The effect of exposure to environmental manganese on the neurobehavioural function of children in Meyerton, South Africa

Abstract

Background Manganese (Mn) is a heavy metal that occurs naturally. Humans are exposed to Mn from a variety of sources. The health effects of environmental exposure to Mn at lower than occupational exposure levels have been the focus of recent research in adults, but only a few studies have investigated the effects in children. No studies in children have been conducted in South Africa where 80% of the world’s Mn reserves are located.

Objective The objective of this research was to investigate the association between exposure to low levels of ambient Mn and neurodevelopmental outcomes, viz. cognitive- and motor-development, in children living and being schooled near a ferro-manganesesmelter in the Midvaal area in Gauteng province, South Africa.

Methods In this cross-sectional study, 204 Grade R to Grade 4 learners were randomly selected from all those registered at Sicelo Primary School in the town of Meyerton. An exposure model developed in a recent study in adults in the same area was used to estimate levels of Mn exposure. Data were collected using questionnaires, and included demographic characteristics, developmental milestones, and residential information. Motor and cognitive skills were assessed using a paediatric balance scale, grooved pegboard, childrens’ memory scale and the Wechsler Intelligence Scale for Children. The study participants were divided into three exposure groups (low, moderate and high) and the outcomes were compared amongst them. Categorical variables were analysed using Fisher’s exact test. Continuous variables were analysed using the Kruskal-Wallis test. Associations were measured using multivariable linear regression analysis.

Results Ninety-one children participated in the study (response rate of 44.6%). The demographic characteristics of the three exposure groups were homogenous - all participants were Black Africans and there were no statistical differences between them regarding home language, grade, age and sex. There were statistically significant differences between the groups regarding mothers’ occupation (p=0.002), fathers’ occupation (p=0.008), duration of pregnancy (p=0.020), birth weight (p=0.045), being breastfed (p=0.003), and achieving developmental milestones (p=0.015). The Mn normalised 4-year mean Mn levels used to categorise the participants into exposure groups were also significantly different between the groups (p=0.001). The mean scores per section for all the cognitive tests were below the standard mean of 100 ± 15. There was a positive relationship between the time taken to complete the grooved pegboard test and Mn exposure group, and a negative relationship between cognitive outcomes and exposure group. The β coefficients from the multivariable linear regression models for motor and cognitive outcomes indicated that there were no statistically significant associations between Mn exposure category and motor and cognitive development.

Conclusion This study laid a foundation in neurodevelopmental testing of children in an Mn exposed community. The findings are in line with those from previous similar studies, in that no significant associations between exposure and either motor or cognitive outcomes were identified. Further research, using larger sample sizes, and unexposed comparison groups should be conducted.