Estimation by a 24-hour Study of the Daily Dose of Intra-oral Mercury Vapor Inhaled after Release from Dental Amalgam

Abstract

The difficulties associated with estimations of daily doses of inhaled mercury vapor released from dental amalgam are considerable. Existing data are often unreliable, especially if they are based on a single or a small series of samples of intra-oral concentrations of mercury vapor before, during, and after chewing stimulation. In the present paper, the aim was to obtain a more representative estimation of the daily dose of mercury vapor inhaled from amalgam fillings by measurement of amounts of mercury vapor released in the oral cavity during 24 h, under conditions that were as normal as possible. A series of measurements was carried out on each of 15 subjects, with at least nine occlusal surfaces restored with dental amalgam, and on five subjects without any amalgam restorations. The subjects had to follow a standardized schedule for 24 h, whereby they ate, drank, and brushed their teeth at pre-determined time periods. The amount of mercury vapor released per time unit was measured at intervals of 30-45 min by means of a measuring system based on atomic absorption spectrophotometry. None of the subjects was professionally exposed to mercury, and all of their amalgam fillings were more than one year old. Study casts were made for each subject, and the area of the amalgam surfaces was measured. Samples of urine and saliva were analyzed so that values for the mercury concentrations and the rate of release of mercury into saliva could be obtained. The average frequency of fish meals per month was noted. The daily release of elemental mercury from dental amalgam was corrected for retention of inspired mercury vapor and for oral-to-nasal breathing ratio. The estimated average daily dose of mercury vapor inhaled from the amalgam restorations was 1.7 μg, i.e., about 1% of the dose obtained from a TLV exposure of 50 μg Hg/m3 air. The threshold limit value (TLV) of a substance is the airborne concentration to which nearly all workers can be exposed eight hours a day, five days a week for prolonged periods without suffering adverse health effects (American Conference of Government Industrial Hygienists: Threshold Limit Values and Biological Exposure Indices for 1985-86, Cincinnati, 1985).