Report for Working Group I: Strategies for Improving the Assessment of Fluoride Accumulation in Body Fluids and Tissues

There are several tissues or fluids that may be used to determine the amount of fluoride in the various compartments of the body. Some are accessible and useful for determining the fluoride currently available in the body fluids. These measurements do not indicate directly fluoride accumulation in the body, but they are to some extent related to the body burden because of an incompletely defined relation between fluoride concentrations in bone and the extracellular fluids. These fluids include urine, plasma, and ductal saliva. Salivary fluoride is related to the concentration in plasma by a factor of about 0.8. Therefore, plasma fluoride can be closely approximated by dividing the salivary concentration by 0.8. Samples taken from fasting subjects are the most meaningful, because the fluoride concentrations in these fluids are influenced significantly by intake during recent hours. Urinary fluoride concentrations are also related to those of plasma, but they are much higher and more variable than those in ductal saliva because of variations in urinary flow and pH. The concentrations of fluoride in nails and hair appear to be proportional to intake over longer periods of time. To the extent that they are, their concentrations reflect the average plasma fluoride concentrations over time. Nails grow at about 0.1 mm/day, so the average level of fluoride intake over a 1-3-week period could be estimated. Hair fluoride could be used to estimate intake for longer periods. There is, however, a lack of sufficient data about the best preparatory methods to eliminate environmental fluoride contamination of these tissues. Further, the development of optimal analytical procedures probably requires attention. Additional research is needed to clarify these potentially important problems as well as the physiological factors that can influence fluoride uptake and accumulation in these tissues. The body burden of fluoride is best reflected in the calcified tissues. Enamel is not the tissue of choice, because most of the fluoride therein is taken up during tooth formation. After eruption, exposure to widely fluctuating concentrations of fluoride in the oral cavity significantly affects fluoride levels in the surface layers of enamel (about 100-200 jjum), where the highest concentrations of fluoride are found. Bone fluoride concentrations are much better indicators of long-term fluoride exposure and body burden. It is known, however, that fluoride is not uniformly distributed throughout bone. In cortical bone, the concentrations are higher at the periosteal and endosteal surfaces than in the interior regions. Also, cancellous bone has higher fluoride concentrations than does cortical bone. Some reports have suggested that the fluoride concentrations of dentin are similar to those of bone, and, as in bone, they tend to increase over the years, provided that fluoride intake does not decline. Dentin, especially coronal dentin, may be the best marker for the estimation of chronic fluoride intake and the most suitable indicator of the body burden. The tissue does not normally resorb, it is more easily obtained than bone, it seems to continue accumulating fluoride slowly throughout life, and it is permeated by extracellular fluid. Dentin is usually protected from exposure to fluoride in the oral cavity by the covering enamel or cementum. Suitable methods for the accurate determination of fluoride in all of these body fluids and tissues, except as noted previously for nails and hair, are available.