Exposure assessments estimates

Risk assessment and epidemiology could be successfully combined to analyze environmental health risks. Exposure assessments estimate concentrations of toxic chemicals in the environment that could be transferred to humans by ingestion, inhalation, or dermal absorption. In the future, there will be a greater need for agreement on how best to simultaneously assess societal risks involved with damage to both ecosystems and the human population (Ruttenber, 1993). 

Exposure assessment estimates the number of exposed persons together with the magnitude, duration and frequency of exposure. A direct possibility to measure the human exposure to toxic substances via ambient air, for example, is the utilization of personal monitors. 

On-line system. Provides support for exposure assessments of toxic substances. Includes chemical properly estimation techniques, siahsiical analysis, multi-media modeling, and graphics display (including models)... 

Exposure assessment, step three, allows a risk assessor to estimate the significance of the effects induced by high doses of a chemical in experimental animals in a human situation. Exposure assessment is, in fact, a prerequisite for quantitative risk assessment because it allows a comparison between effects induced by high dose with those induced by low doses, and also allows... 

Generally, the main pathways of exposure considered in tliis step are atmospheric surface and groundwater transport, ingestion of toxic materials that luu c passed tlu-ough the aquatic and tcncstrial food chain, and dermal absorption. Once an exposure assessment determines the quantity of a chemical with which human populations nniy come in contact, the information can be combined with toxicity data (from the hazard identification process) to estimate potential health risks." The primary purpose of an exposure assessment is to... 

Exposure assessment is the process of measuring or estimating llic intensity, frequency, and duration of human or animtil exposure to an agent present in llie environment... 

Risk characterization is lire process of estimating llie incidence of a health effect under the various conditions of human or animal exposure described in lire exposure assessment. It is performed by combining the exposure (see Cliapter 12) and dose response (see Cluipter 11) assessments. The summary effects of the uncertainties in lire preceding steps should also be described in lliis step. 

One should identify exposure pathways that have the potential to expose the same individual or sub-population at the key exposure areas evaluated in the exposure assessment, making sure to consider areas of highest exposure for each patliway for both current and future land-uses (c.g., nemest down-gradient well, nearest dowiuvind receptor). For each pathway, the risk estimates and hazard indices have been developed for a particular exposure area... 

Thus, tlie focus of tliis subsection is on qualitative/semiquantitative approaches tliat can yield useful information to decision-makers for a limited resource investment. There are several categories of uncertainties associated with site risk assessments. One is tlie initial selection of substances used to characterize exposures and risk on tlie basis of the sampling data and available toxicity information. Oilier sources of uncertainty are inlierent in tlie toxicity values for each substance used to characterize risk. Additional micertainties are inlierent in tlie exposure assessment for individual substances and individual exposures. These uncertainties are usually driven by uncertainty in tlie chemical monitoring data and tlie models used to estimate exposure concentrations in tlie absence of monitoring data, but can also be driven by population intake parameters. As described earlier, additional micertainties are incorporated in tlie risk assessment when exposures to several substances across multiple patliways are suimned. 

Risk characterization is tlie process of estimating tlie incidence of a healtli effect under tlie various conditions of human or animal exposure as described in the exposure assessment. It evolves from both dose exposure assessment and toxicity response assessment. The data are then combined to obtain qualitative and quantitative expression of risk. 

Based upon the various sources of adult consumer exposure to organotin compounds (section 6) and the TDI values derived above, it is possible to estimate the relative exposure from the various organotin compounds expressed as a percentage of the TDI values. The exposure calculations in section 6 were based on a realistic worst-case exposure assessment. Table 26 presents the results of this risk characterization. 

Exposure assessment is the qualitative and/or quantitative estimation of the probable intake of a biological, physical, or chemical agent through food, and exposme from other relevant sources. Only intakes of toxicologically significant amounts can lead... 

Risk characterization is the last step in the risk assessment procedure. It is the quantitative or semi-quantitative estimation, including uncertainties, of frequency and severity of known or potential adverse health effects in a given population based on the previous steps. Risk characterization is the step that integrates information on hazard and exposure to estimate the magnitude of a risk. Comparison of the numerical output of hazard characterization with the estimated intake will give an indication of whether the estimated intake is a health concern. ... 

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-... 

ILSl, A Framework for Estimating Pesticide Concentrations in Drinking Water for Aggregate Exposure Assessments, Workshop Report 5/19/99. International Life Sciences Institute, Washington, DC (1999). 

The internal dose of propoxur was measured by assessing the total amount of 2-isopropoxyphenol (IPP) excreted in the urine, collected over a period of 24 hr from the start of exposure, and described in detail in previous studies (Brouwer et al., 1993 Meuling et al., 1991). Volunteer kinetics studies revealed a one-to-one relationship of absorbed propoxur and excreted IPP on a mole basis. Based on the results by Machemer et al. (1982), a pulmonary retention of 40% was used to calculate the relative contribution of the respiratory exposure to the internal exposure. To estimate the contribution of the dermal exposure, the calculated respiratory portion was subtracted from the total amount of IPP excreted in urine. 

Passive dosimetry, which proved useful for the pursuit of better workplace hygiene in agriculture during the past 40 years (Durham and Wolfe, 1962), yields unvalidated and excessive amounts of worker exposure (Krieger, 1996). Currently, our approach with respect to indoor and agricultural exposure assessments has been the evaluation of exposure estimates using well-known, studied chemicals to first understand the work task and at a later time develop chemical-specific studies as required in the regulatory arena. 

If linear (dose) models without thresholds are to be used for carcinogen (or other) risk assessment, estimation of exposure at specified levels becomes irrelevant to risk assessment or, at least, its use is nonintuitive. For example, a carcinogen risk analysis may be based on a linear, nonthreshold health effects model. The total health risk would thus be proportional to the long-term exposure summed for all affected people for the identified period, and exposure of many people at low concentrations would be equivalent to exposure of a few to high concentrations. The atmospheric dispersion that reduces concentrations would also lead to exposure of more people therefore, increments... 

Because the significance of exposure has only been considered over the past few years, there is not as wide a selection of exposure models available as that for fate models. The latter have been applied for several decades to the calculation of ambient exposure levels compared with some standard values. Papers illustrative of human exposure assessments in this symposium include one on airborne pollutant exposure assessments by Anderson (2), a generic approach to estimating exposure in risk studies by Fiksel (5), and a derivation of pollutant limit values in soil or water based on acceptable doses to humans by Rosenblatt, Small and Kainz (6). 

The absorption efficiency term allows estimation of the effective dose or the amount of pollutant which crosses the membrane of the exposed tissue (e.g., the lung) and reaches a target organ (e.g., the liver). For many pollutants this type of metabolic data is not available and consequently 100% absorption is a common preliminary assumption in exposure assessments. For well-studied substances such as radionuclides, a methodology for calculation of target organ doses has been developed for bone marrow, lungs, endosteal cells, stomach wall, lower intestine wall, thyroid, liver, kidney, testes and ovaries as well as for the total body. 

Population Characterization. An important part of any exposure assessment is the development of a detailed and up-to-date human demographic data base for the area being studied. These data can provide the basis for estimates of subpopulations associated with different exposure pathways. In national exposure assessments it is common to use an average population density for the total U.S. or to simply distinguish between rural and urban densitites. In a geographic exposure assessment in which site-specific data on pollutant releases, environmental fate and ambient levels are measured or estimated, it is important to have equally detailed population data. Population breakdowns by age, sex, housing and... 

Exposure estimation is the next logical step in an exposure assessment. In this step, the data and methods developed in the previous steps dre linked together so that the relationship between pollutant sources and human exposure can be examined. Through estimation of the degree of exposure rather than just estimation of concentrations in environmental media, a more detailed analysis of a pollution problem is possible, including ... 

Exposure assessment is the process of predicting or estimating the concentration or amount of an agent, the frequency, and the duration that reaches the receptor. 

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 ... 

The risk characterization procedure will result in a quantitative comparison per substance of the outcome of the exposure assessment and of the effects assessment. This comparison is made through the ratio PEC/PNEC. The generic name for PEC/ PNEC in EUSES is risk characterization ratio (RCR). Other ratios are used in EUSES for the risk characterization such as the margin of safety (MOS) or the ratio of the estimated no-effect or effect level parameter to the estimated exposure level for human subpopulations and the acceptable operator exposure level (AOEL). 

When determining the impact of a stressor using this approach, there are two main steps, exposure assessment and hazard assessment [28]. The exposure assessment is aimed to determine the concentrations of a substance that a certain organism can be exposed to due to the emission of a certain amount of a substance. The first step is the release estimation. The release estimation aims to determine how much of... 

Verified inhalation and oral slope factors were unavailable from U.S. EPA for dimethylhydrazine. A cancer assessment based upon the carcinogenic potential (withdrawn cancer slope factors) of dimethylhydrazine revealed that AEGL values for a theoretical excess lifetime 10 4 carcinogenic risk exceeded the AEGL-2 values that were based on noncancer endpoints. Because the risk for dimethylhydrazine exposure was estimated from nonverified sources and because AEGLs are applicable to rare events or single once-in-a-lifetime expo... 

In the final phase of risk analysis—risk characterization—one integrates outputs of effects and exposure assessments. Risk is expressed in qualitative or quantitative estimates by comparison with reference values (e.g., hazard quotient). The severity of potential or actual damage should be characterized with the degree of uncertainty of risk estimates. Assumptions, data uncertainties and limitations of analyses are to be described clearly and reflected in the conclusions. The final product is a report that communicates to the affected and interested parties the analysis findings (Byrd and Cothern, 2000). 

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