The Effect of Low-Level Blood Lead on Hematologic Parameters in Children

Environmental Research Section A 82, 150}159 (2000) doi:10.1006/enrs.1999.4011, available online at http://www.idealibrary.com on

The Effect of Low-Level Blood Lead on Hematologic Parameters in Children1 Beate Jacob,*,2 Beate Ritz,- Joachim Heinrich,* Bernd Hoelscher,* and H.-Erich Wichmann* *GSF-Forschungszentrum fuK r Umwelt und Gesundheit, Institut fuK r Epidemiologie, IngolstaK dter Landstra}e 1, D-85758 Neuherberg, Germany; and -UCLA School of Public Health, Department of Epidemiology, Centers for Occupational & Environmental Health, P.O. Box 951772, Los Angeles, California 90095-1772 Received December 29, 1998

1995) and blood pressure (Hense et al., 1993). Deleterious effects of lead were described for heme synthesis at PbB levels of 100}200 lg/L, for vitamin D metabolism below 250 lg/L, for peripheral nerve conduction velocity below 200}300 lg/L, and for cognitive function below 150}200 lg/L (Schwartz et al., 1990; Centers for Disease Control, 1991). Consequently, standards de7ning an acceptable blood lead level have changed in the past 20 years and might be further challenged in the future. In 1975, the Centers for Disease Control (CDC) in Atlanta recommended a maximum permissible PbB level of 300 lg/L (Centers for Disease Control, 1975). Ten years later, this level was lowered to 250 lg/L (Centers for Disease Control, 1985). In 1991, CDC (Centers for Disease Control, 1991) and the World Health Organization (WHO, 1997; European Centre for Environment and Health, 1996) adopted 100 lg/L PbB as the of7cial action or guideline level. In Germany, the Commission for Human Biomonitoring (Kommission ‘‘Human-Biomonitoring’’ des Umweltbundesamtes et al., 1996) in 1996 set the guideline value also to 100 lg Pb/L. Abolishing leaded gasoline in most industrialized countries reduced environmental lead pollution and was accompanied by a substantial decline in average population blood lead levels over the past 20 years (Annest et al., 1983; Grandjean, 1993). Yet, there is still a need to address subtle health effects at low PbB levels in vulnerable populations such as in young children. In the general population, children under the age of 5 have the highest PbB levels due to increased gastrointestinal absorption and exposure through behaviors such as playing outdoors and increased hand-to-mouth activity (Wilhelm et al., 1993). Children are also more susceptible to the harmful effects of lead at low levels (Duggan et al., 1985), including impairment of long-term cognitive

A health survey of school children living in polluted regions of eastern Germany provided us with data necessary to examine the effects of lead on the blood system at levels below current standards for blood lead content. Data collected for 797 children, aged 5+14 years, with low blood lead levels (GM, 33.3 lg Pb/L; range, 7.5+239 lg Pb/L) allowed us to examine the relationship between blood lead content and hematological parameters. Using linear regression analyses and controlling for a number of potential confounding factors, we found that increasing blood lead levels by 10 lg/L were associated with a small increase in the number of red blood cells and in girls with reduced MCV and MCH. The reasons for our observation, especially the gender difference, are still uncertain. In conclusion the morphology and function of erythrocytes might be sensitive parameters of low dose lead toxicity. ( 2000 Academic Press

Key Words: lead; hematologic parameters; epidemiology; children; health effects.

INTRODUCTION

Lead (Pb) is toxic to humans when ingested or inhaled in high doses, causing diseases such as encephalopathy, colic, renal disease, and anemia. Three decades ago, it was recognized that low blood lead (PbB) levels can have subclinical but nonetheless harmful effects such as impairment of the cognitive or neuropsychological development of children (Thacker et al., 1992; Grandjean, 1993; Goyer, 1993) and adverse effects on renal function (Bernard et al., 1

The study was supported by the German Federal Environmental Agency (Umweltbundesamt Berlin), Grant No. 29861724. 2 To whom correspondence should be addressed. Fax:#49-893187-3380. E-mail: [email protected]. 150 0013-9351/00 $35.00 Copyright ( 2000 by Academic Press All rights of reproduction in any form reserved.

LOW-LEVEL BLOOD LEAD AND HEMATOLOGIC PARAMETERS

development and growth (Needleman et al., 1990; Thomson et al., 1989; Grandjean et al., 1991; Mushak et al., 1989; McMichael et al., 1988). Lead impacts many organ systems, but this paper will focus on hematological effects. Lead is known to (1) interfere with heme and hemoglobin syntheses (Mushak et al., 1989), and (2) affect erythrocyte morphology and survival (Piomelli, 1981; Terayama, 1993). A clinical manifestation and cardinal symptom of chronic lead poisoning is microcytic anemia induced by blood system damage at PbB levels above 400 lg/L (Mushak et al., 1989; Centers for Disease Control, 1991). Piomelli indicated that the heme synthesis pathway might start to be affected by lead at blood levels of 150 to 180 lg/L (Piomelli et al., 1982). Schwartz et al. modeling the dose-response relationship between blood lead and hematocrit suggested that a threshold for the depression of hematocrit in young children may not exist (Schwartz et al., 1990). Yet, it is still not clear whether and how blood lead levels lower than 100 lg/L affect hematological parameters such as hemoglobin, hematocrit, and other functional parameters of red blood cells, i.e., whether a threshold for hematological toxicity of lead exists (Poulos et al., 1986). A German survey of school-age children, aiming at the assessment of overall health, respiratory health, allergies, and environmental pollution, provided an opportunity to examine the effects of lead on the hematologic system of these children. Some of the children surveyed lived in industrial and mining centers of the former German Democratic Republic known for elevated environmental levels of heavy metals, including lead. In this report we examine whether blood lead levels below 100}150 lg/L affect red blood cells. METHODS

The data on which the subsequent analyses are based were collected during the 7rst survey of an ongoing cohort study examining the effects of environmental risk factors on respiratory health and atopic diseases in children living at three locations in the Saxony-Anhalt area of eastern Germany (Heinrich et al., 1995, 1999). Prior to 1989, the county of Bitterfeld (population of 117,400 in 1992) (Statistisches Bundesamt, 1994) was a major industrial center involved in the manufacture of chemical products, such as plastics and chlorinated pesticides, as well as coal mining. The county of Hettstedt (population of 52,200 in 1992) (Statistisches Bundesamt, 1994) was a center of mining and smelting of nonferrous metals, including

151

lead and copper, for over 800 years. Hettstedt’s lead smelter was shut down in 1978, but there have been no remediation activities at the site. The coppersilver smelter was shut down in 1991, but was reopened in 1992 with reduced capacity. Most of the residents of Bitterfeld and Hettstedt have lived near these industrial sites. North and upwind from Bitterfeld and Hettstedt is located the town Zerbst, an agricultural community, with a 1992 county population of 36,800 (Statistisches Bundesamt, 1994). Between August 1992 and July 1993, pre- and elementary school children 5}14 years of age were recruited into the study from the towns and neighboring communities. All eligible school children in Zerbst and Hettstedt and children from selected schools chosen to represent the regional district of Bitterfeld were invited to participate. School teachers distributed questionnaires to the children’s parents. The questionnaires elicited information about the health of the children and about social and environmental factors. Overall, 89.1% of the parents, representing 2470 children, returned the questionnaires. All children whose parents consented to participate in the study were physically examined during the 1992}1993 school year (N"2201). Physical examinations were administered and blood was drawn in biweekly cycles at the schools of the three communities. Since some children refused to have blood drawn and some blood specimens were insuf7cient, we do not have measurements of hematological parameters for all 2201 children. Hemoglobin (Hb), Hematocrit (Hct), red blood cell count (RBC), mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH) were determined in blood samples of 1889 children. Due to limited resources, it was furthermore not feasible to analyze the heavy metal content of blood specimens for more than 40% of all participating children. Thus, heavy metal burden was measured in a random subset of 1064 children. Both hematological and heavy metal blood content measures are available for a total of 829 children. The complete dataset with information of all covariates used for analyses included 797 children. Blood specimens were collected from the children between September 1992 and July 1993. After disinfection of the venipuncture site with 70% alcohol, approximately 10 ml of blood was drawn from each subject, 4 ml of which was poured into a K-EDTA tube and mixed for 10 min. All collection materials were tested and certi7ed to be lead free by the Institute for Water, Soil, and Air Hygiene at the Federal Environmental Agency. The blood samples were

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JACOB ET AL.

immediately frozen and stored at !80@C for measurements of lead of K-EDTA-blood 0.2 ml was stored to determine hematologic parameters within 2 h after the blood was drawn. All blood samples were coded to preserve anonymity. The study was approved by the University of Rostock Ethics Committee and carried out in accordance with the institutional guidelines for the protection of human subjects. Informed consent was obtained from all parents of the participating children. Hematologic Assays Using an atomic analyzer (Coulter Counter T 890) red blood cell count, hemoglobin level, and mean corpuscular volume were determined. Hematocrit and mean corpuscular hemoglobin were calculated based on the following arithmetic expressions: RBC * MCV Hct (%)" 1000 and Hb MCH (pg/cells)" 10. RBC * Lead Measurements Frozen blood samples were sent to the laboratory at the end of the survey and analyzed during 1993}1994. The lead concentrations of blood were determined by graphite-furnance atomic-absorption spectrometry (Perkin-Elmer Atomic absorption spectrometer Z-3030 and 5000, respectively) after deproteinization of blood. The minimum detection limit (MDL) for lead was 15 lg/L blood. Whenever the assays yielded a negative 7nding for lead, we assumed the true value to be half the MDL. Assay results below the MDL were reported by the laboratory for 34 (4.1%) of the children. For internal quality control the laboratory used the reference material ‘‘Kontroll-Blute K.-Nr.131’’ (Behring Institut). Furthermore, the accuracy of the analytic methods was con7rmed through an interlaboratory external quality control program. Extensive reports describing the validity of the data have been published elsewhere (Heinrich et al., 1995; Trepka et al., 1997; Meyer et al., 1998). Statistical Methods All statistical analyses were performed using SAS (Statistical Analysis Software 6.09, Cary, NC). We

included in our analyses only those children for whom in addition to outcome and exposure data (N"829) information on all covariate values (potential confounders, such as age, gender, febrile infections, residency, education level of the parents, and season when blood was taken) was available. Thus, after exclusion of 32 subjects with missing covariate data, we performed a complete subject analysis for 797 children. First, we conducted descriptive analyses to explore how covariates such as age, gender, season when blood was taken, febrile infections, educational level of either parent, and place of residency were associated with the blood lead levels and with the levels of hematological parameters (hemoglobin, hematocrit, red blood cell count, mean corpuscular volume, and mean corpuscular hemoglobin). Secondly, linear regression analyses were performed to identify possible associations between blood lead level and hematological parameters after controlling for the in8uence of other risk factors that could confound the association between lead and blood parameters. Age was measured categorically (5}7 years (reference), 8}10 years, 11}14 years), re8ecting the age range of the 7rst-, third-, and sixth-grade children invited to participate in our study. The number of febrile infections experienced within the past 12 months was dichotomized (less or equal to and more than three infections). Blood lead level and hematological parameters were treated as continuous variables in these models. We applied generalized additive models (Hastie et al., 1990) to examine the shapes of association and concluded that it was adequate to use a linear model. We also log-transformed the variable PbB, to account for the skewness of the blood lead distribution. Yet, since this transformation produced no qualitatively different results, we present results for the untransformed PbB variable only. In our 7nal model we adjusted only for age, sex, and febrile infections. These variables were included because they are known to in8uence blood lead levels through mobilization of lead from osseous tissues and, thus, in8uence the distribution of lead between osseous and nonosseous tissues and are also associated with hematologic parameters. We did not need to adjust for factors that affect intestinal absorption, retention in the blood, or excretion of lead unless such factors are known to independently in8uence hematologic parameters as is the case for iron levels. Factors in8uencing blood lead levels such as place of residency and socioeconomic status are correlated with lead load and are most likely proxy measures for lead exposure and, thus, were

LOW-LEVEL BLOOD LEAD AND HEMATOLOGIC PARAMETERS

not included in our models. Note, however, that they did not change the results when added to the models (results not shown). We, furthermore, examined the effect of other potential confounding factors, such as body burden of other heavy metals (arsenic and cadmium) and number of cigarettes currently smoked by household members. Since these later factors did not change the model parameters for lead by more than 10%, they were not included in the 7nal models presented here (Greenland, 1989). RESULTS

Table 1 shows the distribution of demographic and anthropometric variables such as age, gender, height, and weight in our population of children. Since hematologic parameters were found to be approximately normally distributed, we are presenting arithmetic means and standard deviations for them (Table 2). All hematologic parameters increased with age and decreased when children had experienced more than three infections in the previous year. No gender differences are seen for hemoglobin and hematocrit levels. Since we are also 7nding that in our study boys on average had more erythrocytes (RBC) than girls, MCH was consequently slightly lower in boys. Furthermore, MCV was also lower in boys than in girls. MCV and MCH levels were found to be lower in boys than in girls and lower in children whose parents reported 10 or fewer years of formal education (results not shown). MCV and MCH were furthermore lowest for children with blood lead levels above 61 lg/L (90th percentile). The blood lead distribution of the children in our study was distinctly skewed with a geometric mean of 33.25 lg/L (95% CI, 32.09}34.46; range, 7.5}239). The 25th percentile was 25 lg/L and the 75th perTABLE 1 Study Population of Children with Complete Data (N 5 797, 389 Boys and 408 Girls)

Median

(Min}max)

Arithmetic mean

SD

Age (years) Boys Girls

9 9 11

(5}14) (5}14) (5}13)

9.3 9.1 9.5

2.7 2.8 2.6

Height (cm) Boys Girls

143 141 144

(104}180) (104}180) (105}175)

Weight (kg) Boys Girls

34.5 32.0 35.8

(15.0}110) (15.0}110) (15.5}86)

141 139 142 35.5 34.5 36.5

18.0 18.0 18.0 13.2 12.7 13.5

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centile was 46 lg/L. The blood concentrations of lead was much higher in younger than in older children, and boys exhibited higher levels than girls. Only 21 (2.6%) girls had blood lead levels of 61 lg/L (90th percentile) or above, compared to 59 (7.4%) boys. We also observed differences in blood lead levels by region of residency, highest educational level of either parent, season when blood was taken, and by number of febrile infections reported for the past 12 month (see Table 3). The blood lead levels of our study subjects were not correlated with urinary or blood levels of other heavy metals such as arsenic or cadmium (r\0.01) (results not shown). In Table 4 we show the correlation matrix for blood lead levels and hematological parameters, values below the diagonal represent boys and those above the diagonal girls. Age and height were positively associated with all hematological parameters. Blood lead and hematological parameters were not highly correlated, whereas some hematological parameters such as hemoglobin and hematocrit showed a strong correlation (r"0.95 for boys and girls). Multivariate linear regression analyses adjusting for age, sex, and febrile infections showed a signi7cant increase in hematocrit, hemoglobin, and RBC with increasing blood lead concentrations per 10 lg Pb/L (Table 5), especially in boys. The effect for girls on RBC is stronger than for boys and there is also a positive effect on Hb and HCT but somewhat weaker than for boys. For boys we estimated an increase of 0.17% of hematocrit (CI, 0.073}0.269) per 10 lg/L Pb/L. A statistically signi7cant negative association was found for MCV and MCH with increasing blood lead levels in girls. We estimated a decrease of 0.345 lm3 in MCV (CI: 0.546}0.143) and of 0.104 pg in MCH (CI: 0.182}0.026) per 10 lg/L Pb/L in girls (Table 5). Adjusting for socioeconomic status, season at time of examination, region of residency, and blood and/or urinary levels of other heavy metals such as arsenic and cadmium did not change the effect estimates for PbB on hematological parameters (results not shown). DISCUSSION

A cross-sectional respiratory health study of school-aged children living in eastern Germany provided the opportunity to examine the effects of low levels of lead on blood parameters in almost 800 children. According to the international guidelines of the WHO (WHO, 1997), the CDC (Centers for Disease Control, 1991), and the German

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JACOB ET AL.

TABLE 2 Hematological Parameters of Children Aged 5 to 14 Years (N 5 797) by Gender, Age, Infection, and Blood Lead Levels RBC (counts * 106/lm3)

Hb (g/dl)

Hct (%)

MCV (lm3)

MCH (pg)

Factor

N

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Total Gender Boys Girls

797

4.63

0.28

13.22

0.86

39.34

2.38

84.95

3.18

28.54

1.25

389 408

4.67 4.60

0.29 0.27

13.24 13.19

0.89 0.84

39.42 39.26

2.50 2.27

84.42 85.46

2.79 3.44

28.36 28.72

1.14 1.33

Age (years) Boys 5}7 8}10 11}14 Girls 5}7 8}10 11}14

118 97 174 102 98 208

4.52 4.69 4.77 4.52 4.54 4.66

0.25 0.31 0.27 0.24 0.25 0.27

12.62 13.26 13.66 12.57 13.02 13.58

0.74 0.80 0.79 0.70 0.72 0.73

37.68 39.49 40.56 37.56 38.88 40.27

2.13 2.42 2.08 1.99 1.96 1.97

83.49 84.25 85.14 83.20 85.69 86.46

2.74 2.73 2.67 2.85 3.03 3.39

27.97 28.29 28.66 27.85 28.71 29.15

1.12 1.04 1.12 1.16 1.20 1.25

334 55

4.68 4.63

0.29 0.29

13.28 13.02

0.90 0.80

39.51 38.87

2.51 2.34

84.50 83.94

2.81 2.64

28.40 28.12

1.14 1.10

358 50

4.61 4.53

0.27 0.27

13.25 12.81

0.83 0.81

39.42 38.09

2.23 2.23

85.65 84.08

3.45 3.07

28.78 28.28

1.34 1.19

156 174 59

4.66 4.67 4.70

0.29 0.30 0.27

13.23 13.22 13.35

0.90 0.86 0.97

39.36 39.39 39.67

2.54 2.51 2.38

84.49 84.35 84.43

3.05 2.70 2.30

28.40 28.30 28.41

1.20 1.09 1.10

234 153 21

4.58 4.62 4.66

0.26 0.28 0.29

13.24 13.14 13.08

0.83 0.86 0.68

39.43 39.01 39.10

2.22 2.36 2.16

86.15 84.61 83.95

3.39 3.41 2.46

28.92 28.50 28.09

1.31 1.34 1.05

Infections (last 12 months) Boys Less than three times Three times or more often Girls Less than three times Three times or more often Blood lead level Boys \34 lg/L* 34}61 lg/L** '61 lg/L*** Girls \34 lg/L* 34}61 lg/L** '61 lg/L***

Note. RBC, red blood cell count; Hb, hemoglobin; Hct, hematocrit; MCV, mean corpuscular volume; MCH, mean corpuscular hemoglobin. *\50th percentile. **50th to 90th percentile. ***[90th percentile of study population.

Commission for Human Biomonitoring (Kommission ‘‘Human-Biomonitoring’’ des Umweltbundesamtes et al., 1996) blood lead concentrations below 100 lg Pb/L do not require any remedial action. According to these criteria, the blood lead concentrations measured in our study are low (99%\100 lg Pb/L). The geometric mean of the concentration was 33.3 lg/L (95% CI, 32.1}34.5; range, 7.5}239) and only 10 (1.2%) children, 8 boys and 2 girls, exceeded the standard. A comparison with the results of the nationwide. ‘‘Environmental Survey in the Federal Republic of Germany’’ (1990}1992) (Krause et al., 1996), which measured blood lead in 713 German children 6}14 years of age employing the same laboratory as the present study, reveals that the over-

all geometric means for blood lead were almost identical but our study included a few more highly exposed children (the survey mean was 32.3 Pb lg/L blood; 95% CI, 31.3}33.3, range, 7.5}139). Experimental studies revealed that lead interferes with heme biosynthesis at various steps, mainly through the disturbance of the activity of three major enzymes, and it affects the formation and function of red blood cells at blood levels well below 200 lg/L (Mushak et al., 1989). A shortening of erythrocyte life span was observed by Terayama (1993) in experiments with rats. Grandjean et al. (1989) reported a lead-dependent delay of the regeneration of human red blood cells. Lead, furthermore, interferes with iron utilization for heme formation in

LOW-LEVEL BLOOD LEAD AND HEMATOLOGIC PARAMETERS

TABLE 3 Blood Lead Levels of Children Aged 5 to 14 Years (N 5 797) by Age, Gender, Region of Residency, Education Level, Season, and Frequent Infections Blood lead (lg/L) Geometric mean

95% CI

33.25

32.09}34.46

35.73 36.89 30.43

33.07}38.62 34.72}39.19 29.03}31.90

Gender Boys (N"389) Girls (N"408)

37.69 29.51

35.90}39.57 28.09}31.00

Region Zerbst (N"119) Bitterfeld (N"169) Hettstedt (N"509)

32.37 23.20 37.71

30.07}34.85 21.38}25.17 36.21}39.27

Highest educational level of either parent (12 years (N"439) 35.22 512 years (N"358) 31.00

33.51}37.01 29.49}32.58

Season when blood was taken May to October (N"220) November to April (N"577)

41.60 30.53

39.36}43.98 29.26}31.86

Infections during the past 12 month Less than three times (N"692) Three times or more often (N"105)

32.27 40.50

31.08}33.52 36.78}44.62

Total (N"797) Age 5}7 years (N"220) 8}10 years (N"195) 11}14 years (N"382)

the mitochondria (Mahaffy et al., 1986), and radioiron studies showed that lead competes with iron for incorporation into red blood cells (Albahary, 1972). The precise mechanisms of action that lead exhibits on cellular metabolism, however, remain largely unknown (Labbe, 1990). While it has been suggested that lead suppresses heme biosynthesis even at low levels, it is uncertain whether a minimum blood lead level exists at which such an effect occurs. Children are in general known to be at higher risk for toxic lead effects (Duggan et al., 1985; Veerula et al., 1990; European Centre for Environment and Health, 1996). Thus, examining whether low lead levels exhibit any effect on blood parameters in our population of school-aged children provides a sensitive test for a threshold model of low-level lead toxicity. We found that hematocrit and hemoglobin levels increase with blood lead levels, an association that previously has been reported for populations of children and of adults exhibiting comparably low levels of PbB (Hense et al., 1992, 1993; Bernigau et al., 1993; Schwartz et al., 1990). In our population,

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a positive relationship with hemoglobin and hematocrit was suggested for children of both genders, but it was somewhat weaker and statistically not signi7cant in girls. The gender difference might be due to differences in the range of blood lead observed; i.e., the range was wider for boys. Hense et al. (1992) interpreted the positive association between low lead levels and hematocrit or hemoglobin as a direct re8ection of the lead-binding capacity of erythrocytes since more than 95% of blood lead is bound to red cells. At lead levels in the range of 100}400 lg/L, however, in vitro studies found that hematocrit variation has little in8uence on uptake of lead by the blood (Kochen et al., 1973). Furthermore, this positive association is an apparent contrast to the reduction of hematocrit and hemoglobin observed at high blood lead levels (above 400 lg/L) which is thought to re8ect heme synthesis inhibition (Tepper et al., 1975; Betts et al., 1973; Poulos et al., 1986). Thus, all of these observations together suggest that the effect of lead on blood parameters such as hematocrit and hemoglobin might not be linear and driven by complex toxicodynamics. We, furthermore, found a weak positive association of red blood cell count with blood lead levels. Such a positive relationship has previously been described by Rosmanith et al. (1977) and also by Bruin (1971) and was interpreted as a stimulation of blood formation occurring at low lead levels. The stimulating effect of lead on red blood cell formation we observed was somewhat stronger in girls. The lowest observed effect level for lead on erythrocyte protophorphyrin previously reported in the literature also differed by gender, being lower in females (European Centre for Environment and Health, 1996). Thus, lead sensitivity of the heme-synthesizing enzymes may vary according to factors such as gender and result in different susceptibilities to lead toxicity in subpopulations (Goyer, 1990). We saw a negative association between blood lead levels and MCV and MCH. Such an effect has also been observed previously at low-average blood lead levels (average of 65 lg Pb/L) for 6- to 14-year-old children by Rosmanith et al., (1977) and in experiments with rats by Terayama (1993). Lead affected MCV and MCH levels in our population primarily in girls. If not an artifact of our data, this might also suggest a special sensitivity of girls’ erythrocytes. We speculate that lead might interfere with heme synthesis in susceptible individuals resulting in reduced function of the hemoglobin protein and the erythrocyte;as indicated by its small size;rather than a quantitative reduction of hemoglobin levels. Reduced function would explain the need for an

156

JACOB ET AL.

TABLE 4 Matrix of Pearson’s Correlation CoefAcients between Blood Lead and Hematological Parameters StratiAed for Boys (N 5 389) and Girls (N 5 408) Girls Boys

Lead blood*

Lead blood*

Age

Height

!0<17

!0.24 0.93

RBC

Hemoglobin

Hematocrit

MCV

MCH

0.12

!0.01

!0.02

!0.19

!0.16

0.23

0.50

0.49

0.38

0.41

0.21

0.49

0.48

0.39

0.41

0.71

0.75

!0.35

!0.28

0.95

0.34

0.47

0.34

0.35

Age

!0.08

Height

!0.11

0.92

RBC

0.07

0.35

0.39

Hemoglobin

0.08

0.49

0.51

0.81

Hematocrit

0.09

0.48

0.50

0.86

0.95

MCV

0.02

0.27

0.22

!0.23

0.29

0.29

MCH

0.02

0.27

0.25

!0.20

0.42

0.25

0.91 0.85

*Normalized by logarithmical transformation.

increase in the number of red blood cells (the observed stimulation of red blood cell formation) and possibly an initial increase of (functionally impaired) hemoglobin rather than a reduction of overall hemoglobin levels due to heme synthesis inhibition. Recent reports suggested that anemia occurs much less commonly in cases of childhood lead poisoning than previously reported (Bhambhani et al., 1990; Cohen et al., 1981). Cohen et al. (1981) studying children with very high blood lead levels (over 700 lg/L) found that while microcytosis occurred in 46% of these children, microcytosis in combination with anemia occurred in only 2%. Therefore, erythrocyte volume might be a more sensitive parameter for the effects of lead than hemoglobin levels and, furthermore, lead effects on red blood cell function and morphology might depend on other unknown susceptibility factors. As expected, it was essential to control for the in8uence of age and sex to observe the association between blood lead and hematological parameters in our study, since the hematological parameters hemoglobin, hematocrit, MCV, and MCH are known to increase with age (Ciba Geigy, 1968; Lin-Fu, 1973). An inverse association of blood lead levels and age has been described in many epidemiological studies and is most likely due to age-related changes in hand-to-mouth and play behaviors; i.e., younger children are more likely to ingest ambient lead from household dust (Rosmanith et al., 1977; Heyworth

et al., 1991). NHANES II and III data showed that young children between 6 month and 5 years of age had more often elevated blood lead levels than older children (Mahaffy et al., 1986; Brody et al., 1994). Hematologic parameters, especially the mean corpuscular volume (MCV), are furthermore in8uenced by iron (Fe) levels; i.e., iron de7ciency is known to cause a decrease in MCV (Markowitz et al., 1996). Research has also shown that Pb absorption, retention, and toxicity increase in the presence of Fe de7ciency (Markowitz et al., 1990; Watson et al., 1980; Yip et al., 1984; Marcus et al., 1987), although some studies disputed such an association (Sargent et al., 1995, 1996; Markowitz et al., 1996). The ironde7cient state, not uncommon in young children (Oski, 1993; Booth et al., 1997), thus, may constitute a predisposing factor for lead poisoning in childhood (Redondo Granado et al., 1994; Yip et al., 1984). Oski even suggested that the hematological changes previously attributed to lead poisoning might instead be a result of coexisting iron de7ciency, since lead poisoning must be severe to produce anemia ([1000 lg Pb/L) (Oski, 1993). Given the interdependency of iron levels, lead levels, and hematologic parameters, iron de7ciency could be a potential confounder in our study. The range of hemoglobin observed among children in our population (10.2}16.2 g/dl), however, did not suggest severe iron de7ciency. Furthermore, if girls in our study were more iron de7cient than boys, one would suspect that the observed lead

LOW-LEVEL BLOOD LEAD AND HEMATOLOGIC PARAMETERS

TABLE 5 Effect Estimates for an Increase of 10 lg Pb/L on RBC, HB, HCT, MCV, and MCH, Controlled for Age and Infections (Linear Regression Analysis of 797 Children Aged 5 to 14 Years) Effect estimates for an increase of 10 lg Pb/L b RBC (counts * 106/lm3) Boys 0.016 Girls 0.031

CI

P value

(0.004; 0.029) (0.014; 0.047)

0.009 (0.001

Hb Boys Girls

0.062 0.039

(0.027; 0.097) (!0.007; 0.086)

(0.001 0.095

Hct (%) Boys Girls

0.171 0.103

(0.073; 0.269) (!0.023; 0.229)

(0.001 0.110

MCV (lm3) Boys Girls

0.062 !0.345

(!0.061; 0.186) (!0.546; !0.143)

0.322 (0.001

MCH (pg) Boys Girls

0.029 !0.104

(!0.021; 0.080) (!0.182; !0.026)

0.252 0.009

effects in fact must be attributed to iron de7ciency. Yet, we saw no indication that girls suffered from iron de7ciency at greater rates than boys, the mean hemoglobin levels for boys and girls were almost identical and the range quite similar (Table 4). Moreover, the lead effect for girls was consistently observed in all age groups, i.e., was not restricted to the age group of girls 11}14 years old who might already menstruate. Other potential confounding factors we examined but did not include into our 7nal model due to a lack of change in the regression estimates for lead were the occurrence of frequent infections, socioeconomic status, and season. During infections blood lead levels could increase due to the mobilization of lead from bone tissue, and frequent infections can also cause a depression of hemoglobin and hematocrit levels. Lower socioeconomic status represented by parental educational attainment was associated with higher blood lead levels in our population, a result also found in another study of East German children (Begerow et al., 1994). Seasonal variations in blood lead levels have been reported by some authors (Charney et al., 1983; Klein et al., 1975; McCusker, 1997), and not by others (Riegart et al., 1976). Note, that these later two factors might actually be proxy measures for lead exposure rather than independent risk factors, since summer and fall are seasons during which children spend more time

157

outside and lower socioeconomic status might be associated with increased lead house-dust levels. CONCLUSION

Overall, our data show an association between lead and red blood cell parameters at low levels that are in contrast to observations from studies of high lead levels. Although the size of the effects in our study are small, previous studies of low-level lead exposure reported similar results. We speculate that low lead levels (below current standards) might lead to production of red blood cells with reduced volume and function and the observed decrease in MCV and MCH might indicate a qualitative disturbance of the heme synthesis. Thus, the morphology of erythrocytes might be a sensitive parameter for low-dose lead toxicity, and lead toxicity might furthermore depend on susceptibility factors such as gender. We recommend that future investigations attempt to carefully control for iron de7ciency, in addition to other known risk factors such as age and gender that could confound the relation between lead exposure and heme metabolism. ACKNOWLEDGMENTS This study was funded by the Federal Environmental Agency. We thank Mr. H. Schneller and Mrs. K. Honig-Blum for data handling; Dr. H. Adam, Dr. H. Bach, Dr. I. HoK rhold, Dr. D. Bodesheim, Dr. I. Keller, and Dr. S. LoK we for taking the blood samples; Dr. C. Krause Mrs. C. Schulz, Mrs. H. Pick-Fu{, Mrs. L. WindmuK ller, and Mrs. C. Gleue for the laboratory analyses: Mr. G. Burmester, Mr. J. Rudzinski, Mrs. B. Hollstein, Mrs. D. Albrecht, Mrs. H. Machander, and Mrs. C. Boettcher for gathering regional information and local assistance. We thank Prof. Dr. H. Hense, Dr. H. Feyen, and Dr. U. KraK mer for reviewing drafts and for many helpful suggestions. The authors thank also all teachers in Hettsetdt, Zerbst, and Bitterfeld and the local school authorities and health care centers for their support; and all parents and children for their participation.

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