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

MERCURY 228
2. HEALTH EFFECTS
apprehension about relatively benign exposures to environmental substances. The MRL is neither a
threshold for toxicity, nor a level beyond which toxicity is likely to occur. MRLs are established solely as
screening tools for public health officials to use when determining whether further evaluation of potential
exposure at a hazardous waste site is warranted. The relevance of the MRL to public health is discussed
further in the following sections concerning the derivation of the respective mercury MRLs.
ATSDR has established a chronic inhalation MRL of 0.2 µg/m 3 for metallic mercury. Assuming a
ventilation rate of 20 m 3 /day for an average adult, and assuming complete absorption, exposure at the level
of the MRL would result in a daily dose of 4 µg. This level of exposure is thought to represent no health
risk to any element of the human population. No other inhalation MRLs have been derived for mercury or
its compounds.
Oral MRLs have been established for acute (0.007 mg/kg/day) and intermediate (0.002 mg/kg/day) duration
exposures to inorganic mercury. ATSDR has also established a chronic oral MRL of 0.0003 mg/kg/day
(equivalent to 21 µg/day for a 70-kg adult) for methylmercury. This MRL is at least four times the
estimated average daily intake level for methylmercury from the diet. The FDA has estimated that, on
average, the intake rate for total mercury (both inorganic and organic) is 50–100 ng/kg/day (equivalent to
0.05–0.1 µg/kg/day or 3.5–7 µg/day for a 70-kg adult). This figure is based on the FDA total diet study of
1982–1984 (Gunderson 1988). Approximately 80–90% of the mercury in the FDA estimate would be
expected to be in the form of methylmercury. A separate estimate of the average intake of methylmercury
alone, based on a survey of fish eaters and on average levels of methylmercury in fish, places the average
intake of methylmercury at 36 ng/kg/day (equivalent to 0.036 µg/kg/day or 2.52 µg/day for a 70-kg adult),
with a 99% upper-bound estimate at 243 ng/kg/day (equivalent to 0.243 µg/kg/day or 17 µg/day for a 70-kg
adult) (Clarkson 1990). These results indicate that an assessment of total methylmercury intake and body
burden should be conducted when estimating exposure to mercury in populations (especially sensitive
populations) living near hazardous waste sites that have the potential to release mercury to the
environment.
Inhalation MRLs
No inhalation MRLs were derived for inorganic mercury salts or organic mercury compounds due to the
absence of data or to the lack of sufficient information regarding exposure levels associated with the
reported observed effects.