Blood Lead Levels and children’s Behavioral and Emotional Problems: A Cohort Study

Abstract

Importance

The association between lead exposure and children’s IQ has been well studied, but few studies have examined the effects of blood lead on children’s behavior.

Objective

This study examined the association between blood lead concentrations and behavioral problems in a community sample of Chinese preschool children with a mean blood lead level <10 µg/dL.

Design

Prospective cohort study.

Setting

Four elementary schools in Jintan City, Jiangsu Province of China.

Participants

Participants were 1341 children at ages 3–5 years.

Main Outcome Measures

Blood lead concentration was measured at ages 3–5 years. Behavioral problems were assessed using Chinese versions of the Child Behavior Checklist and Teacher Report Form when children at age 6 years.

Results

Mean blood lead concentration was 6.4 µg/dL (SD=2.6), with 75^th and 90^th percentiles being 7.7 and 9.4 µg/dL respectively. General linear modeling showed significant associations between blood lead concentrations and increased scores for teacher reported behavioral problems. One µg/dL increase of blood lead concentration resulted in a 0.32, 0.25 and 0.30 increase of behavior scores on emotional reactivity, anxiety/depressed and pervasive developmental problems, respectively (p <0.05), while adjusting for parental and child variables,. Spline modeling showed that teacher-reported behavior scores increased with blood lead concentration, particular for older girls.

Conclusions and Relevance

Blood lead concentrations, even at mean levels of 6.4 µg/dL, were associated with increased risk of behavioral problems in Chinese preschool children, including internalizing and pervasive developmental problems. This association showed different patterns depending on age and gender.

INTRODUCTION

Lead is now accepted to lower children’s IQ at commonly encountered exposures, and no threshold has been determined for this effect. Accordingly, in May 2012 the US Centers for Disease Control and Prevention eliminated the terminology “level of concern,” which was previously defined as a blood lead level concentration of 10 µg/dL.¹ Rather, children with elevated blood lead concentrations will be identified using a reference value based on the 97.5^th percentile of the NHANES-generated blood lead concentration distribution in children aged 1–5 years old; currently, this value is 5 µg/dL.¹

Blood lead concentrations >10 µg/dL are also linked to behavior problems in children,^2–6 delinquency in adolescents,^7–9 and criminal behaviors in adults.^10–11 Nevertheless, lead’s effect on children’s behavior is less understood compared to its impact on IQ. Few studies have been done at levels of exposure <10 µg/dL,^12–14 and almost all have focused on attention-related problems. Lead exposure is particularly concerning in developing countries where children have higher blood lead concentrations than in the US or Europe.^15–17 The present study examined the association between blood lead concentration (mean = 6.4 µg/dL) and behavioral problems using data from a community sample of Chinese preschool children.

METHODS

Subjects

Data comes from an ongoing longitudinal project, the China Jintan Child Cohort Study, which includes 1656 3–5-year-old children in Jintan city, Jiangsu province, China. Participants were drawn from four preschools chosen to represent the entire city’s geographical, social, and economic profiles between from Fall 2004 to Spring 2005. Detailed information on the Jintan cohort is reported elsewhere.^18–19 The current study used a sample of 1341 children (603 girls, 738 boys) with blood lead data (81% of our original cohort). There were no significant differences in family residence, parental education or occupation, or marital status of parents between children with and without blood lead data.^18–19 Institutional Review Board approval was obtained from the University of Pennsylvania and the ethical committee for research at Jintan Hospital in China. Written informed consent was obtained from parents.

Measures

Blood lead concentrations

Blood lead concentration was tested once for each child, at age 3, 4, or 5 years, between November 2004 and March 2005. Collection was done by nurses using a standard protocol to avoid lead contamination. Each specimen was analyzed twice for blood lead using a graphite furnace atomic absorption spectrophotometer,^20–21 and Kaulson Laboratories in New Jersey provided blood lead reference materials for quality control. Limit of detection (LOD) was 1.8 µg/dL; half of LOD was imputed for 3 (0.2%) samples under the LOD. More details for this process are provided in Liu et al.²²

Behavioral Problems

Children’s behavioral problems were assessed in their last month of preschool, at age 6 years. Parents and teachers assessed the children with Chinese versions of the Child Behavior Checklist (CBCL/1.5–5) and Caregiver-Teacher Report Form (C-TRF/1.5–5) separately. The CBCL and C-TRF are widely used for assessing behavioral and emotional problems in children^23–25; each consists of 99 items dealing with a child’s behavior within the past twelve months. Items are rated on a 3-point scale (0-Not True, 1-Sometimes True, or 2-Often True).²⁶

The CBCL/C-TRF scores are summarized in 3 ways: Two broadband problems (internalizing and externalizing) and Total Problems; seven Syndromes (anxiety/depression, emotional reaction, withdrawn, somatic complaints, sleep problems, attention problems, and aggression); and Five Diagnostic and Statistical Manual (DSM)-oriented Scales (affective, anxiety, attention deficit/hyperactivity, oppositional defiant, and pervasive developmental).²⁷ The problem structure (internalizing and externalizing) is still valid when used in Chinese children. Detailed descriptions of the setting and procedures of CBCL/C-TRF assessment and factor structures are provided in Liu et al.²⁸ We calculated normalized T scores (the ratio of behavior score’s deviation from the population mean to its standard deviation) from raw scores relative to the scores of a Chinese normative sample of peers.^26,28 Children with a T score ≥60 (83rd percentile of Chinese norm group) are more likely to present behavior problems in the borderline/clinical range.²⁶

Cognitive performance

Children’s cognitive performance was assessed during their last year of preschool, at ages 5–6 years, using the Chinese version of the Wechsler Preschool and Primary Scale of Intelligence – Revised (WPPSI–R).Constructed by Wechsler²⁹ to assess the intelligence of children aged 3–7 years, the WPPSI-R was standardized in China in 1988 and is reliable in Chinese children.^30–31 Details are provided in Liu & Lynn³² and Liu et al.³³

Sociodemographic variables

Parents completed a questionnaire on the child’s sex, age at blood lead test, siblings (yes/no), parental education, parental occupation, marital status, and residence (city, suburb, or rural).

Statistical Analysis

Sample characteristics were summarized by descriptive statistics. Student’s t test and ANOVA were used to examine gender and age differences in blood lead concentrations, respectively. We used linear regression to examine the relationship between blood lead concentration (µg/dL) and behavior T scores while controlling for child and family characteristics (gender, residence, parental education, father’s occupation, single child status, marital status of parents, and IQ). We treated blood lead concentration as a continuous linear term with units of µg/dl. Age in months at time of blood test was included in the above analysis, while age at time of behavior assessment was not included since all children were tested at 6 years. The demographic variables were selected based on past studies^22,34 and/or our preliminary analyses indicating these variables were associated with either or both blood lead concentration and behavioral problems. Behavior T scores that were not normally distributed were log transformed.

Since the internalizing and externalizing behavior scores differ by gender in this sample²⁸ and associations between blood lead concentration and behavior assessments may vary with age at blood lead test,³⁵ we tested several interactions of age by gender. Logistic regression analysis did not find any age by gender interaction (p>0.05) on the risk of behavior problem, adjusted by squared lead concentration. We regressed behavior problem scores on spline modeled blood lead concentration,³⁶ while adjusting for child and family characteristics; we further tested two sets of interaction of Blood Lead Concentration by gender, and by 3 age groups (3, 4, and 5 years) separately. Given the significant interaction of Blood Lead Concentration by gender for internalizing problems (p=0.008), we separated the data into six strata by gender and age to examine the relationship between Blood Lead Concentration and behavioral problems.

We finally performed logistic regression analyses to examine the association between blood lead concentration and odds of clinical-level behavioral problems. A p-value<.05 was considered significant. We analyzed data using R, Version 2.15.2 (R Development Core Team, GNU General Public License) for spline modeling and IBM SPSS Statistics, Version 17 (SPSS Inc., Chicago, IL, USA) for all other analysis.

RESULTS

Of the 1341 children with blood lead concentrations available, 1025 children had complete data. Compared with those without complete data, We found no group difference on gender, age group, and residence location. Characteristics of the children and their families, are summarized in Table 1. Mean blood lead concentration was 6.4 µg/dL (SD=2.6). Distribution was as follows: 25% were ≤4.7 µg/dL, 50 % were ≤6.2 µg/dL, 75% were ≤7.7 µg/dL and 90% were ≤9.4 µg/dL. As shown in Table 2, mean concentration was higher in boys than in girls and increased with age at blood lead test for both boys and girls.

Table 1.

Frequency and percentage of baseline characteristics of study population (n=1341) by blood lead categories.

			Blood lead concentrationa category
			(1.8, 4.7]	(4.7,6.2]	(6.1,7.7]	(7.7,32]
	N	Mean	n (%)	n (%)	n (%)	n(%)
Sex
Male	737	6.75	161 (48.06%)	185 (52.71%)	184 (54.76%)	207 (65.51%)
Female	601	6.03	174 (51.94%)	166 (47.29%)	152 (45.24%)	109 (34.49%)
Age at blood lead test (years )
3	314	5.17	158 (47.16%)	61 (17.38%)	55 (16.37%)	40 (12.66%)
4	414	6.46	102 (30.45%)	111 (31.62%)	105 (31.25%)	96 (30.38%)
5	610	7.05	75 (22.39%)	179 (51%)	176 (52.38%)	180 (56.96%)
Residence
City	518	6.33	150 (44.78%)	125 (35.61%)	114 (33.93%)	129 (40.82%)
Suburban	538	6.27	144 (42.99%)	140 (39.89%)	143 (42.56%)	111 (35.13%)
Rural	282	6.91	41 (12.24%)	86 (24.5%)	79 (23.51%)	76 (24.05%)
Father's Education
≤Middle school	486	6.72	100 (31.06%)	123 (38.8%)	119 (37.9%)	144 (45.57%)
High School	408	6.4	113 (35.09%)	95 (29.97%)	103 (32.8%)	97 (30.7%)
College or Higher	375	5.99	109 (33.85%)	99 (31.23%)	92 (29.3%)	75 (23.73%)
Father’s occupation
Unemployed	50	6.22	13 (4.18%)	12 (3.92%)	13 (4.05%)	12 (4.17%)
Physical worker	692	6.46	167 (53.7%)	164 (53.59%)	197 (61.37%)	164 (56.94%)
Professional worker	484	6.37	131 (42.12%)	130 (42.48%)	111 (34.58%)	112 (38.89%)
Mother’s education
≤Middle school	638	6.71	137 (42.41%)	156 (49.37%)	157 (50%)	188 (59.31%)
High School	376	6.08	114 (35.29%)	89 (28.16%)	95 (30.25%)	78 (24.61%)
College or Higher	256	6.08	72 (22.29%)	71 (22.47%)	62 (19.75%)	51 (16.09%)
Marital status of parents
Married	1123	6.34	280 (95.89%)	294 (95.15%)	276 (95.83%)	273 (95.12%)
Divorced	53	6.44	12 (4.11%)	15 (4.85%)	12 (4.17%)	14 (4.88%)
Only Child
Yes	943	6.3	254 (85.52%)	240 (83.92%)	244 (80.26%)	205 (75.09%)
No	217	6.87	43 (14.48%)	46 (16.08%)	60 (19.74%)	68 (24.91%)

^‡Number of children differs across sample characteristics due to missing values.

^aBlood lead concentration is treated as a continuous linear term in units of µg/dl.

Table 2.

Distribution of blood lead concentrations^a by age and gender

Age	Male					Female					p
	N	Mean	Median	SD	(Min.,Max.)	N	Mean	Median	SD	(Min.,Max.)
3 years	174	5.51	5.1	2.47	(2.1,17)	142	4.76	4.4	2.11	(1.8,12.6)	.005
4 years	217	6.72	6.4	3.28	(1.8,32)	198	6.18	6.0	2.28	(1.9,16.2)	.051
5 years	347	7.39	7.0	2.48	(2.5,24.6)	263	6.61	6.3	2.24	(2.3,18.1)	<.0001
Total	748	6.75	6.4	2.83	(1.8,32)	603	6.03	5.9	2.34	(1.8,18.1)	<.0001

^aBlood lead concentration is treated as a continuous linear term in units of µg/dl.

Blood lead concentration and behavior scores

Our linear regression model indicated that all teacher-reported behavior scales scores increased with blood lead concentration after adjusting for children and family factors (Table 3). Blood lead concentration had statistically significant harmful associations with scores on emotional, anxiety and pervasive developmental problems. One µg/dL increase of blood lead concentration resulted in a 0.32, 0.25 and 0.30 increase of behavior scores on emotional reactivity, anxiety/depressed and pervasive developmental problems, respectively (p <0.05). No parent-reported behavioral scales were significantly correlated with blood lead concentration.

Table 3.

Adjusted regression coefficients (95%CI) of blood lead concentration^a on behavioral problems^‡

Behavioral Problems	Parent-reported behavior (CBCL) (n=1026)	Teacher-reported behavior (C-TRF) (n=1025)
Syndrome
Internalizing Problems	−0.029 (−0.28, 0.222)	0.223 (−0.037, 0.484)
Emotionally Reactive	−0.117 (−0.365, 0.131)	0.322 (0.058, 0.587)*
Anxious/DepressedT	0.101 (−0.151, 0.354)	0.001 (−0.001, 0.003)
Somatic ComplaintsT	−0.171 (−0.436, 0.094)	0.001 (−0.003, 0.001)
Withdrawn	0.096 (−0.158, 0.349)	0.001 (−0.001, 0.003)
Sleep Problems	0.016 (−0.241, 0.273)	NA
Externalizing ProblemsT	0.011 (−0.243, 0.265)	0.001 (−0.001, 0.003)
Attention ProblemsP	0.001(−0.002, 0.002)	0.136 (−0.115, 0.386)
Aggressive BehaviorT	−0.018 (−0.264, 0.229)	0.001 (−0.001, 0.003)
Total Problems	0.016 (−0.231, 0.263)	0.202 (−0.051, 0.455)
DSM-oriented problems
Affective ProblemsT	0.019 (−0.234, 0.272)	0.001 (−0.001, 0.003)
Anxiety Problems	0.044 (−0.212, 0.299)	0.253 (0.016, 0.500)*
Pervasive Developmental problems	0.093 (−0.159, 0.345)	0.303 (0.046, 0.560)*
Attention deficit/hyperactivity problemsP	0.001 (−0.002, 0.002)	0.073 (−0.177, 0.322)
Oppositional DefiantT	−0.030 (−0.279, 0.220)	0.223 (−0.037, 0.484)

^‡Adjusting for age at blood lead test, sex, residence, father’s education, mother’s education, father’s occupation, parents’ marital status, single child, and IQ

^TTeacher-reported scale score was log transformed for normal distribution

^PParent-reported scale score was log transformed for normal distribution

^*p<.05

^aBlood lead concentration is treated as a continuous linear term in units of µg/dl.

We used spline modeling to examine the relationships between blood lead concentration and two broadband problems (internalizing and externalizing) ²⁶ in each of the six strata by gender and age groups (3, 4 and 5 years).

Behavior scores for internalizing problems increased with blood lead concentration for girls at age 5 years (p=0.04) and age 4 years (p=0.048) (Figure 1). Behavior scores slightly increased with blood lead concentration for boys at age 5 years and slightly decreased for boys at age 4 years.

Figure 1.

Teacher-reported internalizing problem scales scores among children whose blood lead was measured at age 4 and 5 years. Solid lines are the fitted spline curves. P values of testing significance (F test) are shown in the upper corners of each plot.

Behavior scores for externalizing problems increased with blood lead for boys at age 5 (Figure 2). In the group with blood lead measurements at age 3 years, no clear relationship was observed for either gender (Data available upon request).

Figure 2.

Teacher-reported externalizing problem scales scores among children whose blood lead was measured at age 4 and 5 years. Solid lines are the fitted spline curves. P values of testing significance (F test) are shown in the upper corners of each plot.

Blood lead concentration and clinical-level behavioral problems

Logistic regression was conducted to examine the association between blood lead concentrations) and clinical level behavioral problems (ie., T-score ≥ 60). Odds ratio was used to present the strength of the association. In the unadjusted logistic regression analysis, blood lead concentration was significantly associated with increased odds for attention, internalizing and total problems, as well as for DSM-oriented pervasive developmental and attention deficit/hyperactivity. After adjusting for child and family characteristics, blood lead concentration was still significantly associated with increased odds for emotionally reactive (OR=1.10, 95%CI=1.02 – 1.19), anxious/depressed (OR=1.12, 95%CI=1.03 – 1.23), total internalizing problems (OR= 1.10, 95%CI= (1.03 – 1.18),as well as DSM-oriented anxiety (OR=1.10, 95%CI= 1.01 – 1.19) and pervasive developmental problems (OR=1.16, 95%CI=1.07 – 1.25) (Table 4). For example, a 1 µg/dL increment in blood lead concentration was associated with a 16% increased likelihood of having pervasive developmental problems.

Table 4.

Adjusted odds ratios (OR, 95%CI) of blood lead concentration^a on Teacher-Reported behavioral problems (C-TRF).

	Total (n=1021)	Boys (n=557)	Girls (n=464)
Internalizing problems	1.10 (1.03 – 1.18)**	1.09 (1.00–1.19)	1.12(1.00–1.26)*
Emotionally Reactive	1.10 (1.02 – 1.19)*	1.04 (0.94–1.16)	1.17 (1.03–1.34)*
Anxious/Depressed	1.12 (1.03 – 1.23)*	1.11 (0.98–1.24)	1.14 (0.98–1.32)
Somatic Complaints	1.01 (0.90 – 1.13)	1.08 (0.94–1.24)	0.91 (0.75–1.09)
Withdrawn	1.02 (0.93 – 1.12)	0.98 (0.87–1.11)	1.09 (0.94–1.26)
Externalizing problems	1.02 (0.95 – 1.10)	0.99 (0.91–1.08)	1.08 (0.93–1.25)
Attention Problems	1.07 (0.98 – 1.16)	1.05 (0.95–1.16)	1.09 (0.93–1.27)
Aggressive Behavior	1.07 (0.98 – 1.17)	1.03 (0.93–1.14)	1.21 (0.99–1.47)
Total Problems	1.06 (0.99–1.12)	1.02 (0.95–1.10)	1.11 (0.99–1.23)
DSM-oriented problems
Affective problems	1.04 (0.96 – 1.13)	1.06 (0.96–1.18)	1.03 (0.90–1.18)
Anxiety problems	1.10 (1.01 – 1.19)*	1.05 (0.94–1.17)	1.17 (1.03–1.34)*
Pervasive Developmental problems	1.16 (1.07 – 1.25)***	1.11 (1.00–1.23)*	1.22 (1.07–1.40)**
Attention deficit/hyperactivity problems	1.08 (0.99 – 1.18)	1.04 (0.94–1.16)	1.15 (0.98–1.35)
Oppositional Defiant problems	1.06 (0.98 – 1.15)	1.02 (0.92–1.13)	1.12 (0.97–1.29)

^‡Adjusting for age at blood lead test, sex, preschool residence, father’s education, mother’s education, father’s occupation, parent married status, single child, and IQ

^*p<.05,

^**p<.01,

^***p<.001

^aBlood lead concentration is treated as a continuous linear term in units of µg/dl.

We did not find any significant associations between blood lead concentration and parent-reported clinical-level behavioral problems (p>0.05) (Data available upon request)

DISCUSSION

Key findings

In these 1341 children from the China Jintan Child Cohort Study, we found that blood lead concentration at mean levels of 6.4 µg/dL, when children were at ages 3–5 years, was significantly associated with increased behavioral problem scores at age 6 years. The findings are consistent with previous studies.^{2,5,7–8,14,35,37} We also found that the risk of clinical-level behavioral problems increased with blood lead concentration, including total internalizing problems and DSM-oriented anxiety and pervasive developmental problems. This association showed different patterns depending on age and gender. The results held after adjusting for potential child and family cofounders.

Age, blood lead concentrations, and behavior

Blood lead concentration increased with age in children before school age (Table 2), which is consistent with past studies in China.^38–41 The blood lead concentration and age association in China are different from those in the US, where blood lead concentration increases with age in children up to ages 2–3 years and then declines.⁴² This may be due to different sources of lead exposure in these two countries,²² as well as China’s phasing out of leaded petroleum beginning in 1997 and completed around 2000.^43–44 This transition suggests that our cohort (born between 1999–2001) might have less lead exposure than those born earlier.

The blood lead concentration and behavior association also varied with the age of the child at the time of blood lead measurement. In our stratified analysis by age at the time of blood lead measurement, we found that there was an increased risk for behavioral problems with increased blood lead concentration in older children (5 and 4 years old) but not in 3-year-old children (Figure 1–2). Our findings are consistent with patterns reported by Chen et al.³⁵ where blood lead concentrations at 7 years had a direct effect on concurrent behavioral problems, while blood lead concentration at 2 years was not associated with behavior at age 7 years. Similar effects were reported by Hornung et al.⁴²: blood lead concentrations at age 6 years were more strongly associated with cognitive and behavioral development than those measured in early childhood. Adult gray matter brain volume reduction is also more highly associated with blood lead concentrations at 5 and 6 years compared to those measured at earlier ages.⁴⁵ Taken together, these findings suggest that relatively high blood lead concentrations beyond infancy and toddlerhood can have detrimental effects on behavior.

Gender, blood lead concentrations, and behavior

Though males in this study had higher mean blood lead concentrations than females (Table 2), the association with behavior was more consistent in females when taking into account the time that had elapsed since blood lead measurement (Figures 1 and 2). Separate logistic regression analyses by gender also showed stronger associations between blood lead concentration and increased risk for behavioral problems for girls (Table 4). Results of other studies examining cognitive and behavioral effects of lead exposure between genders have been mixed.^7,14,45–48 Nevertheless, few studies have specifically examined the effects of blood lead concentration on behavior by age and gender or they have treated sex/gender only as a covariate. Our findings could partially reflect biological and sociological differences between genders, such as different sensitivities to lead and its effects on the development of particular brain areas.^49–50 For instance, animal models suggest that whereas lead exposure may alter hippocampal glial metabolism and induce behavioral alterations of hyperactivity in male rats,⁵¹ female rats exposed to lead may have attenuated hippocampus potentiation⁵² and express depressive-like behavior not seen in male counterparts.⁵³ More studies on susceptibility to environmental toxicants between genders are warranted.

Blood lead levels and internalizing behavior problems

Previous research has reported associations between blood lead concentration and externalizing problems in children and adolescents, including attention-related^3,6,12,54 and conduct problems.^7–9,37 Our spline model also indicate that increased blood lead concentration is associated with increased risk for externalizing behavior score in boys at age 5 years (Figure 2). Our logistic regression analysis, however, show significant relationships between blood lead concentration and the internalizing problems of anxiety and depression. Our findings extend those from Liu et al.⁵⁵ that in young Chinese children, adverse outcomes of child temperament, including withdrawal and mood, were associated with lead exposure from electronic waste recycling. Our findings also support past findings of significant problems with anxiety, emotion regulation, and frustration tolerance in lead-exposed toddlers,⁵ as well as anxiety and emotional problems in young children.^14,56 Indeed, recent experimental animal studies suggest that lead exposure may induce anxiety and withdrawal by altering the hypothalamic-pituitary axis and neurotransmitter systems.^57–58

In this study, increased blood lead concentration was associated with an increased risk of DSM-oriented pervasive developmental problems as measured by 13 items like Avoids eye contact, Can’t stand things out of place, Shows little affection, Speech problem, and Withdrawn²⁶ for both genders, though the association was stronger for girls. Pervasive developmental conditions are associated with the development of clinical psychiatric disorders such as autism, ^59–60 and degrees of severity in autism spectrum disorder are reportedly well-correlated with elevations in lead, mercury, and copper concentrations.⁶¹ However, our results should be interpreted with caution as we used only a relatively non-specific CBCL subscale rather than gold standard clinical diagnostic scales. In addition, we cannot conclude whether or not the difference in findings for internalizing compared to externalizing behavior can be contributed to cultural underpinnings.

Because behavior may be influenced the setting in which it occurs, such as the home or school, the use of multiple informants is valuable in assessing children’s behavioral and emotional problems.²⁸ For example, caregivers may be better at detecting disruptive problems, while teachers have been found to be better informants on hyperactivity/inattentiveness⁶² and internalizing problems in preschool children.⁶³ In the present study, we found blood lead concentration was significantly associated with teacher-reported behavioral problems, especially in internalizing and developmental problems, but not with parent-reported problems.One possible explanation is that teachers are exposed to relatively large groups of same-aged children and may be more sensitive to behavioral deviations than parents. This may be particularly relevant in China, where most parents’ understanding of child behavior is limited to their only child due to the single-child policy. It is also possible that teachers’ perceptions of child behavior may be less susceptible to the influence of their own psychopathology and levels of stress, whereas this is not necessarily true for parents.⁶⁴

Limitations

Several limitations of this study should be acknowledged. First, because we measured blood lead concentration once from 3–5 years of age, it is unclear if problems seen at 6 years of age reflect lead exposure at measurement, during the prenatal period, or during the first 2 years of life. Second, we measured behavior using teacher and parent reports rather than clinical diagnosis. Third, although we adjusted for parental education, work, and residence, there may be other cofounders we did not control for that affected teacher report of behavioral problems. Finally, we did not assess behavioral problems at the time blood lead concentrations were measured: it is thus possible that behaviors associated with developmental disorders (e.g. pica) contribute to elevated blood lead concentrations.^61,65

Conclusion and implications

Childhood lead poisoning is well documented and persists as a major, yet preventable, global public health problem. Lead-related health problems cost $43.4 billion annually in the United States.⁶⁶ Lead exposure remains common in Chinese children,^67–68 and nearly one-quarter of children in China have blood lead concentrations >10 µg/dL, with a relatively high mean of 8.1 µg/dL.⁶⁹ The comparatively low mean blood lead concentration (6 µg/dL) in our sample, which is from the small city of Jintan, may be due to better environmental conditions in China’s smaller cities than in its larger metropolises.⁷⁰

Our findings show that frequently encountered blood lead concentrations in preschool children are associated with increased behavioral problems, especially internalizing problems in children at 6 years. To our knowledge this is one of the first studies to comprehensively examine the relationship between blood lead concentrations and behavioral problems in a large sample of young, Chinese children. This study contributes further evidence that blood lead concentrations even below the CDC’s previously defined level of concern, <10 µg/dL, are harmful^71–72 and that exposures now commonly encountered in China may have implications for children’s behavioral developments. As such, continued monitoring of blood lead levels as well as clinical assessments of mental behavior during regular pediatric visits may be warranted. Further examination is needed to more clearly delineate the biological effects of environmental lead exposure and resulting behavioral impairments among children and to assess the long-term clinical significance of these findings.

Abbreviations

CBCL

Child behavior checklist

CI

Confidence Interval

C-TRF

Caregiver-Teacher Report Form

DSM

Diagnostic and Statistical Manual

IQ

Intelligence Quotient

OR

Odds ratio

WPPSI-R

Wechsler Preschool and Primary Scale of Intelligence-Revised