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IV. RECOMMENDATIONS FOR FUTURE RESEARCH

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.

IV.1. SOURCES, FORMATION, CONTROLS AND MONITORING

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:

table Table IV-1. Analysis of air emission sources.
. 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.

expand table Table VX X-X

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 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.

IV.2. ENVIRONMENTAL FATE, TRANSPORT, AND BIOACCUMULATION

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, Bvpa.

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.