revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 466
5. POTENTIAL FOR HUMAN EXPOSURE
In addition, adults may receive potentially higher oral exposures from ingestion of mercury-contaminated
soils from their unwashed hands while working in mercury-contaminated areas. Bioavailability is an integral
factor in the estimation of the internal dose (or dose at the target tissue) of the chemical. Like dermal
absorption, gastrointestinal absorption of various forms of mercury is highly variable (see Section 2.3.1).
The more lipid soluble organic mercury compounds (e.g., methylmercury) are almost completely absorbed,
while the extremely insoluble metallic mercury is poorly absorbed through the gut. The bioavailability of
mercury from soil is likely to vary, since mercury binds tightly to soil, especially to soils with high organic
content. Therefore, the mercury soil concentration alone may not be indicative of the potential for human
health hazard from contaminated soils, and site-specific evaluation of the bioavailability of the various forms
of mercury at the site is essential. However, unless toxicokinetic studies that use soil samples from the
specific site are available, it is difficult to speculate on how much mercury will be bioavailable at any
particular site. Adults may also receive higher doses from routine consumption of mercury-contaminated
home grown fruits and vegetables (Nublein et al. 1995), and from consumption of fish from local waters
receiving runoff or leachate from a waste site. Harnly et al. (1997) studied the impact of inorganic mercury
in soil and dust and organic mercury in fish on a Native American population living near an inactive mercury
mine near Clear Lake, California. These authors reported average methylmercury blood levels of
15.6±7 µg/L (ppb) in individuals that consumed fish from Clear Lake, which was higher than blood levels
reported for individuals that did not consume fish (2 ppb). A significant correlation of methylmercury blood
levels and fish consumption was observed. Mercury has been detected in fish collected at 56 of the 714 NPL
sites where it has been detected in some environmental media (HazDat 1998). Adults may also receive
higher mercury exposures from routine consumption of mercury-contaminated groundwater if this is the
primary drinking water supply. Mercury has been detected in groundwater samples collected at 395 of the
714 NPL sites where mercury has been detected in some environmental media (HazDat 1998).
Individuals living near municipal and medical waste incinerators, power plants fired by fossil fuels
(particularly coal fired plants), or hazardous waste sites may inhale vapors or particulates contaminated with
mercury from ambient outdoor air. Lipfert et al. (1996) evaluated the health risks of methylmercury from
burning coal using a Monte Carlo model to simulate a “baseline” and a “worst case” scenario in which a
population of 5,000 fish eaters in the upper midwestern United States derived the freshwater fish portion of
their diet from local waters near a large, hypothetical coal-fired power plant. The population was
characterized by distributions of body mass, half-life of methylmercury, and the ratios of blood to body
burden and hair to blood methylmercury. Each person’s diet consisted of varying amounts of tuna fish,