Exposure scenario, definition

Definition of the exposure scenarios describing 1° and 2° (direct/indirect) human exposure, the various emissions and the l°/2° environmental compartments exposed. 

NCRP assumes that the risk from disposal of any hazardous substance in waste can be described by means of a dimensionless risk index. The risk index for the ith hazardous substance is defined as the calculated risk from disposal of that substance, based on an assumed exposure scenario, relative to a specified allowable risk for the assumed type of disposal system. Based on this definition, the risk index is written as ... 

Exposure Scenarios for Classifying Exempt Waste. Based on the definition of exempt waste as any waste that would be generally acceptable for disposal in a municipal/industrial landfill for non-hazardous waste, scenarios for inadvertent intrusion appropriate to this type of facility should be used in determining whether a waste would be classified as exempt. 

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. 

Because the objective of an exposure assessment is to characterize both the magnitude and the reliability of exposure scenarios, planning for an uncertainty analysis is a key element of an exposure assessment. The aims of the uncertainty analysis in this context are to individually and jointly characterize and quantify the exposure prediction uncertainties resulting from each step of the analysis. In performing an uncertainty analysis, typically the main sources of uncertainties are first characterized qualitatively and then quantified using a tiered approach (see chapter 4). In general, exposure uncertainty analyses attempt to differentiate between key sources of uncertainties scenario uncertainties, model uncertainties and parameter uncertainties (for definitions, see section 3.2). 

Scenario uncertainty characterization may include a description of the information used for the scenario characterization (scenario definition). This includes a description of the purpose of the exposure analysis. For regulatory purposes, the level of the tiered approach is essential to describe the choice of data, whether defaults, upper-bound estimates or other single point estimates, or distributions have been used. This choice may govern the kind of uncertainty analysis. 

The definition of the scope and purpose of each exposure assessment provides the specifications for building the exposure scenario, which represents the real-life situation that is to be assessed and provides the boundary limits of the assessment. As pointed out... 

For the purposes of this document, the term data is defined broadly based on the description of exposure assessment detailed in IPCS (2004) and covers a wide variety of measurements, methods, modelling and survey information relevant to a given exposure assessment. The definition of data also includes the many elements of exposure assessment, from the development of exposure scenarios to the details of how they are modelled, to the selection of model input parameters and ultimately to how the results and their uncertainties are characterized and communicated to others (see text box for more detailed definitions). 

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... 

The SCENARIO selection activates screens for definition of the transport and exposure scenario to be used in evaluation of the environmental problems. The user can select from a list of scenarios already set up for the current ranking unit or choose to add one. To add a scenario, the user must choose the pathway that most closely represents the actual problem and then select those waste unit constituents that are transported by this particular pathway add or select receptors and match the receptors with exposure routes, such as ingestion, bathing, and direct contact. 

Within the exposure assessment, the definition of the conditions under which the substance is manufactured and used is critical in order to determine the levels of exposure. The information on the conditions under which a substance is manufactured and used is called the exposure scenario under REACH. For each exposure scenario, the exposure levels of humans and the environment need to be determined. 

The PBPK model development for a chemical is preceded by the definition of the problem, which in toxicology may often be related to the apparent complex nature of toxicity. Examples of such apparent complex toxic responses include nonlinearity in dose-response, sex and species differences in tissue response, differential response of tissues to chemical exposure, qualitatively and/or quantitatively difference responses for the same cumulative dose administered by different routes and scenarios, and so on. In these instances, PBPK modeling studies can be utilized to evaluate the pharmacokinetic basis of the apparent complex nature of toxicity induced by the chemical. One of the values of PBPK modeling, in fact, is that accurate description of target tissue dose often resolves behavior that appears complex at the administered dose level. 

Most readers can appreciate that our sense of smell is extremely sensitive and is capable of detecting truly minute concentrations of volatile aromatic substances. It is no surprise that most perfumes are made from the same class of compounds. The scenario presented above is capable of explaining individual reactions to very low concentrations of chemicals in our environment. Some recent studies have even suggested that there may be a unique anatomical pathway in our nose that allows chemicals to directly enter the nerves responsible for smell. These olfactory nerves, with receptors in the nose, then connect to the limbic system of the brain (see below for a definition of the limbic system). This pathway has been clearly demonstrated with manganese transport after inhalational exposure. Note, however, that oral exposure of... 

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