The goal for risk managers is to select an appropriate basis (criteria) for deciding if clearance goals have been met for different materials and surfaces (e.g., air, water). The preference is to have criteria that are expressed in concentration units (e.g., milligram chemical per cubic meter of air, milligram chemical per square meter surface area, milligram chemical per liter of drinking water). In general, two types of health- based (aka risk-based) exposure guidelines may be available to provide a basis for these decisions. In some cases, exposure guidelines are already reported in units of “concentration” and may be used as is – this is usually the case for inhalation exposure guidelines. In other cases, guidelines are expressed in terms of “dose” (e.g., milligrams per kilograms of body weight per day [mg/kg-day], such with EPA’s Reference Dose [RfD]). This estimate of dose is often expressed as a dose associated with absence of adverse health effects (e.g., RfD) or some target risk. These kinds of values are often referred to as toxicity values. These toxicity values can then be combined with exposure targets to derive target material and surface concentrations using risk-based calculations. The decisions on final disposition of the affected site by the IC/UC (e.g., return to full original use or use with some limitations) may have an effect on some of the assumptions applied in determining reasonable clearance criteria, especially if they are derived from the dose-based exposure guidelines.
Some of the values discussed in this appendix are regulatory (e.g., the OSHA PELs and the EPA Maximum 18 Contaminant Levels [MCLs]) and enforceable by law. Others are not. However, all are intended for informational purposes to assist in decision-making during a contamination incident. For simplicity’s sake, this document will generalize all values with the term “exposure guideline.” However, it should not be implied that the use of the word “guideline” denotes, in this case, the existence of or lack of any regulatory significance. Additional information regarding each exposure guideline discussed in this appendix may be found in the EPA’s Graphical Arrays of Chemical-Specific Health Effect Reference Values for Inhalation Exposures, Table 1.1.86
Health-based exposure guidelines are one of the key inputs in the common approach to deriving clearance goals and clearance criteria. These exposure guidelines are derived from equations that combine a level of exposure (dose or concentration in air), a level of acceptable risk, and a set of exposure assumptions for a particular chemical, medium, and exposure scenario. Thus, the resulting health-based exposure guideline will be specific to a particular population and exposure scenario. For example, there are health-based exposure guidelines developed for healthy workers that assume exposures lasting only 15 minutes and other exposure guidelines for exposures up to 8 or 10 hours per day, 40 hours per week for a working lifetime. Conversely, there are exposure guidelines that are based on long-term, or even lifetime exposures to the general population that are meant to be protective of sensitive members of the population such as children and the elderly. Although it is preferred that planners develop scenario-specific cleanup goals and clearance criteria, in the absence of resources and scenario-specific information, pre-calculated, health-based exposure guidelines can provide a useful tool for assessors and decision-makers. However, it is important to clearly understand the basis for any pre- calculated exposure guidelines used so that they are applied appropriately in the response action.
Exposure guidelines have been developed by many different sources and for many purposes. To assist planners, this appendix provides a list of available sources. In general, it is recommended that planners select peer-reviewed exposure guidelines, used in combination with appropriate exposure factors, to arrive at relevant cleanup goals and clearance criteria for the situation of concern.
Inhalation exposure guidelines are often presented as a concentration of a particular chemical in the air and may be directly compared to environmental concentrations. Oral exposure guidelines, however, are presented as applied or administered doses (e.g., mg/kg-day). which are subsequently used in equations to derive acceptable concentrations for drinking water, soil, and surfaces. Due to a lack of dermal toxicity values, oral values are generally used to assess risks from dermal exposure. Depending on the studies from which a value for a chemical was derived, one may need to adjust the oral value to account for the difference between an administered dose and an absorbed dose.87
Some chemicals exhibit both carcinogenic and noncarcinogenic toxicity. For such chemicals, clearance goals and clearance criteria generally are based on the more protective of the cancer- or noncancer- based exposure guidelines. Noncancer values are used primarily to determine a concentration below which no adverse effect is anticipated (threshold), while cancer-based values reflect a degree of increased risk of developing cancer (non-threshold effect).