Public Health Assessment Public Comment Release
Appendix A. Explanation of Evaluation Process
In evaluating these data, ATSDR used comparison values (CVs)
to determine which chemicals
to examine more closely. CVs are the health-based thresholds for contaminant concentrations
in a specific media (air, soil,
or water). They are used
in the selection
further evaluation. A CV incorporates assumptions about daily exposure to a chemical and the
standard amount of air, water, and soil that someone might inhale or ingest each day.
A CV represents a concentration below which no known or anticipated adverse human health
effects are expected to occur. Different CVs are developed for cancer and noncancer health
effects. Noncancer levels are based on valid toxicologic studies for a chemical, with appropriate
safety factors included, and the assumption that small children (22 pounds or less) and adults are
exposed every day. Cancer levels are based on a one-in-one-million excess cancer risk for an
adult eating contaminated soil or drinking contaminated water every day for 70 years. For
chemicals for which both cancer and noncancer levels exist, ATSDR uses the lower of the levels
to be protective of human health. However, exceeding a CV does not mean that health effects
will occur; it merely means that more evaluation is needed.
The CVs used in the evaluation in this document are listed below:
Environmental Media Evaluation Guides (EMEGs) are estimated contaminant concentrations in
a media at which noncarcinogenic health effects are unlikely. EMEGs are derived from the
Agency for Toxic Substances and Disease Registry=s (ATSDR) minimal risk level (MRL).
Cancer Risk Evaluation Guides (CREGs) are estimated contaminant concentrations that would
be expected to cause no more than one additional excess cancer in one million persons exposed
over a lifetime. CREGs are calculated from the U.S. Environmental Protection Agency=s (EPA)
cancer slope factors (CSFs).
Reference Media Evaluation Guides (RMEGs) are estimated contaminant concentrations in a
media at levels at which noncarcinogenic health effects are unlikely. RMEGs are derived from
EPA=s reference dose (RfD).
Region 9 Preliminary Remediation Goals (R9 PRGs) are the estimated contaminant
concentrations in a media at which carcinogenic or noncarcinogenic health effects are unlikely.
The PRGs used in this PHA were derived by use of provisional reference doses or CSFs
calculated by EPA=s Region 9 toxicologists and were last updated in October 2002.
Region 3 Risk-based Concentrations (R3 RBCs) are the estimated contaminant concentrations in
a media at which carcinogenic or noncarcinogenic health effects are unlikely. The RBCs used in
this PHA were derived by use of provisional reference doses or CSFs calculated by EPA's
Region 3 toxicologists and were last updated in October 2003.