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IV.3. CHEMICAL/PHYSICAL PROPERTIES

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.

IV.4. EXPOSURE

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.

IV.5. PHARMACOKINETICS

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.

IV.6. COPLANAR PCBs

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?

IV.7. NON-CHLORINE HALOGENATED FORMS OF DIBENZODIOXIN/FURANS AND COPLANAR BIPHENYLS

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.

IV.8. GLOBAL IMPACTS

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.

REFERENCES FOR RECOMMENDATIONS SECTION
  • Bacci, E.; Calamari, D.; Gaggi, C.; Vighi, M. (1990) Bioconcentration of Organic Chemical Vapors in Plant Leaves: Experimental Measurements and Correlation. Environ. Sci. Technol. 24: 885-889.
  • 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.
  • McCrady, J.K.; Maggard, S.P. (1993) Uptake and photodegradation of 2,3,7,8-tetrachlorodibenzo-p-dioxin sorbed to grass foliage. Env. Sci. Technol. 27:343-350.