was the predominant source of mercury uptake in fish. A major path of human exposure to mercury is from eating fish from contaminated water bodies. The ability to model the pathways of mercury from the source of emissions to human uptake is constrained at present by the difficulty of calculating the interconversion of the elemental mercury and mercuric ion in the atmosphere (which determines the atmospheric lifetime of the mercury), and of determining the rate of methylation of mercury (which determines its uptake by the biota).
Because of the long range transport of mercury, regional average concentrations are uniform within a factor of two to three. The values reported for Wisconsin by Fitzgerald et al. (1991) are representative of continental values. These are a gas-phase concentration of 1.57 ng/m3 and a particulate concentration of 0.022 ng/m3. Mean values for the air over a forested watershed in Tennessee were 5.5 ng/m3 (Schroeder and Fanaki 1988), with the corresponding particle bound concentrations of 0.03 ng/m3. The particle-bound values can be seen to be less than 1% of the vapor values. However, particulate-mercury concentrations are greater in precipitation than in ambient air (ATSDR 1999).
Values are reported for concentration in rain of 10.3 ng/L, and a wet deposition rate of 6.8 µg/m2-year by Fitzgerald et al. (1991) for Wisconsin. Values reported by Glass et al. (1991) of 18 ng/L for rain and 15 µg/m2-year for Minnesota are within a factor of a little more than two of those reported in Wisconsin, and are supportive of the view that the volatile elements are uniformly distributed over wide areas.
Concentrations in freshwater fish are typically in the range of 0.1-1 µg/g fish. For example, trout from Lake Ontario had average values that declined from 0.24 µg/g in 1977 to 0.12 µg/g in 1988 (Borgmann and Whittle 1991). Fish from the Savannah River had concentrations of 0.10 to 0.72 µg/g (Winger et al. 1990). Although the concentration in fish and the water do not always correlate well because of interfering factors (such as age of the fish, pH, and the different bioavailability of various forms of mercury) the concentration in fish is of the order of a million times that in the water. The biological concentration factor of methylmercury in fish in a freshwater lake was 3 million L/kg (Porcella 1994).
Potential sources of general population exposure to mercury include inhalation, ingestion of drinking water and foodstuffs, and exposure through dental and medical treatments. Food, particularly fish consumption, is the major environmental path of exposure for mercury. Studies of the dietary intake conducted by the Food and Drug Administration show an average daily intake for adults of 0.03 µg/kg of daily weight remarkably independent of age and sex, or 2.1 µg/day for a 70-kg adult (Cramer 1994). Using a terrestrial food chain model, Travis and Blaylock (1992) estimated an average daily intake of 6.3 µg/day for adults with over 50% coming from fish intake. Other studies of dietary intake of mercury are presented in ATSDR (1999).