Dioxins are a group of toxic persistent organic chemicals that remain in the environment for a long time.
Dioxins are not deliberately produced, but are released into the environment as a result of combustion and other chemical processes. Human activities involving these processes include power generation and waste incineration, and the manufacture of metals and some chemicals. Dioxins are also generated from natural sources such as bushfires and volcanoes. Dioxins serve no useful purpose.
Dioxins occur in trace amounts as contaminants in air, water and soil throughout the world including remote areas such as the Arctic.
Yes. Scientists use the term 'dioxin' to refer to a group of compounds that share certain similar chemical structures and biological characteristics. Several hundred of these toxic compounds exist and are members of three closely related families:
Twenty-nine of these compounds are believed to be very harmful or toxic to humans and animals. For a given dioxin, furan or PCB it is both the number of chlorine atoms, and their position in the molecule, that determine their physical and chemical properties, as well as their toxicity. The most studied and most toxic PCDD is 2,3,7,8-tetrachlorodibenzo-p-dioxin (or TCDD).
The National Dioxins Program used the term dioxins to include all of the above substances.
The principal human sources of dioxins are combustion and incineration (both industrial and domestic) and chemical processing that may release dioxins into the environment.
In combustion and incineration processes, the presence of chlorine or chlorine compounds and a temperature range between 200-400°C provide the optimum conditions for the formation of dioxins. Dioxins may also form at higher temperatures (800-1200°C) but in much smaller quantities.
A study in 1998 for the Department of the Environment, Water, Heritage and the Arts estimated that prescribed burning and bushfires may contribute almost 75 per cent of dioxins emissions in Australia. As there was little Australian data, these estimates were derived from emission studies in other countries. A study of dioxin emissions from bushfires in Australia commenced in June 2002.
In the environment, dioxins accumulate in soils, sediments and waste disposal sites. They are not present in water because of their insolubility. Dioxins enter the food chain when animals eat contaminated plants and sediments. The dioxins are then absorbed into the animal fat and increase in concentration as they migrate up the food chain. This is referred to as bio-accumulation.
It is estimated that over 96 per cent of dioxins present in the environment have originated from air emissions. They are then deposited onto soil, plant and water surfaces. When released into the air, some dioxins may be transported long distances, even around the world.
When released in wastewater, some dioxins are broken down by sunlight and some evaporate into the air, but most settle in bottom sediment or onto plants. Fish and other aquatic organisms can then ingest and further transport and deposit them.
Dioxins are very toxic to certain animals particularly in the early stages of development. Studies have shown dioxins to have a range of adverse effects on a wide number of animals, including reproductive or developmental effects such as weakened immune responses and behavior changes in offspring.
Dioxins levels generally have been declining since the 1970s when it was first recognised that they are highly toxic chemicals and actions were taken by governments and industry to reduce their production.
However, because dioxins break down very slowly, some of the dioxins produced in the past will remain. These are called "reservoir sources" and play a significant role in the level of dioxins in the environment and people.
Dioxins levels in the environment are unlikely to reach zero because they are also released by natural processes such as bushfires and volcanoes.
To help measure the toxicity of dioxins and dioxins-like substances in the environment the concept of toxic equivalents (TEQs) has been developed. This concept allows the toxicity of a complex mixture to be estimated and expressed as a single number. TCDD is the most toxic dioxin and is assigned a weighting factor (or toxic equivalency factor, TEF) of 1; all other dioxins have a TEF of less than 1. Multiplication of the mass of a specific dioxin by its TEF yields the corresponding TCDD mass (or TEQ). The total toxicity of any mixture is then simply the sum of the individual dioxin TEQs.
Because dioxins are toxic at very low levels, their presence in substances is measured in parts per trillion. Analysis, therefore, requires highly sensitive equipment and is time consuming and costly - costs may be up to $2,000 per sample. Where possible, several samples may be pooled and the analysis done on the pooled sample. The Australian Government Analytical Laboratory is the only laboratory in Australia currently accredited to analyse dioxins at these very low levels.
This is number 2 of 4 Fact Sheets on the National Dioxins Program
For more information contact:
National Dioxins Program
Chemical Policy Section
Department of the Environment, Water, Heritage and the Arts
GPO Box 787
CANBERRA ACT 2601
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Last updated: Wednesday, 27-Jul-2011 13:35:59 EST
Department of Sustainability, Environment, Water, Population and Communities
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