Modern dentures are typically made from acrylic plastic, but at least until the 1980s, porcelain was also very commonly used. In the 1940s, manufacturers began adding uranium to the porcelain powder used to make dentures. The idea was that the fluorescence of the uranium would help mimic the look of real teeth under a variety of natural and artificial light conditions. Uranium had the advantage over some of the alternative materials because its fluorescence is unaffected by the high temperatures (800 – 1400 degrees centigrade) used to bake the porcelain. According to NCRP 95, it seems that manufacturers had stopped adding uranium to porcelain dentures by 1986 or so. Papastefanou et al (1987) measured uranium levels in 22 samples of porcelain powders used in Greece and found that the concentrations ranged from 3.6 Bq/kg (0.1 pC/g) to 5600 Bq/kg (151 pCi/g). A more limited study in 1977 by Binney and Scherpelz indicated an average uranium concentration of 224 ppm (2,760 Bq/kg or 70 pCi/g). In England , O’Riordan and Hunt estimated that the average concentration in the porcelain powders might be as high as 5,060 Bq/kg (136 pCi/g). Sairenji et al measured uranium concentrations in Japanese dental powders ranging from 4.56 Bq/kg (0.12 pCi/g) to 1,000 Bq/kg (27 pCi/g). Note that the naturally occurring potassium-40 in these dentures can contribute significantly to their overall radioactivity. It is likely that the dose to the oral mucosa as a result of the uranium in dentures is primarily due to the beta particles emitted by the decay products of U-238: Th-234 and Pa-234m. The contribution of the alpha particles to the dose is hard to predict due to the tremendous uncertainties in their attenuation. One investigation in Great Britain (O’Riordan and Hunt 1974) estimated that a set of porcelain dentures containing 0.1 % uranium might deliver an annual dose to the oral mucosa of 600 rem by the emitted alphas and 2.8 rem by the beta particles. A 1987 study by Papastefanou et al calculated a similar dose: 400 rem per year. Don Thompson, with the US FDA, calculated a dose of approximately 130 rem per year to the oral mucosa due to the alpha emissions from dentures with a uranium concentration of 0.044 %. NCRP Report No. 95 estimated that the annual beta dose to the basal cells of the mucosa (the most sensitive portion of the oral mucosa) was 0.7 rem. This dose was due to the betas emitted by potassium-40 as well as the decay products of the uranium. When they factored in the percent of the population that did not have uranium-containing dental prostheses, the NCRP estimated that the average beta dose to the basal cells of the mucosa in the US population was approximately 0.13 rem. To permit a calculation of the effective dose equivalent, the NCRP also employed a weighting factor of 0.0001 for the basal cells of the mucosa. The pertinent Nuclear Regulatory Commission regulations (2003), found in 10 CFR 40.13a are as follows: "40.13 Unimportant quantities of source material. (a) Any person is exempt from the regulations in this part and from the requirements for a license set forth in section 62 of the Act to the extent that such person receives, possesses, uses, transfers or delivers source material in any chemical mixture, compound, solution, or alloy in which the source material is by weight less than one-twentieth of 1 percent (0.05 percent) of the mixture, compound, solution or alloy." This limit of 0.05 % (500 ppm uranium) equates to approximately 167 pCi/g or 6,170 Bq/kg. References Binney, S. and Scherpelz, R. Technique for Rapid Analysis of Uranium in Porcelain Dentures. Health Physics 33: 341-343; 1977 Dietz, C. US Patent Number 301,174; 1942. NCRP. Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources. NCRP Report No. 95; 1987. Papastefanou, C., Vitsentzos, S. and Garefis, P. Uranium in Dental Porcelain Powders and Dose Induced in Oral Mucosa. Radiation Protection Dosimetry 19(1): 49-53, 1987. Thompson, D. Recommendations on the Use of Uranium in Porcelain Teeth. in NUREG/CP-0001, Radioactivity in Consumer Products. August 1978.