Although the dioxin-like compounds have probably been studied more than any other set of organic compounds in the environmental field, numerous data gaps remain. Basic questions such as what sources contribute most to human body burdens are still unanswered. This section summarizes the research needs for exposure to dioxin-like compounds.

Research on how CDD/F is formed provides a seminal basis for understanding CDD/F sources. Three basic theories on the formation and emission of CDD/Fs during the combustion of chlorine-bearing wastes and fuels have been advanced by research in the international scientific community and are summarized in Volume II, Chapter 3, Section 3.5. Scientific knowledge on the mechanisms of formation of CDD/F within combustion processes can help to provide answers in a number of important areas, including:

- identification of unknown combustion sources that have yet to be tested for emissions.

- identification of process changes and operating practices that will prevent the formation of CDD/Fs in various combustion sources.

- help with development of engineering controls to reduce CDD/F emissions at known combustion sources.

Further research recommendations relating to sources are outlined below.

• Combustion Source Testing: For purposes of setting priorities on research to better characterize combustion sources, consideration must be given to the estimated size of the source on an individual and collective basis and level of confidence in current estimates. This analysis, given in Table IV-1, suggests that the following source categories are high priority for further testing: 1) medical waste incinerators, 2) cement kilns, 3) industrial

Table IV-1. Analysis of air emission sources.

wood burners, 4) primary metals industry (aluminum, magnesium, iron, copper) and secondary metals industry (aluminum, magnesium, steel) and 5) diesel engine exhaust. For each of these source categories, a field survey is needed involving emissions testing at selected facilities. In planning such a survey, consideration must be given to statistical issues, cost issues, sample collection/analysis, and similar issues.

• Unknown Sources: As discussed earlier in this document, several investigators have speculated that the identification of CDD/F sources may be incomplete on the basis of mass balance analyses comparing emissions to deposition. It is not clear whether this type of mass balance can ever be refined to the point where definitive conclusions can be drawn. However, it remains one of the few methods of evaluating the possibility that unknown sources exist. Thus, research is needed to refine both emission and deposition estimates. Research to better characterize known sources is discussed above. Deposition estimates can be improved via a combination of further field measurements and modeling. Industrial sectors which are likely candidates for dioxin emissions can be identified from knowledge about industrial processes, feed materials and theories on formation.

• Emissions Monitoring: Currently the monitoring of CDD/Fs in stack gas emissions from combustion sources cannot be conducted continuously or on a real-time basis. The test method (EPA Method 23) requires sampling in the stack for 5 or more hours, and several weeks or months lead time in developing laboratory results of the sample. This situation raises concerns about the representativeness of the sample and about the inability to detect variability in emissions. From a public health perspective, a method of continually and instantaneously measuring emissions would be desirable. This situation suggests two areas of research. The first area would be to develop CDD/F stack measurement/laboratory techniques which provide quicker results. The second area would be to identify an easily monitored combustion parameter that strongly correlates with the magnitude of dioxin emissions. Such parameters may be measured inside or outside the furnace, and may include: temperature, carbon dioxide, carbon monoxide, oxygen, total hydrocarbons, and particulates.

• Emission Controls: Engineering research is needed to develop process changes or emission controls which reduce dioxin emissions. For example, pollution prevention research is needed to determine if dioxin releases can be reduced via reductions in chlorine content of feed material, changes in operating temperatures or other techniques.

• Combustor Ash and Scrubber Residues: Municipal waste combustor ash and cement kiln dust/clinker have been tested for CDD/F content. Ash from other combustor types such as coal utilities and medical waste combustors have not been tested. No data was found on CDD/F levels in effluent from scrubbers. Research is needed on the levels of CDD/F in these materials and the potential for their release to the environment.

• Source-Receptor Relations: Studies are also needed to evaluate whether CDD/F sources contribute to human exposure in proportion to their overall contribution to environmental loading, or whether some sources contribute disproportionally to general population exposure. For example, it has been speculated that diesel exhaust emissions which occur as extensive line sources at ground level may cause higher exposure (per unit emission) than stack emissions from stationary sources (Jones, 1993). One way to link sources to receptors is on the basis of congener profiles. Each combustion source technology may routinely emit a distinctive pattern of CDD/F congeners. This has been referred to as a congener profile, and could provide a means whereby emissions from a variety of combustion sources can be distinguished from one another. Thus research is needed to determine whether distinctive congener profiles can be developed for various sources.

• Non-Combustion Sources: The above discussion has focused on combustion sources. It is important, however, to study non-combustion sources. Relatively little effort has been spent characterizing non-combustion sources (one notable exception is the pulp and paper industry). Similarly, little information has been collected on CDD/F levels in most products other than paper. In general this research should parallel the areas identified above for combustors, i.e. formation, source testing, identification of unknown sources, monitoring, controls, process residues/wastes and source-receptor relationships. This research should focus on the following non-combustion sources:

- Chlorophenol production: The two compounds in this class historically of concern are pentachlorophenol (PCP) and trichlorophenol. Although, production and use of these compounds are now limited, new testing is needed of products and waste streams to confirm CDD/F levels.

- Chlorobenzene production: Studies in Germany have measured the presence of CDD/Fs in these compounds. No United States data could be found.

- Aliphatic chlorine production: CDD/Fs can be released during the production of vinyl chloride, however the size of these emissions have not been independently confirmed. As discussed earlier in this document, Greenpeace has suggested that such releases could be large and the vinyl chloride industry have strongly disputed these claims. The Greenpeace estimates are based on information about European plants. No data from the United States could be found.

- Pesticide production: EPA has sponsored data call-ins which has provided some assurance that many pesticides have low CDD/F levels. Not all requested data has been received, however, and independent testing of products and waste streams may be needed to confirm levels.

- Sewage treatment: Effluent and sludge from sewage treatment plants have been shown to contain CDD/F residues. More research is needed characterizing these levels and studying formation mechanisms/controls.

• Reservoir Sources: Rerelease of CDD/F from reservoir sources could occur by dust resuspension, erosion, volatilization, etc. The impact of these reservoir emissions compared to current emissions on the human food chain is unknown. Research is needed to evaluate the magnitude of these releases and their impact on the food chain.

Understanding the environmental fate of CDD/Fs is central to evaluating human exposure. Empirical measurements of inter-media transfers, environmental degradation/clearance rates, and bioaccumulation are fundamental to designing mathematical models that simulate these events. Environmental fate models are a valuable tool for evaluating impacts from specific sources and evaluating the proportionality between magnitude of emissions and subsequent exposures. Although much is known about environmental fate and transport of CDD/Fs, a number of issues remain that require further research. Key areas include:

• Environmental Monitoring: Knowledge of environmental levels is fundamental to understanding how CDD/Fs behave in the environment. More data is needed on CDD/F levels in air, wet/dry deposition, sediments, soils, plants and animals. As discussed below, this information can be used to improve model formulation, parameter assignments and model validation.

• Vapor/Particulate Partitioning: The modeling analysis of Volume III concluded that the transfer of dioxin-like compounds to vegetation which animals consume was the principal cause for terrestrial animal food chain impact. Thus, a better understanding of the extent to which these compounds partition between vapor and particle phases in ambient air in rural and urban environments is important. A second issue is whether this partitioning is different for stack emissions versus volatilized residues from soil. While the volatiles are initially in the vapor form, do they remain as such or do they sorb to airborne particles?

• Vapor Transfers to Vegetation: As noted above, vapor transfers to vegetation largely explain terrestrial food chain impact. Further research is needed to refine the algorithms presented in this document, with particular attention paid to: differences in transfer rates among different congeners, the potential for photodegradation when sorbed onto vegetative surfaces, and the impacts of shifting wind patterns, variable crop densities, sunlight conditions, and other real world conditions.

• Photodegradation/Transformations of Vapor-Phase Dioxins: Some studies have suggested that photodegradation of dioxin-like compounds may occur under natural conditions. This process is not expected to occur for sorbed dioxins, and there is very limited data on photodegradation of dioxins while airborne in the vapor-phase. Laboratory studies have demonstrated that CDD/Fs undergo photolysis, typically following first order kinetics, in the presence of a suitable hydrogen donor such as oil or an organic solvent. Study results, when extrapolated to environmental conditions, indicate half-lives ranging from hours to days. There is some evidence of reductive dechlorination, or the transformation of dioxins of higher chlorine content to dioxins of lower chlorine content. This suggests the possibility that photodegradation can be both a destruction and a formation mechanism. In general, it was decided that these processes are not sufficiently well understood to explicitly incorporate into the procedures of this document. The procedures in Volume III assume no degradation of vapor-phase dioxins during transport from stacks. Photodegradation is partially accounted for in the transfer of vapor-phase dioxins to vegetations in the air-to-leaf transfer factor, B_vpa. The assignment of values for this parameter is based on the air-to-leaf experiments of Bacci, et al. (1990; 1992), with an empirical adjustment developed from the experiments of McCrady and Maggard (1993), who measured the impact of photodegradation in the transfer of vapor phase 2,3,7,8-TCDD to grass leaves. In summary, research is needed which provides 1) photodegradation rate constants for these compounds in the air and on plant surfaces, 2) information on the formation products of photodegradation of dioxins in air and on plant surfaces, and 3) procedures to incorporate this knowledge into fate models. It is important that this research be conducted in ways that convincingly simulate real world conditions and hence provide practical results for incoroprating into fate models.

• Soil Volatilization and Dispersion: The models for soil volatilization and subsequent dispersion to estimate air concentrations for food chain modeling and inhalation exposures have not been verified. Some empirical evidence described in Volume III suggest that these algorithms may be underestimating air concentrations of dioxin-like compounds (see also the entry titled, "Predicted vs. observed air concentrations" in Table III-5 of this Volume).

• Soil Dissipation Rates:  A soil dissipation rate of 0.0693 yr^-1, corresponding to a 10-year half-life, is assumed for all dioxin-like compounds delivered to an exposure site as deposited particles from a stack emission source, or as delivered via erosion from a site of soil contamination. Some empirical evidence described in Volume III suggests that delivered contaminants may be more persistent and that this is a low half-life (see also the entry titled, "Predicted vs. observed beef concentrations" in Table III-5 of this Volume). Further evaluation of this dissipation assumption is recommended.

• Overland Transport Mechanisms: The process of soil erosion was assumed to transport soil-bound residues from a site of contamination to a site of exposure. Soil erosion was also assumed to transport residues bound to watershed soils to surface water bodies. Other mechanisms of soil-bound transport were not modeled, such as wind erosion followed by deposition. Two factors that were modeled but are uncertain is the sediment delivery ratio, which reduced potential erosion based on the deposition of eroded particles prior to their destination, and the enrichment ratio, which increased the concentration of dioxins on eroded soil based on the assumption that eroded materials are finer and higher in organic matter as compared to in-situ soil.

• Water Body Processes: Because of their affinity for organic carbon, the fate and transport of dioxin-like compounds in water bodies is likely to be more a function of sediment-related processes rather than water-related processes. Key sediment processes in water bodies include: sorption/desorption, importance and prevalence of dissolved organic materials in the water column, deposition/suspension/resuspension, and downstream sediment transport. Although procedures for sediment modeling in surface water bodies is presented in the exposure document, the models are fairly simplistic and more development is recommended, especially for evaluating point source discharges.

• Ground Water: The occurrence of these compounds in ground water is expected to be minimal, based on strong sorption to soils. Ground water impacts were not assessed in this document. Dioxin-like compounds, particularly PCBs, have been found, however, in ground water below and near sites of industrial contamination. Co-occurrence with other organic compounds, co-occurrence with solvents, and transport associated with oils have been cited as causes of enhanced mobility in these settings. The possibility that dioxins may impact ground water in certain circumstances should be evaluated further.

• Beef Food Chain Modeling: This document proposes the hypothesis that the air-to-food pathway is the principal mechanism by which dioxin-like compounds enter the food chain. The air-to-beef model developed in this assessment is examined in Chapter 7 of Volume III with a validation exercise which provides preliminary evidence that it will predict beef concentrations that are consistent with observations (see also the entry titled, "Predicted vs. observed beef concentrations" in Table III-5 of this Volume). Given the importance of this pathway, however, further validation work is recommended. More information is needed on several of the components of the model to estimate beef and milk concentrations. Such information includes: cattle soil ingestion rates, pasture grass concentrations and mechanisms of transfer from the air/soil to pasture grass (and other feeds such as corn, hay, etc), the impact of cattle production practices to cattle food product concentrations, models and data to further develop the bioconcentration factor (termed BCF in exposure document) and assessment of differences in bioavailability between soil and vegetative intakes.

• Bioaccumulation in Fish: Several approaches have been suggested for estimating uptake in fish. The approach in this assessment is based on the organic carbon normalized concentration in water body sediments. One parameter used is termed the Biota to Sediment Accumulation Factor, or BSAF. This is defined as the ratio of the concentration in fish lipids to the organic carbon normalized concentration in bottom sediments. The BSAF represents uptake by all mechanisms. Another sediment-based parameter used in this assessment is the BSSAF, or the Biota Suspended Sediment Accumulation Factor. This is defined similarly to the BSAF, except it is based on the organic carbon normalized concentration in suspended sediments. Other parameters that have been used include the Bioconcentration Factor, or BCF, which is based on ratios between levels in fish to levels in water and represents only uptake from water, and the Bioaccumulation Factor, or BAF, which is based on ratios between levels in fish and water and representing uptake by all mechanisms. Further research is needed to develop congener specific values for these factors, develop procedures explaining how to apply these factors and to validate these procedures with field data. A key issue that has been identified is whether BSAFs that have been developed for one species and water body are generalizable to another species and another water body. This question will be difficult to answer because of the several uncertainties associated with BSAF development: fish migratory patterns, variability in fish lipid content and other differences within and between species, study design with regard to fish and sediment sampling, ecosystem differences, and so on. However, after careful examination of existing data sets and considering key differences between species (invertebrates vs. vertebrates, fresh water vs. salt water, bottom feeders vs. water column feeders, etc.), it may be possible to develop a workable system for BSAF assignment based on key considerations.

• Other Food Products: This document did not present site-specific assessment procedures to evaluate all terrestrial exposure pathways. For example, models are not presented to estimate concentrations in such products as eggs, chicken, and pork. Further research is needed to develop these procedures.

Chemical specific inputs are needed for all fate models and can contribute as much uncertainty to impact estimates as the conceptual formulation of the model itself. Throughout the exposure document, the lack of congener-specific data is cited as a major source of uncertainty. For example, congener-specific data is lacking for basic chemical properties such as octanol-water partition coefficients, degradation rates, and vapor pressures. Also, data is lacking for estimation of congener-specific incinerator emission factors, metabolic rate constants, and bioavailability and biotransfer factors. Thus, gathering more data on congener-specific properties is a high priority for further research.

Key areas for exposure research are outlined below.

• Levels in Food Products: This report estimates that about 90% of human exposure to CDD/Fs occurs via food ingestion. Research is needed to determine associations between levels in food to sources and agricultural practices. Data are severely lacking on concentrations in foods identified as critical - beef, milk, other dairy products, eggs, pork, poultry and marine fish. Thus, future exposure research should emphasize issues related to levels in animal product foods. Key questions for further research include:

1) What are representative concentrations of dioxin-like compounds in these food products?

2) Are there regional differences in the level of food contamination? Can these be correlated to local sources or animal raising practices?

3) Are there differences in body burden between: range-fed and feedlot cattle, free ranging or caged chickens, or other alternate practices for other animals?

4) What is the immediate source of animal contamination?

- CDD/F incorporated within grains or other feeds

- surface contamination on grasses and other feeds

- contaminated dirt on grasses and other feeds

- dirt eaten by animals while grazing

- food additives

- other chemicals associated with animals or crops

5) Are there any significant opportunities to reduce exposure to animals by changing feeding practices?

• Other Products: This document presents data showing that, in some circumstances, dioxin can migrate into food from paper products such as milk containers. The paper industry has presented data indicating that recent reductions in dioxin levels in bleached pulp suggest that such migration is minimal. Independent testing of paper products used in food packaging is needed to confirm these claims.

Researchers in Germany (Horstmann and McLachlan, 1994) have found that some textiles contain high levels of CDD/Fs and that they can be transferred from the textiles to human skin. The researchers speculated that the source of these dioxins was pentachlorophenol preservatives used on cotton during sea transport. More research is needed on the levels of CDD/Fs in textiles, the sources of contamination and their potential for human exposure.

• Highly Exposed Populations: This document reports that CDD/Fs have been measured in human breast milk and could contribute a significant portion of a person's body burden. Key questions to address in future research in this area include:

1) What is the relative rates of exposure for nursing infants from breast feeding versus formula feeding?

2) Is there much variation in CDD/F levels for mother's milk and if so, do these variations correlate with any observable factors?

3) Is there anything nursing mothers or women of child-bearing age can do to reduce exposure to their children?

Other subpopulations, such as subsistence fishers and farmers, have been identified as potentially highly exposed. More research is needed to identify these groups and determine their level of exposure. Finally, studies should also be conducted examine whether socio-economic factors can influence dioxin exposure.

The use of pharmacokinetics in body burden analysis has shown great potential for estimating exposure levels. In order to reduce the uncertainty in these procedures, increased collection of biological samples and improvements in PK model structure and input parameters are recommended. In addition, further research should be conducted on the application of these procedures to estimating target organ dose, absorbed dose, lactational/placental transfers, and effects on offspring.

This document does present some information on the chemical/physical properties of some coplanar PCBs, brief qualitative information on possible sources, some information on environmental occurrence levels, and nothing on background exposures. The fate and transport models presented in the document would be generally applicable to these compounds, but the chemical specific inputs need further development.

The available information does suggest that total PCB levels are commonly much higher in soils and sediments than the other dioxin-like compounds. Most environmental data are reported as total PCBs or as an Aroclor mixture. Since congener specific data are largely unavailable, it is not clear what portion of these PCBs are coplanar. Congener specific sampling and analysis protocols need to be evaluated. Also, there is not yet a concurrence on Toxicity Equivalency Factors (TEF) schemes, so even if estimates of concentrations of coplanar PCB were made, it is not yet clear how to convert these to a 2,3,7,8-TCDD comparable basis. Thus the first goal of this research would be to derive preliminary estimates of what portion of the total PCBs present in the environment are the coplanar congeners. This would involve reviewing the limited congener specific data that is currently available and evaluating how representative it may be of PCBs in other locations. The various TEF schemes that have been proposed could be used to further assess the potential importance of these compounds. The next logical step would be to conduct a large sampling and analysis program to confirm the levels of these compounds in the environment. As TEF schemes are refined they should be incorporated into this effort.

Other research questions specific to PCBs include:

1) Are there any current sources releasing coplanar PCBs to the environment? Under what conditions are coplanar PCBs formed in industrial and combustion processes? What are the emission factors are what are the locations for major sources?

2) What are the background exposure levels to these compounds? Evaluation could be done using both a forward analysis, starting with diet information, and in a reconstructive manner, starting with body burdens.

3) How persistent are the coplanar PCBs relative to the other PCBs?

4) Is most of the body burden derived from "old PCBs" recirculating around in the environment or is current and future body burden significantly effected by more recently released materials?

5) What is the relative contribution of controlled large sources (HD electrical equipment) versus the more uncontrolled dispersed small sources such as small capacitors and fluorescent light ballasts?

6) Are the pathways of exposure for dioxin-like PCBs different than for CDD/Fs?

7) Do PCB sources contribute to human exposure proportional to the overall contribution to environmental loading, or do some sources contribute disproportionally to general population exposure?

Considerable uncertainty remains concerning the health effects of these compounds as well as basic exposure issues such as environmental occurrence, background exposure levels, chemical/physical properties, and sources. Other than some discussion on chemical/physical properties, these compounds are not addressed in the current document. The fate and transport models presented in the document would be generally applicable to these compounds, but the chemical specific inputs would need further development. No TEF schemes have been published or adopted for these compounds. As with the coplanar PCBs, the first goal of the research in this area would be to estimate the levels of these compounds in the environment and human body burdens. This estimate should initially be attempted on basis of existing data, but very likely a sampling and analysis program will be needed to collect sufficient data for even initial estimates. Congener specific sampling and analysis protocols need to be evaluated. The next steps would be to identify/evaluate sources and pathways of exposure and to estimate background exposure levels.

This document presents environmental and human body burden data showing that the dioxin-like compounds are found all around the world. Atmospheric deposition has been measured in remote locations such as the Arctic indicating that long range transport of these compounds occur. It is important to better understand the geographic extent of exposure to these compounds and how far impacts from particular sources may spread. Thus, further research is needed to compare local, regional and global impacts.

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Bacci, E.; M.J. Cerejeira; C. Gaggi; G. Chemello; D. Calamari; M. Vighi (1992) Chlorinated Dioxins: Volatilization from Soils and Bioconcentration in Plant Leaves. Bull of Env. Cont. and Tox. 48(3):401-408.

Horstmann, M.; McLachlan, M.S. (1994) Textiles as a source of polychlorinated dibenzo-p-dioxins and dibenzofurans (CDD/CDF) in human skin and sewage sludge. Environ. Sci. and Poll. Res. 1(1):15-20.

Jones, K. (1993) Diesel truck emissions, an unrecognized source of CDD/CDF exposure in the United States. J. Risk analysis 13(3):245-252.

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