Silver-coloured dental fillings containing mercury, a neurotoxin, do not appear to cause brain damage in children, two studies suggest.

Dentists have used amalgam fillings containing 50 per cent mercury by weight for more than a century.

Use of amalgam fillings has dropped off after mercury-free white composite resin fillings came on the market.

The silver amalgam contains mercury, silver, tin, copper and zinc.

More sophisticated testing developed over the years can now detect lower levels of mercury, raising concerns about health effects from continuous exposure to mercury vapours from fillings.

One research team in the U.S. and another in Portugal compared the neurological effects of giving children amalgam fillings versus composite ones.

Dr. Sonja McKinlay of the New England Research Institutes in Boston looked for changes in intelligence, memory and other mental functions, as well as kidney function, in 534 New England children ages 6 to 10.

The youngsters all had untreated decay in their permanent back teeth, and none had previously received amalgam fillings. Half received each type of filling, and they were followed up for about five years.

"In this study, there were no statistically significant differences in adverse neuropsychological or renal [kidney] effects observed over the five-year period in children whose caries [cavities] were restored using dental amalgam or composite materials," the study concluded.

The Portugese team found similar results after conducting tests of memory and attention in 507 children aged eight to 10, who were followed for seven years.

"These findings ... suggest that amalgam should remain a viable clinical option in dental-restorative treatment."

Long-term effects unknown

Drawing similar conclusions on the safety of amalgam from two large, geographically separate groups of children supports beliefs that mercury is safe, said Dr. Lawrence Tabak, director of the U.S. government-funded institute that sponsored the New England study.

However, people shouldn't conclude that mercury amalgam in dentistry is risk-free, cautioned Dr. Herbert Needleman in a related commentary also appearing in Wednesday's edition of The Journal of the American Medical Association.

Effects of early toxic exposure on health later in life are not addressed by the studies, said Needleman, a professor in the department of psychiatry at the University of Pittsburgh School of Medicine.

Smaller effects may also be undetected, but could be meaningful if mercury had an adverse impact in just one per cent of children with amalgam fillings, he said.

Two anti-amalgam groups in the U.S. said they are filing complaints against Harvard University and the University of Washington where the research was conducted, calling it unethical to experiment on children.

Authors of both studies said participants were told about the purpose of the research. The children were given standard dental treatment, added Dr. Timothy DeRouen, a University of Washington professor of biostatistics and dental public health sciences who led the study on Portugese children.