Exposure Scenario

In aquatic test systems, exposure is usually a whole-body exposure. That means that the toxicant can enter the organism through the skin, cell wall, respiratory system (gills, stomata), and ingestion. Occasionally, a toxicant is injected into an aquatic organism, but that is not usually the case in toxicity tests to screen for effects. Whole-body exposures are less common when dealing with terrestrial species. Often an amount of xenobiotic is injected into the musculature (intramuscular), peritoneum (intraperitoneal), or into a vein 

Schematic of a proportional diluter with flow controlled by solenoid valves. This mechanism ensures that an accurate concentration of the test material is reliably introduced to the test organisms at a specified rate. 

Other routes of exposure include inhalation exposure for atmospheric borne pollutants. In many cases of an originally atmospheric exposure, dermal exposure may occur. An alternative method of ensuring an inhalation exposure is to provide an air or watertight seal limiting exposure to the respiratory apparatus. In the case of rodents, nose-only exposures can be used to limit coat 

Plant-, soil-, and sediment-dwelling organisms have other potential routes of exposure that may be used in toxicity testing. Plants are often exposed through the soil or to an atmospheric deposition. Soil invertebrates are often placed in a standardized soil laced with a particular concentration of the test substance. Sediment tests are usually with contaminated sediments or with a material added to a standardized sediment. 

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The models in the THERdbASE CD are Chemical Source Release, Instantaneous Emission, Chemical Source Release, Timed Application, Indoor Air (2-Zone), Indoor Air (N-Zone), Exposure Patterns for Chemical Agents, Benzene Exposure Assessment Model (BEAM), Source Ba.sed Exposure Scenario (Inhalation + Dermal), and Film Thickness Based Dermal Dose. 

Metal Fate and Effects in Different Metal Exposure Scenarios. 48... 

Risks to human health and the environment will vary considerably depending upon the type and extent of exposure. Responsible authorities are strongly encouraged to characterize risk on the basis of locally measured or predicted exposure scenarios. To assist the reader, examples of exposure estimation and risk characterization are provided in CICADs, whenever possible. These examples cannot be considered as representing all... 

Table 27 presents the results of the child consumer exposure scenario in the same manner as for the adult scenario. Again, the exposure from each source is expressed in relation to the TDI. 

Developmental Effects. Adverse effects of methyl parathion on hirman fetal development have not been reported. Based on studies in animals, such effects appear to be possible if pregnant women were exposed during the first trimester to high concentrations of methyl parathion that resulted in significant depression of cholinesterase levels, particularly if concomitant signs and symptoms of organophosphate intoxication occur. Such an exposure scenario may occur with occupational exposure, exposure in homes or offices illegally sprayed with methyl parathion, or accidental exposure to methyl parathion, but is less likely as a result of low-level exposure. 

Tables (3-1, 3-2, and 3-3) and figures (3-1 and 3-2) are used to summarize health effects and illustrate graphically levels of exposure associated with those effects. These levels cover health effects observed at increasing dose concentrations and durations, differences in response by species, minimal risk levels (MRLs) to humans for noncancer end points, and EPA s estimated range associated with an upper- bound individual lifetime cancer risk of 1 in 10,000 to 1 in 10,000,000. Use the LSE tables and figures for a quick review of the health effects and to locate data for a specific exposure scenario. The LSE tables and figures should always be used in conjunction with the text. All entries in these tables and figures represent studies that provide reliable, quantitative estimates of No-Observed-Adverse-Effect Levels (NOAELs), Lowest-Observed-Adverse-Effect Levels (LOAELs), or Cancer Effect Levels (CELs).

The degree of confidence in the final estimation of risk depends on variability, uncertainty, and assumptions identified in all previous steps. The nature of the information available for risk characterization and the associated uncertainties can vary widely, and no single approach is suitable for all hazard and exposure scenarios. In cases in which risk characterization is concluded before human exposure occurs, for example, with food additives that require prior approval, both hazard identification and hazard characterization are largely dependent on animal experiments. And exposure is a theoretical estimate based on predicted uses or residue levels. In contrast, in cases of prior human exposure, hazard identification and hazard characterization may be based on studies in humans and exposure assessment can be based on real-life, actual intake measurements. The influence of estimates and assumptions can be evaluated by using sensitivity and uncertainty analyses. - Risk assessment procedures differ in a range of possible options from relatively unso-... 

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. 

Protocols to determine exposure scenarios should require that application be made using normal practices. Test substance application must be thoroughly documented by researchers. Documentation should include weights and volumes of materials added to... 

Risk assessment pertains to characterization of the probability of adverse health effects occurring as a result of human exposure. Recent trends in risk assessment have encouraged the use of realistic exposure scenarios, the totality of available data, and the uncertainty in the data, as well as their quality, in arriving at a best estimate of the risk to exposed populations. The use of "worst case" and even other single point values is an extremely conservative approach and does not offer realistic characterization of risk. Even the use of arithmetic mean values obtained under maximum use conditions may be considered to be conservative and not descriptive of the range of exposures experienced by workers. Use of the entirety of data is more scientific and statistically defensible and would provide a distribution of plausible values. 

The MOE calculations for the various exposure scenarios with chlorpyrifos are presented in Table 3. Listed in this table are the calculated results for the workers involved with mixing and loading and the application of chlorpyrifos,... 

The calculated MOEs in Table 3 demonstrate some of the problems with using this approach for characterizing risks and making regulatory decisions. Several of the exposure scenarios have MOEs of less than 10, which is the... 

The results from using the Student s f-test for a distributional analysis are presented in Table 4. These results indicate the probability of a given worker in the listed scenario exceeding the NOEL of the toxicity endpoint. The probability of exceeding the LOEL and of thus experiencing a depression of plasma cholinesterase activity is not given (except for chronic exposure scenarios in the "100 ug/kg bw/day" column). Hence, even these probabilities may be considered to be conservative and not fully representative of the probability of a worker actually experiencing a toxic effect. 

During application, temperatures ranged from 17 to 27°C and from 16 to 28°C for the work clothing and protective clothing trials, respectively. During harvesting, temperature ranges were 19 to 26°C and 15 to 25°C for both exposure scenarios. 

Relative humidity ranges were 45 to 97% and 44 to 98%, and 54 to 97% and 56 to 92% for application and harvesting in both exposure scenarios, respectively. No significant differences were observed between both temperature and relative humidity for both exposure scenarios. 

Concentrations of propoxur in the breathing zone ranged from 0.6 to 25 (median, 0.7) pg/m3, and from 0.4 to 29.4 (median, 1.2) pg/m3 for applicators in exposure scenarios without and with protective clothing, respectively. For... 

Brouwer, D.H. and van Hemmen, J.J. (1994) Fitting personal protective equipment (PPE) to the hazard selection of PPE for various pesticide exposure scenarios in greenhouses, in Book of Abstracts of the American Industrial Hygiene Conference Exposition, American Industrial Hygiene Association, Anaheim, CA. 

Exposure assessments have become an essential element of contemporary risk assessment (NAS/NRC, 1983). The primary purpose of exposure assessment is to qualitatively and/or quantitatively determine exposure and absorbed dose associated with a particular use practice or human activity. Contemporary exposure assessors and risk managers place a high premium on accurate data obtained by monitoring chemical exposure scenarios and critical human activities or work tasks. 

Compilation of residue data for various exposure scenarios.109... 

Interroute extrapolation. The values for pharmacokinetic variables in the O Flaherty Model are independent of the route of exposure. However, the model does incorporate media-specific estimates of absorption from the gastrointestinal tract. Different exposure scenarios have been evaluated with the O Flaherty Model for children and adults (O Flaherty 1993, 1995a). 

Recent studies provide evidence for rapid dermal absorption of inorganic lead in adults however, these studies have not quantified the fraction of applied dose that was absorbed (Stauber et al. 1994). The quantitative significance of the dermal absorption pathway as a contributor to lead body burden remains an uncertainty. In children who experience extensive dermal contact with lead in soil, sand, or surface water and suspended sediment (e.g., beach or shoreline exposure scenario), even a low percent absorption... 

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