Exposure pathway

States have made substantial recent progress in the adoption, and EPA approval, of toxic pollutant water-quahty standards. Furthermore, virtually all states have at least proposed new toxics criteria for priority toxic pollutants since Section 303 (c) (2) (B) was added to the CWA in February of 1987. Unfortunately, not all such state proposals address, in a comprehensive manner, the requirements or Section 303 (c) (2) (B). For example, some states have proposed to adopt criteria to protect aquatic hfe, but not human health other states have proposed human health criteria that do not address major exposure pathways (such as the combination of both fish consumption and drinking water). In addition, in some cases final adoption or proposed state toxics criteria that would be approved by EPA has been substantially delayed due to controversial and difficult issues associated with the toxic pollutant criteria adoption process. 

Aggregate the hazard for multiple chemical agents and exposure pathways as a hazard index, where appropriate. 

For each subclironic exposure pathway (i.e., two weeks to seven year exposure), calculate a separate subclironic hazard index from die ratios of subclironic daily... 

There are two steps required to detennine whetlier risks or haaird indices for two or more pathways should be combined for a single exposed individual or group of individuals. The first is to identify reasonable exposure pathway combinations. The second is to examine whcUier it is likely that the same individiuils would consistently face the "reasonable ma.ximum exposure (RME) by more than one pathway. 

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... 

To assess die overall potential for non carcinogenic effects posed by several exposure pathways, the total haziird index for each exposure duration (i.e., chronic, subchronic, and shorter-term) should be calculated separately. This equation is described below ... 

Hazard Index (exposure pathway ) + Hazard Index (exposure pathways) +. Hazard Index (exposure pathway,)... 

When the total hazard index for an exposed individual or group of indii iduals exceeds unity, dicrc may be concern for potential non cancer health effects. As indicated before, for multiple exposure pathways, the hazard index can exceed unity even if no single exposure padiway hazard index exceeds unity. If the total hazard index exceeds unity and if combining exposure... 

There are insufficient data to determine potential daily inhalation and dermal exposure levels. However, based on the information presented in Seetions 6.3 and 6.4, exposure levels for the general population are probably very low by these routes. Inhalation exposure is not important for the general population, with the possible exception of those individuals living near areas where methyl parathion is frequently sprayed. Since methyl parathion is readily adsorbed through the skin, dermal eontact may be the most relevant exposure pathway. Dermal eontaet is most likely to oeeur in people who are occupationally exposed. 

Simcox NJ, Fenske RA, Wolz SA, et al. 1995. Pesticides in household dust and soil Exposure pathways for children of agricultural families. Environ Health Perspect 103 1126-1134. 

Children can be exposed to endosulfan by eating food contaminated with the pesticide, by accidentally ingesting the pesticide if it is stored around the house, or by breathing air contaminated with the pesticide if it is sprayed on nearby fields. There are no known unique exposure pathways for children. We do not know if children s intake of endosulfan per kilogram of body weight is different than that of adults. 

Fish are often the focal point of interest for methylmercury contamination, representing the main exposure pathway for humans and wildlife. Unfortunately, longterm data sets with records of both mercury deposition and fish mercury concentrations over time are limited. In Sweden, Johansson etal. (2001) estimated that... 

Collecting and analyzing existing data. Existing data (Table 16.1) are collected and analyzed to develop a conceptual site model that can be used to assess both the nature and the extent of contamination and to identify potential exposure pathways and potential human health or environmental receptors. 

Developing remedial action objectives specifying the contaminant and media of interest, exposure pathways, and preliminarily remediation goals that permit a range of treatment and containment alternatives to be developed on the basis of chemical-specific ARARs when available, other available information, and site-specific, risk-... 

The exposure pathways of concern identified during the baseline risk assessment include direct contact, with the possible ingestion of contaminated soil (1 x 10 3 4 associated excess cancer risk), and potential ingestion of contaminated groundwater in the future through existing or newly installed offsite wells (2 x 11 0 2 associated excess cancer risk). 

Model Types. If it is determined that exposure pathways of interest intersect more than one of the media, the analyst is faced with the need to link together single media models (or to apply existing multimedia models). Despite claims to the contrary, there is probably no single model that is appropriate to all problems. Thus, a hybrid combination of boundary... 

For each category of land or water body use, one may envision a simplified scenario. In each scenario, only those activities most likely to lead to toxic exposures are considered. For example, In the Industrial scenario, Indoor workers would not be exposed to levels of dust bearing high concentrations of soli contaminants outdoor workers who stir up dry soli with heavy machinery, however, could expect to Inhale contaminant-laden dust. A scenario could Involve more than one exposure pathway. Thus, the Industrial worker might drink water from a contaminated well, In addition to breathing contaminated dust these exposures might represent not only different pathways but different sources. 

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

The purpose of an Exposure Route and Receptor Analysis is to provide methods for estimating individual and population exposure. The results of this step combined with the output of the fate models serve as primary input to the exposure estimation step. Unlike the other analytic steps, the data prepared in this step are not necessarily pollutant-specific. The two discrete components of this analysis are (1) selection of algorithms for estimating individual intake levels of pollutants for each exposure pathway and (2) determination of the regional distribution of study area receptor populations and the temporal factors and behavioral patterns influencing this distribution. 

Estimation of Intake. For each exposure pathway, the general equation for estimation of the individual pollutant intake is ... 

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... 

For a limited number of exposure pathways (primarily inhalation of air in the vicinity of sources), pollutant fate and distribution models have been adapted to estimate population exposure. Examples of such models include the SAI and SRI methodologies developed for EPA s Office of Air Quality Planning and Standards (1,2), the NAAQS Exposure Model (3), and the GEMS approach developed for EPA s Office of Toxic Substances (4). In most cases, however, fate model output will serve as an independent input to an exposure estimate. 

Multimedia model for fate analysis and extensive analysis of exposure pathways Human toxicity... 

FUN tool is a new integrated software based on a multimedia model, physiologically based pharmacokinetic (PBPK) models and associated databases. The tool is a dynamic integrated model and is capable of assessing the human exposure to chemical substances via multiple exposure pathways and the potential health risks (Fig. 9) [70]. 2-FUN tool has been developed in the framework of the European project called 2-FUN (Full-chain and UNcertainty Approaches for Assessing Health Risks in FUture ENvironmental Scenarios www.2-fun.org). 

---