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Low-level Lead Exposure and Behavior

CITATION:  Mendelsohn, AL, Dreyer, BP, et al. (1998).  Low-Level Lead Exposure and Behavior in Early Childhood.  Pediatrics 1998;101(3).

ABSTRACT:

Objective. To assess whether small elevations in blood lead level were associated with measurable behavioral changes in a group of poor children between 1 and 3 years old.

Methods. The study population consisted of children presenting for routine well-child care to the pediatric clinic at Bellevue Hospital Center, a large urban public hospital. The following inclusion criteria were used for entry into the study: age 12 to 36 months; capillary lead screening result <1.21 mmol/L (25 mg/dL); no known prior history either of blood lead level >1.21 mmol/L (25 mg/dL) or lead exposure requiring chelation therapy; Latino or African-American; English or Spanish spoken in the home; biological mother as primary caretaker; child not presently attending day care; full-term, singleton gestation; birth weight at least 2500 g; no known neurologic or developmental disorder; and no severe chronic disease, including human immunodeficiency virus infection. Study enrollment was simultaneously stratified by capillary lead level and age.

All children between 12 and 36 months attending the pediatric clinic during the study period received screening capillary blood measures of lead level following the recommendations of the Centers for Disease Control and Prevention and the American Academy of Pediatrics as part of routine primary care. During periods of enrollment, consecutive lead measurements performed in the pediatric clinic were reviewed by one of the researchers.

For those children meeting entry criteria based on lead level and age, further eligibility based on the remainder of the inclusion criteria was determined through parental interview and review of the medical record.  Lead exposure was assessed with a single capillary blood specimen, using atomic absorption spectrophotometry.  Subjects were considered to be lead-exposed if their lead level was between 0.48 and 1.20 mmol/L (10 and 24.9 mg/dL) and nonexposed if their lead level was between 0 and 0.48 mmol/L (0 and 9.9 mg/dL).

Behavior was assessed using the Behavior Rating Scale (BRS) of the Bayley Scales of Infant Development, second edition. The BRS in this age group consists of three components: an Emotional Regulation Factor that measures that measures hyperactive/distractible/easy-frustration behaviors; an Orientation-Engagement Factor that measures fear/withdrawal/disinterest behaviors; and a Motor Quality Factor that assesses the appropriateness of movement and tone. The BRS is scored as a percentile; lower scores reflect more problematic behaviors. Researchers performing the BRS were blinded to capillary lead results. 

Information was collected concerning factors that might confound the relationship between lead and behavior. Demographic factors were collected, including: child’s age, gender, and country of origin; mother’s age, marital status, parity, country of origin, and primary language spoken; parental education, and occupation and receipt of public assistance. Socioeconomic status was determined using the Hollingshead Two-Factor Index of Social Position. Maternal verbal IQ was assessed using the Peabody Picture Vocabulary Test-Revised. Maternal depression was assessed using the Center for Epidemiologic Studies-Depression Scale. Cognitive stimulation provided in the home was assessed using a new officebased instrument, the StimQ, which measures the quantity and quality of play materials and parent-toddler activities in the child’s home. To assess the child for iron deficiency, we performed a hematocrit and mean corpuscular volume at the time of the capillary lead evaluation. A presumptive diagnosis of iron deficiency was made if the child was either anemic (defined as a hematocrit <32) or had a mean corpuscular volume <72.

Results. The study sample consisted of 72 children. Children in the lead-exposed group (n 5 41) had a mean BRS behavior score that was 15.8 points lower than that of children in the nonexposed group (n 5 31), which was significant by the Student’s t test. For the emotional regulation factor measuring hyperactive/impulsive/easy frustration behaviors, children in the exposed group had a mean score that was 14.6 points lower than that of the nonexposed group, which was significant by the Student’s t test. For the orientation-engagement factor measuring fear/withdrawal/disinterest behaviors, children in the exposed group had a mean score that was 14.1 points lower, significant by the Student’s t test.

Multiple linear regression analyses were used to examine the independent relationship between BRS (total and factor scores) and lead group, after adjusting for potential confounders. Six variables were related to either lead group or BRS behavior score in unadjusted analysis and were, therefore, included as potential confounders in each of the multiple regressions: child’s age and gender, and mother’s age, verbal IQ, depression score, and provision of cognitive stimulation.

In the analysis of the relationship between the BRS total score and lead group, the adjusted mean BRS behavior score in the exposed group was 17.3 points (95% confidence [CI]: 3.3, 31.3) lower than that of children in the nonexposed group (sr 5 -0.27). In the analysis of the relationship between the emotional regulation factor and lead group, the adjusted mean factor score in the exposed group was 16.6 points (95% CI: 2.1, 31.2) lower than that for the nonexposed group (sr 5 -0.25). In the analysis of the relationship between the orientation-engagement factor and lead group, the exposed group had an adjusted mean score that was 14.2 points (95% CI: 22.1, 30.5) lower than that for the nonexposed group (sr 5 -0.20). In these multiple regression analyses, mother’s depression score was significantly associated with a lower total BRS score (sr 5 -0.25) and with lower emotional regulation factor (sr 5 -0.23). Older children had higher BRS scores (sr 5 0.20), and had significantly higher emotional regulation factor scores (sr 5 0.22). A relationship was observed between male gender and lower emotional regulation scores that did not reach significance (sr 5 -0.21). Iron deficiency, cognitive stimulation provided in the home and mother’s verbal IQ were not related to any measures of behavior.

Conclusions. Low-level lead exposure is associated with adverse behavioral changes in very young preschool children. This association may be particularly important for poor children, who are also at risk for behavior problems on the basis of other environmental factors such as maternal depression. Clinicians should consider screening for behavioral problems in very young children with low-level lead exposure.

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