Influence Lower Level Heavy Metal Exposure on Childhood Behaviour

The participants were drawn from the ongoing Environmental Exposures and Child Health Outcomes (EECHO) study, a research project investigating the relationships among exposures to environmental toxicants and cardiovascular risk indices in 9–11 year old children living in low- to middle-income neighborhoods in a midsize city in upstate New York.

As part of a more extensive blood draw protocol, a certified phlebotomist drew 5-mL venous blood into a plastic lavender-top (EDTA), certified by the analyzing laboratory for measurement of blood levels of Pb and (total) Hg concentrations. Blood specimens were immediately placed on ice and within 2 hours of the blood draw the samples were transferred into 5-mL cryovials (certified by the analyzing laboratory) and frozen at −80°C pending shipment to the trace elements section of the Laboratory for Inorganic and Nuclear Chemistry at the New York State Department of Health’s Wadsworth Center, Albany, NY.

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Children were selected from this study from a larger, ongoing research project that investigates the relationship between environmental exposures and children’s health, focusing on 9-11 year-olds of lower to middle socioeconomic brackets from New York state. A trained professional drew a small amount of blood from each child using a needle, collected in special tubes designed to preserve the blood for measuring the presence of heavy metals. The samples were kept cold and then eventually frozen to further preserve them, before being sent to a state health laboratory that had the equipment and expertise to measure mercury and lead levels.

Effects of Pb exposure also have been examined for behaviors such as delinquency; however, much of this work has focused on children with identified exposure routes (e.g., proximity to a battery factory) and thereby higher Pb levels.

More needs to be learned about possible consequences of high normal Pb exposure. Exposure at these low levels is of particular importance to public health, as high normal exposures would affect a much larger proportion of children.

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According to the authors, past studies on behavioural changes in relation to heavy metal exposure were only conducted on children whose exposure routes were known and high level, prompting medical investigations. In this study, they planned to do a more general assessment of children at lower levels of heavy metals to see if behavioural changes were still noteworthy.

After controlling covariates, these associations remained significant with increasing blood Pb associated with significantly higher levels of hostile distrust (t (31) = 3.10, p = 0.004) and increasing total hostility (t (31) = 2.39, p = 0.04), as reported by the child. In the analysis of parent-reported disruptive behaviors (using scores rather than clinical cut-points), increasing blood Pb was associated with increasing ODD behaviors in the model without covariates (t (34) = 2.16, p = .04) as well as with covariates (t (30) = 2.20, p = 0.03; see Figure 1). Increasing Pb was also associated with significantly greater ADHD-hyperactivity in the base model (t (34) = 2.31, p < 0.05) but was no longer significant after controlling for covariates (t (30) = 1.50, p > 0.10).

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After taking statistical factors into account, higher lead levels in the children’s blood were linked to higher levels of suspicious, mistrustful attitudes and greater overall hostility, based on what the children reported. When looking at behaviour problems reported by parents, higher lead levels were linked to more oppositional and defiant behaviours. Higher lead levels were also linked to more hyperactive ADHD-type behaviours at first, but once other influencing factors were considered, the link was no longer strong enough to be considered meaningful from a statistics standpoint.

The present study suggest that harmful neuro-behavioral effects of blood Pb may occur at levels of exposure that are significantly lower than the levels usually studied, and that they may be detected across the different physiological, psychological, and social regulatory systems that underlie and enable competent everyday functioning. The pattern of significant associations observed here suggests that Pb is significantly associated with impairments in a child’s ability to regulate emotions, form trusting and supportive interpersonal relationships, and regulate aggressive and oppositional behaviors that undermine adjustment in family, neighborhood, and school settings.

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The study contributed affirming evidence to the hypothesis that even lower levels of heavy metals can have significant impacts on childhood behaviour.

The study had a small sample size of 203 children, and some of the findings which were talked about in the discussion section did not align with previous studies about mercury, lead, and childhood developmental impairment. This could suggest that either lower mercury levels don’t cause developmental problems, or such problems were not captured in the data. A future repeat study could benefit from a much larger cohort of children to increase statistical significance.

Because this study was cross-sectional, a snapshot in time, a future study focusing on longitudinal research could help delineate behavioural issues that may simply be a product of the child’s development or if there really could be a heavy metal factor. Cross-sectional research is traditionally not great at proving causation.

Many of the terminologies related to statistics were difficult to grasp and integrate into my overall understanding of the paper. These were mainly mentioned in the methods section. Instead, I had to “read between the lines” and trust that the authors did their due diligence determining the significance of the data. Although I looked up the definitions of many of the statistics terms, I could have benefited from someone with higher level statistics knowledge to give me an executive summary of the measures.