An IQ comparison shows that Canadian regulations on manganese in drinking water should be updated to protect children, Quebec researchers say.

The average IQ of children whose tap water was in the upper 20 per cent of manganese concentration was six points below children whose water contained little or no manganese, the researchers found.

National and international guidelines for safe manganese levels in water should be revised, Quebec researchers say. (CBC)

The study looked at 362 children aged six to 13. The amount of manganese from tap water and food was estimated, based on the results of a questionnaire.

Manganese is a naturally occurring metal found in groundwater. It is an essential nutrient, but in excessive amounts, it can damage the nervous system. It occurs in naturally high levels in several parts of Quebec, New Brunswick and other regions, researchers say.

Their study, published in Monday's online issue of the journal Environmental Health Perspectives, focused on manganese levels in drinking water in eight communities along the St. Lawrence River between Montreal and Quebec City.

All the towns chosen for the study rely on ground water for drinking, either from municipal or private wells that aren't specially treated for manganese.

"I studied mercury, lead, PCBs, and usually the associations we see are more in the range of one IQ point, two, maybe three IQ points," said the study's lead author, Maryse Bouchard of the University of Quebec at Montreal's Centre for Interdisciplinary Studies in Biology, Health, Society and Environment.

"But this is a very strong effect," she added in an interview.

The manganese concentrations were well below current guidelines, the researchers said.

Cognition, motor skills

The towns were chosen because of the high concentrations of naturally occurring manganese in the area.

Each child was also assessed with a range of tests of cognition, motor skills and behaviour.

Factors such as family income, maternal intelligence, maternal education and the presence of other metals in the water were taken into account.

"The findings from the present study support the hypothesis that low-level, chronic exposure to manganese from drinking water is associated with significant intellectual impairments in children," the study's authors concluded.

"Because of the common occurrence of this metal in drinking water and the observed effects at low manganese concentration in water, we believe that national and international guidelines for safe manganese in water should be revisited."

The researchers also called for the findings to be tested in another population.

Manganese is not on the list of inorganic substances in the Quebec Environment Ministry's drinking water standards.

The Canadian Institutes for Health Research funded the study.