Identifying Potentially Exposed Populations

Exposure is one element in the environmental health framework depicted in Figure 13 that links contaminant sources to health effects. Exposure assessment evaluates the processes for identifying potentially exposed populations, identifying potential pathways of exposure, and quantifying the magnitude, frequency, duration, and time pattern of contact with a contaminant. 

All potentially exposed populations should be identified. The exposed populations should be associated with the activity, task, or source of environmental releases that lead to the exposure. Highly exposed or highly susceptible populations should be addressed whenever possible. All routes of exposure should be included. 

If actual or potential exposure has been identified, a quantitative exposure assessment is necessary. Exposure levels/concentrations for each potentially exposed population need to be derived from the available measured data and/or from modeling. A range of exposure values to characterize different subpopulations and scenarios may result. These results are taken forward to the risk characterization where they are combined with the results of the effects assessment in order to decide whether or not there is concern for the human population exposed to the substance. In some cases all three types of exposure estimates may contribute to an overall exposure value (combined exposure), which should be considered in the risk characterization. 

In tliis step, tlie exposure assessor identifies tliose patliways by ihich the previously identified populations may be exposed. Each exposure pathway describes a unique niechanisni by which a population may be exposed to the chemical at or originating from tlie site. E. posure patliways are identified based on consideration of the sources, releases, types, and location of chemicals at the site, the likely environmental fate (including persistence, partitioning, tnuisport, and intermedia transfer) of tliese chemicals, and the location and acti ities of the potentially exposed populations. Exposure points (points of potential contact with the chemical) and routes of e.xposure (e.g., ingestion, iiilialation) are identified for each exposure patliway. 

Step 5. The potentially exposed population and characterization of the population that may come or have come in contact with the contaminant are identified. 

Exposure assessment determines the type and magnitude of exposure to a chemical or chemicals exposure. Exposure assessments identify the exposed population, describe its composition and size, and present the magnitude, frequency, type, and duration of exposure. It is used to predict effects of exposure and recommend strategies for reducing that exposure. Once the exposure assessment has been determined, the results are combined with toxicity information to characterize potential risks. 

Some animal studies indicate that dietary exposure to methyl parathion causes decreased humoral and cellular responses (Shtenberg and Dzhunusova 1968 Street and Sharma 1975). A more recent, well-designed animal study that included a battery of immuno/lymphoreticular end points showed few effects at the nonneurotoxic doses tested (Crittenden et al. 1998). No adequate studies are available in humans to assess the immunotoxic potential of methyl parathion. Therefore, studies measuring specific immunologic parameters in occupationally exposed populations are needed to provide useful information. Further studies are also needed to investigate the mechanism for methyl parathion-induced immunotoxicity since this information would help to identify special populations at risk for such effects. 

Individuals occupationally exposed to coal tars or the naphtha fraction of coal-tar distillate have potentially high exposure to 2,3-benzofuran. Persons living near industrial sources or hazardous waste sites contaminated with 2,3-benzofuran may be exposed to 2,3-benzofuran. There are insufficient data to identify any other populations with potentially high exposure to this compound. 

Although the notion of normal is not appropriate to biomonitoring, there are two axes of potential reference around which comparisons may be pertinent and indeed important. The first is in relation to comparable subjects. For example, where adequate information exists, it would be entirely appropriate to express values in relation to one or more groups of subjects. For example, it would be reasonable to report an individual or group of blood lead concentrations in children as comparable with those measured (extensively) in populations of children not known to have a specific exposure source or suggestive of an identifiable source of exposure. In adults, it might be reasonable to express lead concentrations as comparable with concentrations seen in the population not occupationally exposed to lead within the range of concentrations seen in workers exposed to lead under well-controlled conditions, or the like. Those are not assessments of normality or health effects, merely comparisons with other potentially relevant populations that could lead to research inferences or clinical or public-health interventions. 

Exposure Levels in Environmental Media. Tetryl has been detected in seepage water, groundwater, and surface and subsurface soil at military installations (Army 1980, 1981b, 1986a, 1988, 1990b ATSDR 1987 HazDat 1994). More data are needed regarding levels of tetryl in surface water, groundwater, soil, and air in and around these sites. Quantitative information is needed to assess the potential for human exposure and to better identify exposed populations. 

Human exposure data are evaluated to identify populations that might be exposed, to identify potential pathways of exposure, and to estimate the range of exposure so that quantitative estimates of exposure can be made that are associated with each pattern of use. Exposure conditions that are unique for reproductive and developmental toxicity should be considered because the embryo, fetus, neonate, juvenile, young adult, and older adult differ in susceptibility. Human exposure data are important for accurate evaluation of the risk potential of an agent, but data of sufficient quality and quantity are often unavailable. 

Occupational exposure can be significantly higher than the exposure of the general population. NIOSH (1988) identified about 10,600 workers (mainly chemists, equipment servicers, and janitorial staff) as potentially exposed workers in facilities where nitrobenzene is used. Because nitrobenzene is readily absorbed through the skin, as well as taken up by inhalation and ingestion, industrial exposure necessitates worker protection, and this has been recognized for decades. At an industrial exposure level of 5 mg/m (1 ppm), a worker would receive about 25 mg during an 8- hour day (Dunlap 1981). 

Effect. Several potential biomarkers for the effects of zinc have been identified. These include increased levels of serum amylases and lipase, indicative of pancreatic damage non-iron responsive anemia and decreased HDL cholesterol levels (Suber 1989). However, these biomarkers of effect are not specific for zinc. These biomarkers cannot be used for dosimetry. A potential biomarker of exposure for recent exposures to zinc is increased erythrocyte metallothionein concentrations (Grider et al. 1990). Further investigation of serum biomarkers of effect, particularly for chronic exposure, in zinc-exposed populations would be useful to determine whether exposed populations may be experiencing adverse health effects as the result of zinc exposures. 

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... 

Immunotoxicity. No information on specific immunological effects of 1,2-dibromoethane is available for humans or animals exposed via inhalation, oral, or dermal routes. Some effect on the immune system can be inferred from a report of lymphoid neoplasia associated with exposure of workers to various chemicals including 1,2-dibromoethane (Alavanja et al. 1988). Epidemiological and animal studies would be useful to investigate the immunotoxic potential of 1,2-dibromoethane. Furthermore, if 1,2-dibromoethane proves to be a potential immunosuppressant, further research into this area could help identify populations at higher risk because of pre-existing permanent immunosuppression. 

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