Ecological risk assessment of dioxins in Australia

Executive summary

This ecological risk assessment is a component of the National Dioxins Program initiated by the Australian Government to assess the impact of polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF) and "dioxin-like" coplanar polychlorinated biphenyls (PCB) on Australian native fauna exposed in aquatic and terrestrial environments to ambient levels.

PCDDs, PCDFs and coplanar PCB, collectively called "dioxins" in this report, are among a group of twelve persistent organic pollutants, which were identified in the Stockholm Convention Treaty on Persistent Organic Pollutants (POPs), (2001) as priority substances for elimination or restriction of release worldwide. The 2001 Stockholm Convention on Persistent Organic Pollutants (POPs) entered into force on May 17, 2004, marking the start of international efforts to rid the world of dioxins, furans, PCBs and nine pesticides.

Dioxins are aryl hydrocarbon receptor agonists, which cause a wide spectrum of adverse toxic effects in many vertebrate species. They are particularly potent developmental toxicants at low concentrations and can disrupt the development of the endocrine, reproductive, immune and nervous system of the offspring of fish, birds and mammals when exposed from conception through postnatal or post hatching stages. In addition to their high toxicity, dioxins are widespread environmental contaminants, found worldwide, even at remote locations. They are resistant to biological and chemical breakdown, and have the ability to bioaccumulate in organisms.

The risk assessment comprises three main parts: the hazard assessment, the exposure assessment, and the risk charatcerisation. The hazard assessment was conducted using excisting published studies examining the toxic effects of dioxins, which are available for a limited number of test species and classes of organisms. Toxicity reference values (TRVs) derived from these studies were adopted to assess the potential risk to native wildlife, for which no toxicity data is available. The exposure assessment was based on the results of the National Dioxin Program's data gathering surveys where dioxin levels were measured in soil, sediment and fauna at locations representative of airsheds and catchments throughout Australia. The risk characterisation was performed by combining information from the hazard and exposure assessments. The findings are summarised as follows:

• Low levels of dioxins were found in Australian soil, sediment and fauna samples, although the levels were highly variable, particularly in fauna. Dioxin levels in soil were on average highest in urban and industrial areas, while dioxin levels in sediment were on average highest in urban and industrial estuaries
• The levels in fauna reflect the animal's position in the food chain, with high trophic level aquatic and terrestrial organisms having the highest levels of dioxins in their bodies relative to other organisms in the same environment. Low trophic level herbivores accumulated the lowest levels relative to other organisms in the same environment. On average, birds of prey had the highest levels found in any organism
• PCDD/PCDFs and PCBs contributed equally to the toxic load in birds and terrestrial mammals, while for marine mammals, PCBs contributed over 90% of the toxic load in dolphins and the seal, and over 80% in whales
• The most sensitive toxicity endpoints (or toxicity reference values, TRVs) with ecological relevance applicable to this risk assessment were taken to be the no-observed-adverse-effect-level (NOAEL) for reproductive and developmental effects in early life-stage fish, birds, and mammals
• A low risk to fish was indicated from exposure to ambient dioxin levels found in the Australian aquatic environment, when using the NOAEL for embryo mortality in the most sensitive fish species tested in the laboratory
• A low risk to terrestrial mammals was indicated from exposure to ambient levels of dioxins when assuming TRVs derived for placental mammals (i.e. rats) exposed during gestation. However, the absence of data on the toxicity of dioxins to native marsupials and monotremes add significant uncertainties to this risk determination. The ramifications of the differing reproduction strategies between placental mammals and marsupials for dioxin exposure at sensitive life stages are not known
• Based on a very small data set and limited toxicity information, a potential risk is indicated for the two dolphins living in the vicinity of urban/industrial estuaries, which had higher TEQs in their bodies than mammals living in the open ocean. Tissue TEQ levels in these animals were within the threshold range TRVs found to cause toxic effects in laboratory and semi-field studies with mink, seal, and otters, which are the only aquatic mammalian wildlife for which TRVs are available. The risks to dolphins in other regions and dwelling in the open ocean are not known.
• No risk is indicated for marine mammals living in the open ocean environments of Australia, which had low levels of dioxins in their bodies
• Recognizing the inherent uncertainties in the models and assumptions, the data is sufficient to signal a potential risk to raptors exposed to ambient levels of dioxins, at least in the regions and subpopulations of birds from where the samples with the highest dioxins loads were collected. Approximately 30% of egg exposure concentrations were higher than the TRV for the most sensitive species - the domestic chicken, while approximately 20% of egg exposure concentrations were above the threshold for the bald eagle, and about 10% of eggs were above the threshold for embryo mortality in the American kestrel. Birds with the highest dioxin loads were collected in urban environments.

All risk assessments have uncertainties associated with them. The uncertainties arise from the inevitable knowledge and data gaps, which require the adoption of assumptions to cover these gaps. The above risk estimations are no exception. The conclusions are based on a small fauna data set, comprising a limited number of species and trophic levels, and whose sensitivity to the toxic effects of dioxins is not known. These inherent uncertainties should be taken into account when interpreting the results of the risk assessments. A conservative approach has generally been adopted at all stages of the risk assessment to prevent underestimation of the risk, and this should also be kept in mind when interpreting the results.

More reliable risk estimations would require information on the toxicity of dioxins to Australian wildlife species. Australian ethical committees and current State Government legislation generally do not allow toxicity testing on native species. More targeted sampling of the eggs of raptors and other high trophic level birds, in association with field population studies of potentially exposed bird populations, would help to clarify whether dioxins are having a real impact on wild bird populations.

Further information

• More about dioxins