Calculated exposure

The ICRP (1994b, 1995) developed a Human Respiratory Tract Model for Radiological Protection, which contains respiratory tract deposition and clearance compartmental models for inhalation exposure that may be applied to particulate aerosols of americium compounds. The ICRP (1986, 1989) has a biokinetic model for human oral exposure that applies to americium. The National Council on Radiation Protection and Measurement (NCRP) has also developed a respiratory tract model for inhaled radionuclides (NCRP 1997). At this time, the NCRP recommends the use of the ICRP model for calculating exposures for radiation workers and the general public. Readers interested in this topic are referred to NCRP Report No. 125 Deposition, Retention and Dosimetry of Inhaled Radioactive Substances (NCRP 1997). In the appendix to the report, NCRP provides the animal testing clearance data and equations fitting the data that supported the development of the human mode for americium. 

More traditional approaches have calculated exposure doses from a particular medium via a specific route (ATSDR, 1992). Such exposure doses can then be compared with a reference value derived for the same substance via the same route of exposure. Usual assumptions are ingestion rates of 100 mg dust/day and 200 mg soil/day, child body weight of 15 kg, and continuous exposure scenarios. This approach assumes a threshold for the effects of lead and does not reflect the fullest possible use of the wealth of human data on PbB levels. 

In calculating exposure times, adjustment needs to be made for light/dark cycles in artificial weathering apparatus. 

D to wheat. The resultant data were used to calculate exposure doses and urinary excretion relationships. 

In summary, the PFOA risk assessment is a good example of biomonitoring-led risk assessment. There is no attempt to calculate exposure dose with pathway analysis, because the sources of human PFOA exposure are too uncertain. Instead, the biomonitoring data served as the sole source of human exposure information. Those data could be interpreted in a risk-assessment framework with the aid of PK mod-... 

In addition, existing databases where environmental media and biomonitoring data are collected (such as NHEXAS) could be further studied to estimate exposure and explore the relationships between biomarker concentration and exposure. That information can be used to apportion chemical intake into the different exposure pathways to assist in interpreting population variability, to calculate exposure by combining environmental measurements with survey information to verify estimates of exposure from pharmacokinetic models, and to identify research needs on the basis of discrepancies between estimates obtained from the exposure-pathways analysis and biomonitoring results. 

Pharmacokinetic calculations yielded estimates of chlorpyrifos intake of 0.05-1 pg/kg per day in the general population. The model estimates compare favorably with pathway analysis estimates of aggregate chlorpyrifos exposure from numerous dose routes, including indoor inhalation, dermal contact, and food ingestion (Shurdut et al. 1998 Pang et al. 2002). The calculated exposure doses ranged from 0.02 to 1 pg/kg per day. Further... 

The term model is used to describe exposure including all the circumstances, the scenarios and their mathematical expressions. The term model is also used for computer programs to calculate exposure. In clinical pharmacology, a model characterizes the mathematical expression of the uptake, distribution and elimination of a drug from the body. WHO defines exposure model as a conceptual or mathematical representation of the exposure process (IPCS, 2004). This means that the model includes both concept and mathematical description of the exposure process. 

The calculated exposure results are expressed in the language of numbers. The level of uncertainty is described qualitatively or quantitatively. A numerical approach has the advantages of precision, leaving less room for misinterpretation and providing more input for the decision-maker (Covello Merkhofer, 1993). 

Exposure models relate the concentration of a substance in soil to the potential for exposure or uptake in a human or ecological receptor. They can be described and calibrated on the basis of land use (Figure 5.3), but we recommend that there is a base set of data that would be used in calibrating minimum function in any land use. Calculated exposure differed substantially among models in a comparison of 7... 

Many models are available for calculating exposure, but the European Union System for the Evaluation of Substances (EUSES) is the most commonly used in the EU. Variations in human populations across Member States are considered in terms of body weight, diet, and activities [133]. Consideration is also given to susceptible individuals such as children and the elderly [133]. More specific models are used in conjunction with EUSES to assess occupational dermal exposure (DERMAL), occupational inhalation (EASE) and consumer exposure (CONSEXPO) (see [134]). 

The described models each use a different set of input variables for calculating exposure levels. The format of study data and the procedure for estimating the exposure levels vary for each model. The German and the UK models have used only local and mainly unpublished studies. The PHED is largely based on studies... 

The data on which the estimated levels of exposure are based vary greatly. Some of the differences in these calculated exposure levels might be explained by the nse of different statistics, differences between pre-registration studies (UK model, German model and PHED) and surveillance (post-registration studies), but it is likely that some other (local) factors contribute substantially to the variance. 

As implied by the name, distributional risk assessments use the entire data distribution to calculate exposure and risk. As described for the EPA Tier 1 assessment [5], a distributional analysis is not necessarily a probabilistic analysis. The EPA Tier 1 acute dietary assessment produces a dietary exposure distribution using the entire food consultation distribution and point estimates of residue concentration. There is no probability sampling in the Tier 1 assessment. 

When migration limits for substances are set a conventional system is applied to calculate exposure. It is assumed that a 60 kg person will consume 1 kg of packaged food per day. However, a different convention is sometimes necessary for some circumstances. One such arises in the case of lipophilic substances. Lipophilic substances migrate readily into fatty foods. The consumption of fatty foods is usually only 200 g or less per day. For these substances a reduction factor is therefore planned for use in compliance testing, taking into account the lower consumption of fat. 

Two SI units refer to doses of radioactivity and these are used when calculating exposure levels for a particular source. The sievert (Sv) is the amount of radioactivity giving a dose in man equivalent to 1 gray (Gy) of v-rays 1 Gy = an energy absorption of 1 Jkg. The dose received in most biological experiments is a negligible fraction of the maximum permitted exposure limit. Conversion factors from older units are given in Table 35.3. 

Table 1 Cosmogenic nuclides used for calculating exposure ages.

EPA has recently released the first volume containing the results of an effort to obtain the most reliable possible values for a variety of exposure parameters. This document, The Exposure Factors Handbook (4), covers 12 commonly considered exposure scenarios. The exposure assessment module of Risk Assistant incorporates the algorithms for calculating exposures under each of these scenarios, for all environmental media for which the scenario is applicable. The user can select any or all of the exposure scenarios that are relevant to the environmental media that are contaminated at a site. Where more than one contaminated medium could influence a scenario, the user has the option of selecting the most appropriate medium. 

Referenced Calculations. Exposure assessment involves numerous calculations, covering both cross-media transfers of chemicals and the derivation of exposures from concentrations and scenario-specific parameters. In general terms, such calculations can be viewed as the limiting case (in simplicity) of either theoretical or empirical models. All calculations in Risk Assistant for deriving exposures from concentrations in an appropriate medium (e.g. inhalation exposures from air concentrations) are obtained from the Exposure Factors Handbook (4). Equations for evaluating cross-media transfer for particular exposure scenarios (e.g. volatilization from domestic water to household air) are obtained from literature sources. For such equations, the original reference is provided for the user. 

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