Dioxin can harm tooth development.

In the early 1980s, a Finnish dentist noticed that an unusually large share of her young patients had soft, discolored molars. Because the affected teeth had emerged bearing structural defects, she suspected that during infancy, when the teeth were forming beneath the gums, the children had been exposed to some toxic compound.

The culprit now appears to be breast milk tainted with dioxins, says Satu Alaluusua of the University of Helsinki Institute of Dentistry. She explained her team's findings in an interview this week.

Several groups of epidemiologists worldwide have reported a similar yellowish-brown discoloration of teeth in children exposed to extremely high concentrations of dioxinlike compounds. To probe whether the defects that she was observing might reflect a more moderate exposure to the same pollutants, Alaluusua began exposing adult rats to TCDD, the most potent dioxin. "We saw a similar mottling of teeth," as well as malformations in their mineral structure, she told Science News.

To affect human molars, exposures must begin in early childhood. So, Alaluusua tracked down children whose mothers' breast milk had been analyzed 6 years earlier in a World Health Organization study. She correlated the incidence of the defects in children's newly emerging molars with milk concentrations of dioxins and furans, a class of dioxinlike compounds. Overall, 17 of the 102 children studied bore soft, mottled teeth, which will remain permanently vulnerable to cavities.

In the Jan. 16 Lancet, Alaluusua's team reports that children who had encountered the most dioxin in their mother's milk had the highest rate of these tooth defects. Breast-milk exposure to polychlorinated biphenyls, another group of dioxinlike chemicals, played little if any role.

The group has also homed in on dioxin's target. The pollutant affects cellular receptors for epidermal growth factor (EGF), a hormonelike substance that contributes to development of many tissues.

The researchers removed embryonic tooth tissue from normal mice and from those lacking active genes for the EGF receptor. After exposure to high concentrations of TCDD, only the tissue able to produce EGF receptors matured into structures with the telltale defects, they reported in the December 1998 Laboratory Investigation.

The Finnish team's new data "are very exciting in a scientific sense--and very concerning in a public-health sense--because they demonstrate effects from [dioxin] exposures at background levels," says Linda Birnbaum, a toxicologist with the Environmental Protection Agency in Research Triangle Park, N.C.

Indeed, she notes, the average 50 parts per trillion (ppt) dioxin concentration--and the maximum of 258 ppt--that the team measured in the fat of Finnish women's breast milk are within the range at which dioxin taints fat, including that in breast milk, in the U.S. population.

Moreover, Birnbaum notes, the EGF-receptor link to tooth defects "is perfectly plausible" given that a number of studies--including some of her own--have shown that dioxin can alter the number of EGF receptors in various developing tissues, including the tooth buds of experimental animals.