The health risk of putting manganese in drinking water is that too much exposure to manganese mainly affects the nervous system by slowing movement and causing mood and behavioral changes, respiratory irritation and problems with the reproductive system, according to the Agency for Toxic Substances and Disease Registry. There is also evidence that manganese impairs brain development in children. Both nervous and reproductive damage occur in animals after ingestion.
Manganese occurs naturally in many rocks and is a trace element in the environment, says the Agency for Toxic Substances and Disease Registry. Small amounts of manganese in food or water are actually essential to human health; however, overexposure to manganese has health risks, mainly neurological.
In 2010, researchers from the University of Quebec assessed children between the ages of 6 and 13 to determine the effect of manganese on brain development, reports ScienceDaily. The researchers measured the amount of manganese and other metals present in the children's drinking water and tested them in cognition, motor skills and behavior. Children's scores on intelligence tests suffered at higher manganese concentrations, with children exposed to higher levels of manganese averaging six points lower on IQ tests than children with little to no manganese exposure.