Exposure pathway defined

One important purpose of monitoring is to provide data that permit the analysis and evaluation of human radiation exposure. For this purpose, programmes for monitoring radionuclides in the environment should focus on pathways of human exposure. An exposure pathway defines routes from a source of radionuclides and/or radiation to a target individual or a population through media in the environment. There are two main categories of exposure pathway external exposure pathways (the source of exposure remains outside the body) and internal exposure pathways (the source of exposure is incorporated into the body). 

Once the exposure pathways of concern have been defined, an exposure and risk assessment can be performed. 

Exposure assessment is one of the most important steps in risk assessment. It is the process that predicts or estimates the amount of the substance under study that reaches the human body. To assess the exposure, it is necessary to define in detail the exposure pathway, the route of exposure, the concentration of the pollutant in the particular media, the contact rate, the frequency of exposure, and the population exposed (age, gender, and vulnerable population, among others). A general equation (Eq. 1) to determine the exposure dose is as follows ... 

Exposure assessment should be a component of population-based biomonitoring studies to facilitate interpretation of the data. Typically, large-scale biomonitoring studies do not evaluate potential sources of exposure. That often leads to the question, Where is the exposure coming from For some chemicals, exposure pathways may be well defined from previous... 

Route of exposure is defined as the portal of entry to the body. Pathway is defined as the course that the contaminant takes from its source to the exposure medium, and then to the portal of entry. For a given source, exposure media and exposure routes can define the pathways. Depending upon the life stage of the child, exposure media can include amniotic fluid, breast milk, air, water, soil/dust/ sediments, food, and objects/surfaces. Exposure routes include transplacental transfer, inhalation, ingestion, dermal absorption, and indirect (non-dietary) ingestion. 

To address these questions, an exposure assessment should begin with a definition of the assessment objective (section 2.2.1). From this follows the need to define and evaluate the conceptual exposure model (section 2.2.2). The goal of the conceptual model is to establish exposure links via exposure pathways to exposure routes and relative magnitude of uptake or intake by different exposure routes. These questions are discussed in more detail in section 5.4 with respect to interpretation of the results of a probabilistic exposure assessment. 

Scenarios may be defined under the umbrella of this conceptual model. There are different levels of scenarios, such as that describing the release of formaldehyde from furniture. Each of the exposures from one of the sources may be characterized by a particular scenario, but all of these scenarios may be combined to yield bigger and more complex scenarios, such as the inhalation exposure pathway. In this concept, the scenario describing the whole exposure including all sources and paths represents a very complex construction. 

In the USA and Canada, aggregate exposure is defined as the combined exposure of a target to a single pesticide via all relevant pathways and sources . 

Cumulative exposure is defined as the combined exposure of a target to two or more pesticides (with a common mechanism of action) via all relevant pathways and sources . The US FQPA and the Canadian PCPA mandate the consideration of aggregation of pesticide chemical residues from dietary and residential exposures, and cumulative exposures of pesticide chemical residues with common mechanisms of action. This presents a formidable methodological challenge to exposure assessors. 

Make a conceptual model that ties the stressors to the receptors and to the assessment endpoints. The conceptual model delineates the potential connections between sources, stressors, habitat, and endpoints that will be used in each risk region. The conceptual model is an extension of the basic framework for a regional risk assessment with sources providing stressors identified by particular habitats. In this instance, the habitats are broadly defined as terrestrial and aquatic to capture the exposure pathways and location within the region of our endpoints. In this instance, there are numerous interconnected endpoints to show both the valued ecosystem components and the interdependence and potential indirect effects. 

The Remedial Action Priority System (RAPS) and Multimedia Environmental Pollutant Assessment System (MEPAS) are different names for an objective exposure pathway evaluation system developed by Pacific Northwest Laboratory to rank chemical and radioactive releases according to their potential human health impacts. Constituent migration and impact are simulated using air, groundwater, overland, surface water, and exposure components based on standard assessment principles and techniques. A shell allows interactive description of the environmental problem to be evaluated, defines required data in the form of problem-specific worksheets, and allows data input. The assessment methodology uses an extensive constituent database as a consistent source of chemical, physical, and health-related parameters. 

Because the major human exposure pathway is food, human exposures to SCCPs as indirect exposures via food are evaluated. The risk of SCCPs to human health is estimated as MOE and MOE is compared with uncertainty factors. The MOE is defined by the following equation ... 

Following the 1985 edition of the Regulations, the application of the Q system as described here treats the derivation of each Q value, and hence each potential exposure pathway, separately. In general this will result in compliance with the dosimetric criteria defined earlier, provided that the doses incurred by persons exposed near a damaged package are dominated by one pathway. However, if two or more Q values closely approach each other this will not necessarily be the case. For example, in the case of a radionuclide transported as a special form radioactive material for which = Qg, the effective dose and skin dose to an exposed person could approach 50 mSv and 0.5 Sv, respectively, on the basis of the Q system models. Examination of Table 1.2 shows that this consideration applies only to a relatively small number of radionuclides, and for this reason the independent treatment of exposure pathways is retained within the Q system. 

The activities to which protection and safety provisions apply are defined as practices or interventions . A practice involves the introduction of a new source of exposure or a new exposure pathway, resulting in an increase in exposure or the likelihood of exposure, or in the number of persons exposed. Practices are those human activities involving radiation exposure, or the potential for exposure, for which radiation... 

Once potential receptors cu e identified, the ways in which they may contact chemicals need to be defined. This is done by identifying complete exposure pathways. There are four components that must all be present for an exposure pathway to be complete ... 

Wildlife indicators can establish baseline conditions, act as early warning signals of environmental problems, identify the extent of contamination, define critical pathways and responses at multiple trophic levels, as well as integrate biological exposure with the physical and chemical environment (Farrington 1991). Indicator selection is based on a combination of criteria or characteristics that include (Jenkins 1981) ... 

Biotic Transport Biotic transport can be defined as the actions of plants and animals that result in the transport of a radioactive material or other substance from a waste site to locations where it can enter pathways that may result in exposure to humans. Small mammals are ubiquitous and inhabit areas containing radioactive contamination or radioactive waste sites. Mammals inhabiting these areas may become contaminated with americium by consuming contaminated soil or plants and disturb americium-contaminated soil through their burrowing and excavating activities. These animals may therefore affect the distribution of americium within the waste site or transport americium to previously uncontaminated areas. In addition, small mammals may be consumed by animals higher in the food chain such as hawks and coyotes, which would add to the dispersal of americium from disposal areas. However, results of... 

When chemicals are released in the environment, their hazard potential to human or ecological receptors depends upon the extent of contact between the receptors and the chemical. This exposure level is not only influenced by where, when and how much of the chemical is released, but also on its movement and changes in air, water, soil or biota relative to the locations of the receptors. Risk is defined as the probability of some adverse consequence in the health context, or as the probability times the extent of the consequence in the technology context. In this paper we shall examine and discuss how mathematical models are used to generate estimates of risk when more than one of the environmental media must be considered in tracing pathways connecting sources with receptors. The principal objective here is to place in perspective the... 

ALAD, an enzyme occurring early in the heme pathway, is also considered a sensitive indicator of lead effect (Hemberg et al. 1970 Morris et al. 1988 Somashekaraiah et al. 1990 Tola et al. 1973). Because there is no well-defined blood lead threshold at which inhibition of ALAD does not occur, it allows measurement of the effect on the general population at environmental lead levels and does not require high exposure levels as with occupational workers (Hemberg et al. 1970). However, ALAD activity may also be decreased with other diseases or conditions such as porphyria, liver cirrhosis, and alcoholism (Somashekaraiah et al. 1990). 

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