Public Health Assessment
NAVAL WEAPONS STATION YORKTOWN, CHEATHAM ANNEX
WILLIAMSBURG, YORK COUNTY, VA
CERCLIS NO. VA3170024605
ATSDR health assessors use comparison values (CVs) as screening tools to evaluate environmental data that are relevant to the exposure pathways. CVs represent media-specific contaminant concentrations that are much lower than exposure concentrations observed to cause adverse health effects. In that way, CVs are protective of public health in essentially all exposure situations. If the concentrations in the exposure medium are less than the CV, the exposures are not of health concern and no further analysis of the pathway is required. However, while concentrations below the CV are not expected to lead to any observable health effect, it should not be inferred that a concentration greater than the CV will necessarily lead to adverse effects. Depending on site-specific environmental exposure factors (for example, duration of exposure) and activities of people that result in exposure (time spent in area of contamination), exposure to levels above the CV may or may not lead to a health effect. Therefore, ATSDR's CVs are not used to predict the occurrence of adverse health effects. Rather, they are used by ATSDR to select contaminants for further evaluation to determine the possibility of adverse health effects.
CVs used in this PHA include:
Cancer Risk Evaluation Guide (CREG)
Estimated contaminant concentrations that would be expected to cause no more than one excess cancer in a million (10-6) persons exposed over a 70-year life span. ATSDR's CREGs are calculated from EPA's cancer slope factors (CSFs).
Environmental Media Evaluation Guide (EMEG)
EMEGs are based on ATSDR minimal risk levels (MRLs) and factor in body weight and ingestion rates. An EMEG is an estimate of daily human exposure to a chemical (in mg/kg/day) that is likely to be without noncarcinogenic health effects over a specified duration of exposure to include for acute (< 14 days), intermediate (15-364 days), and chronic (> 365 days) exposures.
Reference Media Evaluation Guides (RMEG)
ATSDR derives RMEGs from EPA's oral reference doses (RfDs). The RMEG represents the concentration in water or soil at which daily human exposure is unlikely to result in adverse noncarcinogenic effects.
EPA's Region III Risk-Based Concentration (RBC)
The U.S. Environmental Protection Agency (EPA) combines RfDs and CSF with "standard" exposure scenarios to calculate risk-based concentrations (RBCs), which are chemical concentrations corresponding to fixed levels of risk (i.e., a hazard quotient of 1, or lifetime cancer risk of 10-6, whichever occurs at a lower concentration) in water, air, fish tissue, and soil.
EPA Maximum Contaminant Level (MCL)
The MCL is the drinking water standard established by the EPA. It is the maximum permissible level of a contaminant in water that is delivered to a free-flowing outlet. MCLs are considered protective of human health over a lifetime (70 years) for individuals consuming 2 liters of water per day.
CVs are derived from available health guidelines, such as ATSDR's MRLs, EPA's RfDs, and EPA's CSFs. These guidelines are based on the no-observed adverse effect levels (NOAEL), lowest-observed adverse effect levels (LOAELs), or the cancer effect levels (CELs) reported for a contaminant in the toxicologic literature. A description of these terms is provided:
Minimal Risk Levels (MRL)
MRLs are estimates of daily human exposure to a chemical (i.e., doses expressed in mg/kg/day) that are unlikely to be associated with any appreciable risk of deleterious noncancer effects over a specified duration of exposure. MRLs are calculated using data from human and animal studies and are reported for acute, intermediate, and chronic exposures.
Reference Dose (RfD)
The RfD is an estimate, with safety factors built in, of the daily, life-time exposure of human populations to a possible hazard that is not likely to cause harm to the person.
Cancer Slope Factor (CSF)
Usually derived from dose-response models and expressed in mg/kg/day, CSFs describe the inherent potency of carcinogens and estimate an upper limit on the likelihood that lifetime exposure to a particular chemical could lead to excess cancer deaths.
Lowest Observed Adverse Effect Level (LOAEL)
The lowest dose of a chemical that produced an adverse effect when it was administered to animals in a toxicity study or following human exposure.
No Observed Adverse Effect Level (NOAEL)
The highest dose of a chemical in a study, or group of studies, that did not cause harmful health effects in people or animals.
Cancer Effect Level (CEL)
The CEL is the lowest dose of a chemical in a study, or group of studies, that was found to produce increased incidences of cancer (or tumors).