Hg Emission from Dental Amalgam as Related to the Amount of Sn in the Ag-Hg (γ1) Phase

Abstract

Previous studies have shown that: (1) different dental amalgams emit different amounts of Hg vapor after surface films have been removed by abrasion (Boyer, 1988); (2) Sn oxide may be the predominant factor in these surface films that inhibit Hg vapor loss (Okabe et al., 1989); and (3) Sn in the Ag-Hg (γ₁) phase may influence the vapor pressure of γ ₁ (Reynolds, 1974). The purpose of this study was to explore the relationship between Hg vapor emission and the Sn content of the Ag-Hg (γ₁) phase in dental amalgam. Eleven commercial amalgams were selected whose γ ₁ phases contain different amounts of Sn. Amalgam specimens were ground on 600-grit carborundum paper and immediately placed into an apparatus designed around a gold film Hg vapor detector. Hg vapor loss in air over a 30-minute period was determined, and the log₁₀ Hg loss/volume fraction γ ₁ was plotted vs. the Sn content of the γ₁ phase for the 11 test amalgams. A linear regression of these data, showing that of the higher the Sn content in γ₁, the lower the Hg vapor loss, produced a highly significant R² = 0.94 (p < 0.001). To examine for differences in vapor pressure among alloys, we eliminated the oxidation effect by conducting these same tests in an argon atmosphere. Although the Hg loss was significantly greater in argon than in air, the same differences among alloys were observed. Thus, from the standpoint of both vapor pressure and oxidation, the amount of Sn in the γ₁ phase of dental amalgam has a significant and specific influence on the potential for Hg vapor emission.