Manganese is a metallic element that gets into water after contact with rocks and minerals in the ground. It's a natural compound and is also found in low levels in foods such as green vegetables, tea and cereals.

The levels in the UK have been investigated a number of times, but in most research the levels found in drinking water have been judged to be too small to have an effect on public health.

When high levels of manganese are breathed in rather than drunk, however, it can have a disastrous effect. Occupational health researchers have found that when miners and battery workers inhale manganese, it can lead to manganism, a condition similar to Parkinson's disease. Symptoms of this condition can include lethargy, tremors, mental disturbances, and even death.

Researchers at Wake Forest University School of Medicine say that while agencies have worked out safe regulatory levels for eating, drinking and inhaling manganese, no one has looked at the effects on the central nervous system from inhaling aerosols while showering with manganese-contaminated water.

"At first glance, this may seem to be a trivial delivery vector,'' they say. "Nonetheless, extrapolating animal data suggests that it may actually be a serious public health consideration.''

They say that compared to eating and drinking, inhaling is far more effective at delivering manganese to the brain. The report says that any manganese that does get through to the brain may have a cumulative effect, and it's suggested that some groups, including the elderly, pregnant women and people with mineral deficiencies such as anaemia are at increased risk from absorbed manganese.