In childhood environmental and dietary exposure to toxic and non-toxic elements may affect development and growth. The elemental concentrations of scalp hair and toenails may reflect chemical uptake via the diet, or environment. Very little is known about the relationship between elemental concentrations in scalp hair and toenails and behavioural and growth characteristics of children in New Zealand.
The Pacific Islands Families (PIF) study has provided a unique opportunity for children of age six and nine years to explore such relationships. The overall aim of this research was to collect and analyse the scalp hair and toenails of Pacific children (and in some cases from their mothers) resident in South Auckland, New Zealand (as part of the PIF population study) for calcium, magnesium, iron, manganese, zinc, copper, selenium, iodine, cobalt, chromium, nickel, molybdenum, antimony, arsenic, aluminium, boron, mercury, lead and cadmium. Three questions were addressed: (1) what is the relationship between mercury in scalp hair, reported to originate from fish consumption to behavioural problems; (2) what is the relationship between mercury in toenails and behavioural characteristics and specific behavioural domains (particularly aggression, rule breaking, attention and social problems); and (3) in a population where three out of four children are defined as obese or overweight is there any relationship between elemental concentration in toenails and current body-size?
The first study was a nested case-control study which recruited children (with and without behavioural problems) and their mothers to study the effects of mercury through seafood diet and dental amalgams on child behaviour. Hair samples were collected from both mothers and their children as a biomarker for mercury exposure. The second and third studies were cross-sectional studies designed to explore the association of elements on behavioural problems as identified by mothers (using the child behaviour checklist) and body-size categories (using the International Obesity Task Force’s (IOTF) cut-off points). Toenail clippings were collected as a biomarker for elemental assessment for these studies. The biological measurements (scalp hair and toenail) were carried out using inductively coupled plasma mass spectrometry (ICP MS). The existing multi-disciplinary, longitudinal PIF information provided additional data on confounding factors and co-variants on these samples.
In the first study (n=92 mother and child pairs), almost 20% of both mothers (median hair: 0.32 µg/g mercury) and 18% of children (median hair: 0.43 µg/g mercury) exceeded the Environmental Protection Agency’s (EPA) threshold for mercury of 1 µg/g Hg but were lower than the World Health Organization’s (WHO) threshold of 1.6 µg/g Hg. There was no conclusive evidence on child behavioural problems and hair mercury concentrations. A direct correlation was observed between mothers and their children's hair mercury concentrations (r = 0.79 (95% CI 0.65, 0.88).
In the next study (n=278 children; 160 boys, 118 girls), 21% of children had toenail mercury concentrations above 1 µg/g Hg with girls having higher mercury concentrations (24%) than boys (18%). Aggressive behaviour and seafood diet was associated with toenail mercury exposure after adjusting for gender, ethnicity and income levels (OR: 2.15 95% CI 1.45, 3.18 p-value< 0.05; OR 1.38 95% CI 0.83, 1.2 p value <0.05, respectively).
Within the final study, three out of four children were defined as obese or overweight; however, no significant correlation was found between body-size categories and toenail elemental concentrations. The elemental interaction of selenium-mercury and zinc-copper had an association with the different body-size categories (p value 0.03; and p value 0.02, respectively). It was observed that the mean toenail selenium (0.35 µg/g Se), calcium (868 µg/g Ca) and zinc (129 µg/g Zn) concentrations were lower than the required optimal health concentrations for toenails (selenium 0.75 µg/g Se; calcium 900 µg/g Ca; zinc 160 µg/g Zn) amongst these children. Toenail mean lead (0.86 µg/g Pb), cadmium (0.21 µg/g Cd) and mercury (0.72 µg/g Hg) concentrations were also higher than the optimal health requirements. Ethnic differences in relation to toenail elemental concentrations were observed for manganese, cobalt, iron, chromium, antimony, aluminium, mercury, lead and cadmium. Boys had higher concentrations for calcium, magnesium, manganese, copper, zinc, iron, antimony arsenic, aluminium, boron and lead than girls. Toenail clippings are a better biomarker for elemental status within Pacific people than scalp hair samples.
Overall, this research contributes to the understanding of the elemental concentrations for Pacific people using scalp hair and toenail clippings as biomarkers in terms of associations with health outcomes (particularly child behavioural problems and body-size categories). Mercury in toenails demonstrated a moderate association with a specific behavioural domain – aggressive behaviour – while elemental interactions such as zinc-copper and mercury-selenium seemed to influence the body-size categories in these children even though single elements did not show any associations on body-size categories. Furthermore, some possible explanations for both the conclusive and inconclusive results that have so far emerged, and suggestions for potential ways forward in this area of research are discussed.