Exposure estimation

After intakes have been estimated, they arc organized by population, as appropriate. Then, tlie sources of uncertainty (e.g., variability in analytical data, modeling results, parameter assumptions) and their effect on tlie exposure estimates are evaluated and sunuiumzed. Tliis information on uncertainty is important to site decision-makers who must evaluate tlie results of the e.xposure... 

Extrapolating exposure estimates to all urban cities In the world would Indicate that as many as 625 million people, mostly In developing countries, are exposed to unacceptable levels of 02 pollution and about 1.25 billion people live In urban areas where SPN levels exceed acceptable guidelines. This means that less than 20 percent of city dwellers live In environments that can be considered to have acceptable outdoor air quality levels. 

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... 

Most of the exposure estimates are based on modelling, which is highly dependent on physicochemical properties of the compormds actual monitoring is minimal in most cases. 

This study, like that of Fisher and Allen (1993), incorporated a linear multistage model. However, the mechanism of trichloroethylene carcinogenicity appears to be non-genotoxic, and a non-linear model (as opposed to the linearized multistage model) has been proposed for use along with PBPK modeling for cancer risk assessment. The use of this non-linear model has resulted in a 100-fold increase in the virtually safe lifetime exposure estimates (Clewell et al. 1995). 

NOAEL (no-observed-adverse-effect level) is defined as the highest dose at which no adverse effects are observed in the most susceptible animal species. The NOAEL is used as a basis for setting human safety standards for acceptable daily intakes (ADIs), taking into account uncertainty factors for extrapolation from animals to humans and inter-individual variabilities of humans. The adequacy of any margin of safety or margin of exposure must consider the nature and quality of the available hazard identification and dose-response data and the reliability and relevance of the exposure estimations. In some cases, no adverse endpoint can be identified such as for many naturally occurring compounds that are widespread in foods. In that case, an ADI Not Specified is assigned. ... 

R.C. Honeycutt, The Usefulness of Farm Worker Exposure Estimates Based on Generic Data, in Dermal Exposure Related to Pesticide Use Discussion of Risk Assessment, ed. R.C. Honeycutt, G. Zweig, and N.N. Ragsdale, ACS Symposium Series 273, American Chemical Society, Washington, DC, pp. 369-375 (1985). 

Exposure of the general population to diisopropyl methylphosphonate is expected to be highly unlikely and to occur at extremely low levels, but data are insufficient for exposure estimates. Diisopropyl methylphosphonate has been detected in the groundwater and, to a lesser extent, in the surface water and soil at or near the RMA. If exposure of the general population to diisopropyl methylphosphonate were to occur, water would be the most likely source. 

In a report comparing community responses to low-level exposure to a mixture of air pollutants from pulp mills, Jaakkola et al. (1990) reported significant differences in respiratory symptoms between polluted and unpolluted communities. The pollutant mixture associated with the pulp mills included particulates, sulfur dioxide, and a series of malodorous sulfur compounds. Major contributors in the latter mixture include hydrogen sulfide, methyl mercaptan, and methyl sulfides. In this study the responses of populations from three communities were compared, a nonpolluted community, a moderately polluted community, and a severely polluted community. Initial exposure estimates were derived from dispersion modeling these estimates were subsequently confirmed with measurements taken from monitoring stations located in the two polluted communities. These measurements indicated that both the mean and the maximum 4-hour concentrations of hydrogen sulfide were higher in the more severely polluted community (4 and 56 g/m3 2.9 and 40 ppb) than in the moderately polluted one (2 and 22 g/m3 1.4 and 16 ppb). Particulate measurements made concurrently, and sulfur dioxide measurements made subsequently, showed a similar difference in the concentrations of these two pollutants between the two polluted communities. 

Table 7 Comparison of Exposure Estimated Levels (Total Absorbed Dose of Chlorpyrifos) Dosimetry vs. Urine Monitoring...

The third method used to interpret the level of risk associated with chlorpy-rifos use is Monte Carlo simulation. This method provides a range of exposure estimates for the evaluation of the uncertainty in a risk estimate based on ranges of input variables. The first step in performing a Monte Carlo simulation is determination of a model to describe the dose. This model describes the relationship between the input parameters and dose, and a specific model is presented here for one group of workers. 

In 1985, Berteau and Mengle (1985) of the California Department of Health Services and Maddy of the Department of Food and Agriculture conducted a preliminary review of pesticides used indoors. They noted several cases (six) from the Pesticide Illness Surveillance system in which illness was reported after structural pest control. Hypothetical exposure estimates for infants, children, and adults following label use for propoxur, DDVP, and chlorpyrifos were sometimes greater than toxic levels. In 1987, Berteau et al. (1989) reiterated the concern about the potential magnitude of indoor exposures, particularly for children. 

Table 2 Summary of Human Chlorpyrifos Exposures Estimated Using Whole-Body Dosimeters and Biomonitoring...

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. 

Modeling re-entry exposure estimates techniques and application rates... 

Haan de, M., Brouwer, D.H., and van Hemmen, J.J. (1996) Re-entry Exposure Estimates Application Technique, Foliar Surface and Re-entry Time as Critical Predictors for Dislodgeable Foliar Residue, Report V96.384, TNO Nutrition and Food Research Institute, Zeist, The Netherlands. 

In the early 1980s, the whole-body dosimeter (WBD) was introduced as a superior method for passive dermal dosimetry monitoring. A standard protocol was described by the World Health Organization (1982), and Abbott et al. (1987) described some additional options. Chester (1993) reported refinements that permitted exposure estimation by passive dermal dosimetry and biological monitoring simultaneously. 

In the mid-1980s, Fenske et al. (1986a,b 1993) described the use of fluorescent compounds coupled with video-imaging measurements to produce exposure estimates over virtually the entire body. The technique has been applied to pesticide mixers and applicators by Fenske and his coworkers, but has not been widely used by other investigators. 

In the SRI report (2) the release information on benzene was used with atmospheric dispersion models and data on geographic distribution of population to obtain aggregate exposure estimates (shown in Table IV). 

Each of the main risk analysis elements consists of three interactive studies. Exposure estimates result from the integration of pollutant dispersion patterns and human population patterns. The dispersion patterns, in turn, result from the joint action of emissions and dispersion processes. 

Another case of multimedia fate modeling may be exemplified by human inhalation exposure estimates for PCB spills. The spill size is estimated considering both spread and soil infiltration. Volatilization calculations were carried out to get transfer rates into the air compartment. Finally, plume calculations using local meteorological statistics produced ambient concentration patterns which can be subsequently folded together with population distributions to obtain exposures. 

Receptor Exposure. Exposure modeling should produce a statistically representative profile of pollutant intake by a set of receptors. This is done by combining the space/time distribution of pollutant concentrations with that of receptor populations (whether they be people, fish, ducks or property made of some material that is vulnerable to pollutant damage). The accuracy and resolution of the exposure estimates are chosen to be consistent with the main purposes of decision making. These purposes include the following ... 

It is clear that these goals place widely differing requirements on both the resolution and the accuracy of exposure estimates thus, the approach selected should be optimized to fit the requirements. 

---