Exposure scenarios, site-specific

For the purposes of these field studies, a test system is defined as a specific tract of land managed in part through use of pesticides. Test systems are normally limited to one crop or land use type and may include row crops, grains, fruits or golf courses. The tract of land, of course, has associated biota that are present naturally or as part of the management practices. These biota are also part of the test system and are normally described as test species or species of interest. Selection of test systems is critical to evaluate wildlife exposure scenarios in a sufficient number of sites within appropriate geographic regions. 

Risk assessments vary widely in scope and application. Some look at single risks in a range of exposure scenarios such as the IPCS Environmental Health Criteria Document series, others are site-specific and look at the range of risks posed by an installation. 

NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at disposal sites should be used in classifying waste. This recommendation is based on two considerations. First, scenarios for inadvertent intrusion can be applied to an assumed type of disposal system at any site, whereas scenarios for exposure of members of the public due to release and transport of hazardous substances to locations beyond the boundary of a disposal facility are highly site-specific and, thus, are not appropriate for use in generally classifying waste. 

The dominance of the risk to inadvertent intruders at near-surface waste disposal sites allows the use of this type of scenario to develop a risk-based waste classification system. However, NCRP recognizes that exposures of the public and protection of the environment also are of concern in determining acceptable disposal practices at specific sites. The potential for off-site releases of hazardous substances is the primary reason that classification of waste based on risks to hypothetical inadvertent intruders does not obviate the need for site-specific risk assessments to determine waste acceptance criteria in the form of limits on disposal of particular hazardous substances. 

In principle, for any type of disposal system that could be assumed for purposes of classifying waste, such as a near-surface disposal facility for hazardous wastes, a multitude of exposure scenarios might be considered. However, NCRP believes that only a single type of exposure scenario should be considered in classifying waste. Specifically, NCRP believes that the concept of a hypothetical inadvertent intruder at waste disposal sites provides a suitable basis for... 

The maximum time after disposal over which generic exposure scenarios should be evaluated for the purpose of classifying waste should not be any longer than the maximum time over which potential exposures of members of the public are evaluated for the purpose of determining the acceptability of specific wastes for disposal at specific sites. This time could range from... 

In addition, the reliance on generic scenarios for inadvertent intrusion in classifying waste cannot, by definition, represent site-specific risks. However, this is not a serious shortcoming because such scenarios have been used in establishing subclasses of low-level radioactive waste for disposal in near-surface facilities (NRC, 1982a). Furthermore, as emphasized in this Report, establishment of a risk-based waste classification system using particular exposure scenarios does not obviate the need to perform site-specific risk assessments for the purpose of establishing waste acceptance criteria at each disposal site. 

Site-specific exposure scenarios based on site-specific factors are evaluated in conjunction with relevant toxicologic, epidemiologic, and medical information. This involves assessing site-specific information aboutthe likelihood, frequency, routes, and levels of exposure to contaminants, and the populations that are likely to be exposed. 

The typical end-use product and application method chosen as representative of the extreme-case exposure scenario must be used to attain the highest permissible rate allowed by label directions. Sampling for indoor residues should consider all potential sites where appreciable residues are expected and are accessible. For instance, dermal contact may come from exposure to the pesticide as a residue on carpets, vinyl tile, upholstery and counter tops, while airborne residue (vapor- or particle-phase) may provide the source of inhalation exposure. The measurements taken are linked specifically to the method of application. 

Quantified values for human exposure factors are best determined on a case-by-case basis, with site-specific and source-specific circumstances driving choices about appropriate values for intake rates, exposure duration and frequency, body weight, averaging time and other related variables affecting calculation of the ADD. Each exposure assessment is unique and the assessor must construct a scenario and tailor related human exposure factors to fit the conditions at hand. 

MRLs are, by definition (Chou et al. 1998), substance-specific and do not include effects attributable to interaction (whether additive, synergistic, or antagonistic) with other chemicals or environmental substances. Their relevance to the mission of ATSDR is to assist public health officials in the identification of chemicals/elements of potential health concern at hazardous waste sites. The ATSDR MRL is not intended to be used in the regulatory or site clean-up process, but is instead intended to serve as a basis of comparison with actual measured levels of environmental exposure. Further, the role of informed biomedical judgment is crucial in the application of any MRL, or the media-specific health guidance values (HGVs) derived from them, in any given exposure scenario (Risher and De Rosa 1997). MRLs for a particular substance are based upon the most sensitive effect/endpoint in that portion of the human population considered to be most susceptible to injury from exposure to that substance. Thus, the... 

As a further aid to the risk assessor, Risk Assistant incorporates a database of the "average" and "reasonable worst-case" values for the parameters applicable to each of the exposure scenarios. The "average" value is automatically provided as a default. However, such "average" values may not be appropriate for particular locations or populations, and the user has the ability to substitute the "reasonable worst case" parameter value, or any other appropriate value for the specific population under consideration, for each parameter in each scenario. Such an approach provides for risk assessments that are responsive to the context of specific sites, yet retain a fundamental consistency of approach. 

The CCME protocol was also used to derive human health ESIs, using a scenario adapted for military sites. The military-specific exposure scenario was developed... 

The second element of health risk assessment for lead is the identification of dose—response relationships for the various adverse health effects noted in the previous chapter. Dose—response relationships, like health hazard characterizations, are general in nature and independent of site-specific lead exposure scenarios. 

Part 2 and its chapters presented the topic of human lead exposure in global and categorical terms, addressing the technical areas of lead intakes, uptakes (absorption), toxicokinetics, integration of toxicokinetics into in vivo disposition in a manner allowing quantitative assessments of lead exposure, etc. In contrast to these broadly descriptive aspects of human Pb exposme, the applied health discipline of quantitative risk assessment requires prescriptive approaches for site-specific, case-specific, and environmental scenario-specific lead exposure characterizations. Data from such specific exposure characterizations are combined with available data for lead dose—response relationships to arrive at some quantitative risk characterization indexed as some endpoint for human health risk. 

The EPA has used interim guidance of 500-1000 ppm for abatement decisions in the Comprehensive Environmental Response Compensation and Liability Act (EPA) (superfund) and the Resomce Conservational and Recovery Act (EPA) decision-making processes. There is an increasing trend to make these decisions on a site-specific basis using the EPA Uptake/Biokinetic model, which uses exposure information fi-om all sources to develop an exposure scenario to predict blood lead distribution for the exposme population. 

Many other OECD activities on hazard/risk assessment are undertaken within programs such as Existing Chemicals, New Chemicals, and Pesticides and Biocides, which deal with specific types of chemicals. The work on exposure assessment methods is undertaken by the Task Force on Environmental Exposure Assessment, consisting of experts. Most of the outcome of this work is published in the Series on Testing and Assessment or in Emission Scenario Documents, which are available at the OECD Web site (OECD 2006a). 

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