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It's Your Health DIOXINS AND FURANSIntroduction Dioxins are a family of toxic substances that scientists call polychlorinated dibenzo-para-dioxins. A second family of closely related toxic substances -- known as polychlorinated dibenzofurans -- is very often present with dioxins. Recent research has indicated that elevated doses of dioxins and furans can significantly damage the health of laboratory animals. The impact of these substances on humans and wildlife is less certain. Dioxins and furans are therefore the subject of considerable controversy, both in the public realm and within the scientific community.
Dioxins and furans are by-products of the production of certain chemicals (such as those used to produce some pesticides and wood preservatives), of the chlorine bleaching process used in some pulp and paper mills, and of the incomplete combustion of materials that contain both chlorine atoms and organic matter. Although they are most often associated with industrial activities, some natural occurrences such as forest fires are believed to make a small contribution to the presence of dioxins and furans in the environment. All dioxins feature the same basic chemical "skeleton" on which one to eight chlorine atoms can be attached in a variety of positions. This is also the case for the furans. The different combinations yield 75 distinct dioxins and 135 different furans. It is generally acknowledged that 2,3,7,8-tetrachloro-p-dibenzo-dioxin (2,3,7,8-TCDD) is the most toxic of the dioxins and furans. The toxicity of mixtures of dioxins and furans is usually expressed in terms of this substance, that is, in 2,3,7,8- TCDD toxic equivalents.
Dioxins and furans originate from a variety of sources and can be transported in the atmosphere over long distances. As a result, they are found at very minute levels throughout the environment. Important sources of dioxin and furan releases to the environment include municipal waste incinerators, iron manufacturing, cement kilns, and some chemical dumps. High levels in the environment can generally be linked to specific sources, such as chemical dumps. Polychlorinated biphenyls (PCBs) used to be an important source of furans, which are contaminants in commercial PCB mixtures. Today, most PCBs are contained in secure facilities, and would be a source of furan releases to the environment only in the event of accidental leakage or fires.
Dioxins and furans persist in the environment and accumulate in living organisms. People living in industrialized nations around the world are constantly being exposed to minute amounts of dioxins and furans through their presence in food, air, water, soil or some consumer products. Scientists have shown that food is the major source of dioxins and furans in humans. Recent developments in research have enabled scientists to measure dioxins and furans at levels as low as several parts per trillion or even parts per quadrillion (a part per quadrillion is equivalent to one cent in 10 trillion dollars).
Because dioxins and furans are soluble in fat they can accumulate in the bodies of all animals, including humans. No long-term effects have been found in fish, wildlife or domestic animals that can be definitively linked to exposure to the low levels of dioxins and furans typically found in the environment. However, the remarkable loss of reproductive capability in fish-eating birds in the Great Lakes area in the 1970s may have been associated with dioxins and furans. The toxic effects of dioxins and furans on vegetation are not known. On the other hand, a large number of scientific studies do indicate that elevated levels of dioxins and furans can significantly damage the health of laboratory animals. Impacts vary widely from species to species. For example, guinea pigs are very susceptible to single doses of dioxins and furans while hamsters require 5,000 times as much to be similarly affected. Dioxins and furans are not unique in this regard; many animal species, including humans, react differently to a given substance, although the range in toxicity is particularly wide for dioxins and furans. In the studies referred to above, laboratory animals exposed to 2,3,7,8-TCDD experienced a number of adverse health effects, including weight loss, skin disorders, effects on their immune system, impaired liver function, impaired reproduction and development (including birth defects) and increased numbers of tumours. While the compound 2,3,7,8-TCDD has been shown to have a range of long-term effects on laboratory animals, effects on humans are more equivocal. In studies of humans who have apparently been exposed to high levels of dioxins and furans through their work or accidentally, the health effect which has been most consistently observed is a skin condition called chloracne. Chloracne resembles acne and is caused by exposure to various chlorinated organic chemicals. It is not exclusive to dioxins and furans, and usually disappears several months after the affected person ceases to be in contact with the contaminant. Some of the people exposed to chemicals contaminated with dioxins and furans have also displayed other adverse effects on the skin, liver, thyroid, behavioural development, and reproductive and immune systems. There are also several reports of increased numbers of cancers in populations exposed to high levels of 2,3,7,8-TCDD. While the evidence for these effects in humans is not conclusive, the findings generally support the results of animal studies.
On the basis of laboratory and exposure data available at this time, toxicologists have been able to derive an intake limit applicable to dioxins and furans below which no hazard to health is considered likely to occur. Establishing levels of exposure to dioxins and furans which might be considered acceptable for the general population is, however, controversial. Many countries have derived tolerable exposure guidelines that are similar to Canada's. However, using different approaches for assessing the risk to human health of exposure to 2,3,7,8-TCDD, some agencies in the United States have arrived at widely contrasting conclusions. Agencies responsible for estimating risk are constantly reviewing their risk assessments as new data becomes available.
Despite the lack of direct evidence that current exposure to dioxins and furans in Canada contribute to health problems in humans, the federal government recognizes that these compounds are undesirable environmental contaminants and that, where possible, their unintentional production should be limited. Investigations by both government and industry are continuing on several fronts. These include sampling and analysis of air, sediments, wildlife, fish, industrial effluents and sludges and pulp from pulp and paper mills using chlorine bleaching. The Health Protection Branch of Health Canada has investigated and continues to monitor dioxin levels in a variety of foods and in human breast milk. Scientists at Health Canada are also investigating the possible role of 2,3,7,8- TCDD in reproductive disorders such as endometriosis. Dioxins and furans were declared “toxic” under the Canadian Environmental Protection Act in 1990. This health and environmental assessment concluded that current exposures do not pose a risk to the majority of the general population. However, some exposures may put certain groups at risk, particularly persons who eat large amounts of fish contaminated with high levels of dioxins and furans. Following on this conclusion, regulations were passed requiring the virtual elimination of discharges of the most toxic of these compounds from pulp mills. The federal government regulates the content of dioxins in pesticides in Canada. Pesticides which had high levels of 2,3,7,8- TCDD, including products containing hexachlorophene, trichlorophenol, and the herbicide 2,4,5 - T (trichlorophenoxyacetic acid) have not been permitted for use in Canada for many years. Good progress has been made to reduce levels of dioxins in other pesticides. Monitoring programs indicate that the dioxin content in currently registered pest control products is very low. The use of PCBs in new electrical equipment has been banned for a number of years, and their use in existing equipment is being phased out through attrition. PCB regulations were also put in place to control export, destruction and storage. The federal government has been using mobile high- temperature incineration facilities to safely dispose of federal stores of PCBs. The federal government, in conjunction with the provinces, has also established codes of practice to reduce contamination by the wood preservation and protection industries. Incinerator emission technology continues to improve and the Canadian Council of Ministers of the Environment (CCME) has issued operating and emission guidelines to minimize releases of dioxins and furans from municipal solid waste and hazardous waste incinerators. These efforts to control sources of dioxins and furans have had a positive impact. Levels of dioxins and furans in Great Lakes fish-eating birds have declined. Cross-Canada surveys of breast milk indicate that from 1981 to 1992 concentrations of dioxins, furans and PCBs decreased by half. As a result of Canadian regulatory and guideline initiatives, dioxin and furan releases from municipal solid waste and hazardous waste incinerators have decreased by 80%, and from bleached Kraft pulp mills by 97%.
Despite the positive news about declines in exposure to these chemicals, exposures of the developing fetus and newborns remain a concern. Recent reports of adverse effects ascribed to some dioxins, furans and PCBs on hormonal activity, neurobehavioural development, and reproductive system development and maturation need to be carefully reviewed. Health Canada is participating in a review of the available information on dioxins and furans under the International Programme on Chemical Safety, to determine whether existing tolerable exposure guidelines need to be revised. Under the recently adopted federal Toxic Substances Management Policy, dioxins and furans have been targetted for virtual elimination, along with a number of other toxic, persistent and bioaccumulative substances. At the time of publication, the federal government is co-leading a federal- provincial task force which has updated the inventory of sources for these substances in Canada, with the ultimate goal of virtually eliminating them from the environment.
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