Manganese exposure in the workplace is known to have neurotoxic effects, but manganese occurs naturally in soil and sometimes in groundwater. One region where the groundwater contains naturally high levels of manganese is Quebec. A study involving 362 Quebec children, aged 6-13, has measured both the concentrations of metals (manganese, iron, copper, lead, zinc, arsenic, magnesium and calcium) in their tap water, and their cognitive abilities.
Although manganese concentrations were well below current guidelines, the average IQ of those whose tap water was in the upper 20% was 6.2 points below children whose water contained little or no manganese. The association was more marked for Performance IQ than Verbal IQ (Performance IQ reflects perceptual organization and processing speed). The analysis took into account factors such as family income, maternal intelligence, maternal education, and the presence of other metals in the water. No association was found between manganese in their food and IQ.
(2010). Intellectual Impairment in School-Age Children Exposed to Manganese from Drinking Water.
Environmental Health Perspectives.