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

MERCURY 232
2. HEALTH EFFECTS
Employment of the Chronic Inhalation MRL for Metallic Mercury
ATSDR emphasizes that the MRL is not intended to be used as an estimation of a threshold level.
Exceeding the MRL does not necessarily mean that a health threat exists. However, the greater the amount
by which the MRL is exceeded and the longer or more frequent the individual exposures, the greater the
likelihood that some adverse health outcome may occur. Secondly, the chronic inhalation MRL is, by
definition, a level that is considered to be without appreciable (or significant) health risk over a lifetime of
exposure at that level. It is further considered to be a "safe" level for all factions of the exposed human
population, when exposure exists for 24 hours a day, 7 days a week for an extended period of years. The
employment of the MRL, therefore, must be geared to the particular exposure scenario at hand. For
example, people may be able to "tolerate" metallic mercury levels above the MRL for intermittent periods
of exposure (e.g., 1 or 2 hours per day, 5 days per week) without any adverse health sequelae, either overt
or covert. The use of the "contaminated area" (e.g., storage versus exercise room versus day care) will
largely influence the use of the MRL. Finally, the MRL is intended primarily as a "screening value" for
public health officials to use in their assessment of whether further evaluation of the potential risk to public
health is warranted in a hazardous waste site scenario. The MRL is not intended, nor should it be
indiscriminately used, as a clean-up or remediation level, or as a predictor of adverse health effects. While
it is considered to afford an adequate degree of protection for the health of all potentially exposed
individuals, it might be unnecessarily stringent for application to some exposure situations (i.e., higher air
concentrations might afford a similar degree of protection in some exposure scenarios); thus, its relevance
in any specific environmental situation is intended to be determined by an experienced public health or
medical official.
Oral MRLs
Metallic Mercury
No oral MRLs were derived for metallic (elemental) mercury due to the lack of data. Oral exposure to
liquid metallic mercury would be expected to present little health risk, since it is so poorly absorbed
(