels of lead were likely to have occurred. Hobby and employment histories were each rated by a board-certified occupational physician (HH) as to the likelihood of significant lead exposure, using 337

21 0

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11

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RO 100 c H t M I t ~ i r i i ~ w OR ~ i 1141

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120

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Fig. 2. Lead concentration in lead-doped phantoms (as formlated) and cadaver limbs (as measured by atomic absorption spectroscopy) K-XRF measurements. Sfraighf line represents linear regression slope, and dotted lines represent 95% confidence limits to regression slope.

three categories: (1) none suspected; (2) possible small amounts, e.g., light soldering; and (3) definite significant exposure, e.g., employment in leadrelated industry such as battery manufacturing, radiator repair, paint removal. All questionnaire and interview data were collected prior to the completion of the K-XRF measurement. ~~

Results Thirty-four employees of 55 who were eligible agreed to participate. These employees consisted of 26 (76%) men and 8 (24%) women ranging in age from 21 to 58 y (mean = 3 6 . 7 ~ )In . comparison, nonparticipants were proportionately more female (33%), but were approximately the same age (mean = 34y). None of the participating subjects had medical history suggestive of pediatric or adult clinical lead toxicity. N o subject had a history suggestive of definite significant job or hobby-associated lead exposure. Nine subjects reported a history of light soldering i n industry or at home that was suggestive of possible light lead exposure. Variability of response was noted for questions pertaining to the environment and housing i n which subjects spent their childhood years and for questions regarding smoking (Table 1). One subject grew up in the Soviet Union, of which little i s known about environmental lead contamination, and was excluded from subsequent analyses. (This subject's K-XRF measured tibia lead level was below the measurement of uncertainty.) Eighteen (53%) of the subjects had K-XRF estimated bone lead levels that were sufficiently low to include zero (or less) within the estimate of uncertainty. The highest bone lead level was 21 5 4 mcg lead/g bone mineral. The distribution of bone lead levels by age demonstrates generally higher concen338

trations among older individuals (Fig. 31, although each decade of age included at least one individual with a bone level close to zero. A linear regression of bone lead on age yielded a statistically significant (p = .02) positive slope estimate of 0.31 mcg lead/g bone mineral . year (95% confidence interval 0.19 to 0.441, with a y-axis intercept (extrapolated to age = 0) of -6.71 mcg lead/g bone mineral and xaxis intercept of 21.45 y. The average estimate of uncertainty for the group was 6.13, with a low of 4 and a high of 9 mcg lead/g bone mineral. Females had a larger mean uncertainty (6.88) than males (5.921, a difference that bordered on statistical significance (Student's t test H:O

p

= .07). A cross-tabulation of subjects by categories of bone lead level, sex, and responses to the medical/ environmental history questionnaire i s presented in Table 2. The proportion of subjects with a relatively high lead burden was greater for older subjects. There were no clear trends for bone lead level by sex, hobby or occupation, smoking, or neighborhood. A difference, however, i s suggested i n lead burden distribution among the 16 young adult subjects who were born during or after 1955 (34 y of age or younger) according to whether they spent their subsequent childhoods in newly constructed housing (estimated to have been built during or after 1955) or older housing. None of the 6 subjects who grew u p in newly constructed housing had lead burdens greater than the measurement uncertainty vs. 6 of the 10 subjects who grew up in older housing, 4 of whom had burdens up to 10 mcg lead& bone mineral, and two of whom had burdens greater than 10 mcg lead& bone mineral (Fig. 3). The mean age of

Table 1.-Responses to Selected Questions on the Environmental History Questionnaire

Type of neighborhood lived in as child Urban Suburban Rural Approximate year of construction of the house in which subject spent childhood < 1955 2 1955 Occupational history Not suggestive of lead exposure Possible small amount of lead Definite significant exposure Hobbies Not suggestive of lead exposure Possible small amount of lead Definite significant exposure Current smoker No Yes

16 (47%) 12 (35%) 6 (18%)'

27 (79%)' 7 (21%) 30 (88%)' 4 (12%)

0 (0%) 25 (74%)' 9 (26%) 0 (0%) 24 (71%) 10 (29%)'

'Includes subject who grew up in the Soviet Union (of which little is known about environmental lead contamination).

Archives of Environmental Health

variables for age and year of home construction had positive but statistically insignificant ( p > .05) coefficient estimates (Table 3).

I20

Discussion 10

0

-10

1

I

I 1

-20

20

30 AGE

40 [YEARS)

I

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60

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Fig. 3. Age vs. K-XRF measurements of tibia lead concentration. Bars around each point represent measurement uncertainty, straight line represents linear regression slope, and dotted lines represent 95% confidence limits to regression slope.

young adult subjects who grew up in newly constructed housing was 28.8 y (standard deviation = 4.621, which was not statistically different from the mean age of the young adult subjects who grew up in older housing (29.ly, standard deviation = 4.23). In a multiple linear regression model of the 16 young adult subjects, using bone lead levels as the dependent continuous variable, both the

Table 2.-Lead

The range and age trend of tibia lead levels described in the present in vivo K-XRF work are consistent with those seen in previous studies of skeletal bones from cadavers in the general population using atomic absorption spectroscopy metho ~ s , ~as~well , ~ as~ in J two ~ previous in vivo studies of K-XRF measured bone tibia lead le~els.~,~O In a population of 22 normal subjects, Somervaille et al.9 found that a linear regression of K-XRF-measured bone tibia bone lead on subject age yielded a coefficient of 0.34 (mcg lead/g bone mineral). y with a y intercept of -7.8 mcg lead/g bone mineral (To convert concentrations of lead in tibia bone@bone mineral [or bone ash] to concentrations per g wet bone, a nominal conversion of 0.56 may be used. Values in the literature report a range from 0.53 to 0.58 [g bone mineral]/[g wet bone] for cortical b ~ n e . ~ r ' ~ - ' ~ ) . In a study of 59 subjects from a city with a history of metallurgical industry, Morgan et a1.l0 fit K-XRFmeasured tibia bone lead concentrations to a quadratic function of subject age with no linear term. They found a coefficient of .0081 (mcg lead@ bone mineral)/year2with a y intercept of - 10.78 mcg lead/

Burden According to Sex and Questionnaire Responses

Juestion and response Sex Male Female Age

< 30y 30 -39 y > 39 y Childhood neighborhood Rural Suburban Urban Estimated year of home construction' 2 1955 < 1955 Current smoker No Yes Occupational lead exposures None suspected Possible small amount Lead exposures from hobbies None suspected Possible small amount

Lead < M.U.'

Lead > M.U.' up to 10 ppm

Lead > 10 ppm

13 5

7

5

0

3

6 7 5

1 4 2

1 3 4

4 7 7

1

2 4

1 3 4

6 4

0 4

0 2

11 7

5 2

7 1

14 4

6 1

a

11 7

5 2

7 1

0

*Estimate of measurement uncertainty, i.e., one standard deviation of the counting statistics for that particular measurement (see Fig. 2). The average uncertainty for the whole group was 6 ppm. 'This analysis was restricted to individuals who had been born during or after 1955.

November/December 1990 [Vol. 45 (No. 6 ) ]

339

Table 3.-K-XRF Bone Lead Multiple Regression Results, Including Age and Housing Age, for Subjects Born During 1955 or After

Va r iabIe Age (y) Year of home' const r uct io n

Regression coefficient

t statistic

p

0.39 5.62

1.05 1.75

.31 .I0

'0 = estimated year of construction 2 1955, and 1 estimated year of construction < 1955.

* * * * * * * * * * =

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g bone mineral. If our data were inserted into the same model, we would obtain a coefficient of .OM1 (mcg lead/g bone mineral). y2 with a y intercept of -1.17 mcg lead& bone mineral. The K-XRF procedure used in this study was convenient and well accepted by the study subjects. Scan times were limited to 30 min to minimize disruption of work schedules. The negative values seen in seven-point estimates of bone lead (see Fig. 3) are reflective of the measurement uncertainty of the KXRF procedure at bone lead levels close to zero. Longer scan times and/or a more powerful ' T d source than was used in this study would further decrease the uncertainty of the measurement with a relatively minimal increment in radiation exposure and risk.15The larger uncertainties in the K-XRF measurements of women vs. those of men that were seen in this study probably occurred because women generally have smaller skeletal masses. This population lacked any indication of any ongoing lead exposures. Blood lead levels in individuals with a history of previous elevated lead exposure, but no ongoing lead exposures, have been shown to be poor indicators of accumulated lead burden.21Nevertheless, it would have been helpful in this study to have had blood lead levels to compare with bone lead levels. The differences in distribution of lead burden that was suggested between the two groups of young adult subjects according to estimated age of childhood housing was an interesting finding in this study. Because no subject was aware of his or her lead burden prior to the history taking, recall bias was unlikely. The similar mean ages for the two groups mitigates against confounding by age. Pope has estimated that 70 to 99% of all houses built prior to 1959 have painted surfaces containing significant amounts ( 20.7 mg/cm2)of lead.22Older housing i s also more likely to contain lead plumbing23 Ingestion of lead-contaminated dust and water as a child has been identified in many studies as a major contributor to lead exposure." Conclusion

A sensitive K-XRF technique can potentially distinguish between low levels of lead burden in a popu340

lation study. In individuals without any known significant occupational or adult environmental exposures to lead, age and, among younger subjects, having grown up in housing built prior to 1955 may predict an increased risk for higher bone lead levels. K-XRF may be a useful technique for assessing the toxicologic implications of low-level lead burdens in epidemiologic studies.

This research was supported, in part, by National Institute for Environmental Health Sciences Environmental Training Grant ST 32 ES 07069 and National Institutes of HealthiSmall Business Innovation and Research Grant 2R44 ES03918-02. Thanks go to Dr. Tor Tosteson for statistical advice and to Drs. Scott Weiss and Frank Speizer and the anonymous reviewers for helpful comments on the manuscript. Submitted for publication January 8, 1990; revised; accepted for publication June28, 1990. Requests for reprints should be sent to: Howard Hu, M.D., Channing Laboratory, 180 Longwood Ave., Boston, MA 02115.

* * * * * * * * * * References

1. Landrigan PJ. Toxicity of lead at low dose. Br J Ind Med 1989;593-96. 2. Hu H, Milder FL, Burger DE. X-ray fluorescence: issues surrounding the application of a new tool for measuring burden of lead. Environ Res 1989;49:295-317. 3. Barry PSI, Mossman DB. Lead concentrations in human tissues. Br J Ind Med 1970;27:339-51. 4. Schroeder HA, Tipton, IH. The human body burden of lead. Arch Environ Health 1968;17:965-78. 5. Zaric M, Prpic-majic D, et al. Exposure to lead and reproduction. In: Summary proceedings of a workshop: selected aspects of exposure to heavy metals in the environment. Monitors, indicators, and high risk groups; April 1985. Washington, DC: National Academy of Sciences; Yugoslavia: Council of Academies of Sciences and Arts 1987; pp. 119-26. 6. Manton WI. Total contribution of airborne lead to blood lead. Br J Ind Med 1985;42:168-72. 7. Silbergeld EK, Schwartz I,Mahaffey K. Lead and osteoporosis: mobilization of lead from bone in postmenopausal women. Environ Res 1988;47:79-94. 8 . Wedeen RP, Batuman V, Quinless F, et al. In vivo x-ray fluorescence (XRF) for assessing body lead stores. In: Ellis KJ, Yasumura S, Morgan W, (Eds). I n vivo body composition studies. London: The institute of Physical Sciences in Medicine, 1987; pp. 357-62. 9. Somervaille LJ, Chettle Dr, Scott MC. In vivo measurement of lead in bone using x-ray fluorescence. Phys Med Biol 1985; 30: 929-43. 10. Morgan, WD, Ryde SJS, Jones SJ, et al. In-vivo measurements of cadmium and lead in occupationally exposed workers and in an urban population. International Conference on Nuclear Analytical Methods in the Life Sciences, Gaithersburg MD, April 17-21,1989, Biol Trace Elem Res (in press). 11. Bellinger D, Leviton A, Waternaux C, Needleman H, Rabinowitz M. Longitudinal analyses of prenatal and postnatal lead exposure and early cognitive development. N Eng J Med 1987;316 :1037-43. 12. McMichael AJ, Baghurst PA, Wigg NR, Vimpani GV, Robertson EF, Roberts RJ. Port Piric cohort study: environmental exposure to lead and children's abilities at the age of four years. N Engl J Med 1988;319:468-75. 13. Pirkle JL, Schwartz J, Landis R, Harlan WR. The relationship between blood lead levels and blood pressure and its cardiovascular risk implications. Am J Epidemiol 1985;121:246-58. 14. Harlan WR, Landis JR, Schmouder RL, Coldstein NG, Harlan LC. Blood lead and blood pressure: relationship in the adoArchives of Environmental Health

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1985;253:530-34. 15. Burger DE, Milder FL, Morsillo, Adams BB, Hu H. Automated bone lead analysis by K-XRF for the clinical environment. Proceedings of the Second international Symposium on In Vivo Body Composition Studies (in press). 16. Wittmers LE Jr, Aufderheide AC, Wallgren J, Rapp G, Alich A. Lead in bone.iV. Distribution of lead in the human skeleton. Arch Environ Health 1988:43:381-91. 17. Wittmer LE, Alich A, Aufderheide AC. Lead in Bone. I. Direct analysis for lead in milligram quantities of bone ash by graphite furnace atomic absorption spectroscopy. Am J Clin Pathol

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21. Batuman V, Landy E, Maesaka JK,Wedeen RP. Contribution of lead to hypertension with renal impairment. N Engl J Med

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22. Pope A. Exposure of children to lead-based paints. Research Triangle Park, NC: U.S. Environmental Protection Agency, Strategies and Air Standards Division; EPA contract no. 68-02-

4329. 23. Agency for Toxic Substances and Disease Registry. The nature and extent of lead poisoning in children in the United States: a report to Congress. Public Health Service. U.S. Department of Health and Human Services. July, 1988.Section 11-5.

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Press, 1982;pp. 6-7. 19. Barry PSI. A comparison of concentrations of lead in human tissues. Br J ind Med 1975;32:119-39. 20. Gross SB, Pfitzer EA, Yeager DW, Kehoe RA. Lead in human

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X-ray fluorescence measurements of lead burden in subjects with low-level community lead exposure.

A k-x-ray fluorescence (K-XRF) instrument that can measure in vivo bone lead at low levels was used on a population of 34 adults with no known history...
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