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Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans - PSL1
Environmental Fate and Levels
Dioxins and furans occur throughout the Canadian environment and are found in all compartments of the ecosystem, including air, water and soil. These contaminants are highly persistent. They are transported to remote parts of the country by the prevailing winds and are deposited in sediments and soils. They accumulate in animals and have been found in most species of wildlife surveyed. They also appear in the natural and human food chains. All Canadians have detectable concentrations in their body fat. In this respect, Canadians are no different from individuals living in other industrialized countries.
Transport and persistence
Dioxins and furans do not move readily through soils and sediments because they generally attach to the particles. Soils and sediments represent the most significant "sink" for dioxins and furans. Once dioxins, particularly 2,3,7,8-tetrachlorodibenzodioxin, enter the soil and sediments, they are very slow to degrade. 2,3,7,8-Tetrachlorodibenzodioxin has a degradation half-life of ten years or longer. Dioxins and furans also accumulate in biological tissue where they also have long half-lives. Recent studies of aerial transport14 reveal that prevailing winds can play a significant role in environmental contamination. Contamination of wildlife in the Far North appears to be primarily the result of aerial transport.
Levels
Since the 1981 report of the NRCC,15 further research has confirmed that dioxins occur everywhere in the environment. A summary of many recent data for air, water, soil, foods and consumer products was prepared by the Federal-Ontario Ad Hoc Working Group on Multimedia Standards.16 The summary's findings are condensed in Table 6.
1ND: not detectable
2fw: fresh weight
Dioxin and furan levels in North American air, as summarized in Table 6, ranged from 0.4 to 36.7 picograms per cubic metre. In drinking water, one sample Out of 800 contained 46 picograms per litre of octachlorodibenzo-dioxin. Soil values varied from a low of 50 picograms per gram to a high of 14,100 picograms total dioxins and furans per gram of soil in highly contaminated areas.
The high water-octanol partition coefficients of dioxins and furans indicate a potential for considerable accumulation in biological tissues. The compounds which are substituted in the 2, 3, 7 and 8 positions are not only accumulated but are also metabolized slowly and therefore persist in the body tissues for considerable lengths of time. More than 100 species of invertebrates, fish, reptiles, amphibians, birds and mammals across Canada have been demonstrated to contain detectable levels of dioxins and furans.17
Environment Canada surveys revealed that levels, of 2,3,7,8-tetrachloro-dibenzodioxin ranged up to 37 nanograms per kilogram in mammals; up to 1996 nanograms per kilogram in birds; up to 474 nanograms per kilogram in reptiles; and up to 35 nanograms per kilogram in amphibians.
The department of Fisheries and Oceans coordinated a National Dioxin and Furan Fish Sampling Program to assess the level of contamination in fish and shellfish from the vicinity of 47 Canadian pulp mills employing the chlorine bleaching process. Levels of 2,3,7,8-tetrachlorodibenzodioxin reported in this survey ranged up to 662 nanograms per kilogram in Dungeness crab hepatopancreas; up to 31 nanograms per kilogram in shellfish tissue; and up to 137 nanograms per kilogram in fish.18 Mean values of 2,3,7,8-tetrachlorodibenzodioxin for whole fish lake trout samples from Lake Ontario averaged more than 35 nanograms per kilogram from 1977 through 1988.19 Much of the Lake Ontario contamination is related to upstream losses from chlorophenol manufacturing waste dumps.
A long-term study of 2,3,7,8-tetrachlorodibenzodioxin residue levels in herring gull eggs from Lake Ontario showed that the highest level of 1996 nanograms of 2,3,7,8-tetrachlorodibenzodioxin per kilogram was found in eggs collected in the early 1970s and the levels declined steadily to 50 nanograms per kilogram by 1980.20 During the 1980s, levels have been fairly constant and similar to background levels found in wildlife across the country.
Levels of dioxins and furans in the eggs of great blue herons from British Columbia exceed those currently found in herring gull eggs from the Great Lakes. This indicates the extent of the contamination of certain near shore zones in the southern Strait of Georgia off Canada's West Coast.21 Studies have linked the 2,3,7,8-tetrachlorodibenzodioxin in the herons to emissions from pulp mills using chlorine in the bleaching process, whereas the pentachloro-dioxins and hexachloro-dioxins were related to a chlorophenol source, possibly the use of chlorophenol-contaminated wood chips in the pulping process. Dioxin and furan contamination in some species of shellfish in the vicinity of certain pulp mills resulted in the closure of certain fisheries in November of 1988 and again in 1989.
Dioxins and furans, particularly 2,3,7,8-tetrachlorodibenzodioxin and octa-chlorodibenzodioxin, have been found in the tissues of polar bears, ringed seals and beluga whales from areas in the Canadian Arctic that are far from sources of these compounds. This indicates distribution by long-range transport mechanisms such as the prevailing winds.22
Dioxins and furans move along the natural food chain and are found in the food consumed by humans. Canadian foods that have been tested include fish, beef, pork, poultry, eggs, milk, fruits, vegetables and wheat-based products (Table 6). Detectable levels of octachlorodibenzodioxin have been reported in all of these food types; hexachloro- and heptachloro-dioxins and furans, and octachlorodibenzofuran have been reported in some fatty foods. Maximum total dibenzodioxin concentrations in whole fish samples from the Great Lakes have been reported at greater than 200 nanograms per kilogram. 23
A recent report indicates that smoking represents another potential source of exposure to dioxins. The total concentration of dioxins in cigarette smoke was determined to be approximately 5 micrograms per cubic metre (or 1.8 nanograms toxic equivalents per cubic metre).24
Human fat tissues sampled from several industrialized countries, including Canada, contain primarily dioxins and furans substituted in the 2, 3, 7 and 8 positions.25 The average background concentrations of tetrachloro-dioxins range from approximately 3 to 10 nanograms per kilogram; pentachloro-dioxins from 10 to 15 nanograms per kilogram; hexachloro-dioxins from 20 to 100 nanograms per kilogram; heptachloro-dioxins from 100 to 250 nanograms per kilogram; and octachlorodibenzodioxin from 250 to 1000 nanograms per kilogram. In general, the concentrations for furans are considerably lower than those for dioxins.
Persons exposed to substances containing dioxins and furans through occupational or accidental exposures have elevated body fat burdens of the 2,3,7,8-substituted compounds to which they were exposed. Some victims of a 1968 mass poisoning in Japan, the "Yusho" incident, had concentrations of up to more than two orders of magnitude higher than background levels in the general population (see Effects on Humans).
The types and concentrations of dioxin and furan compounds in human milk fat are similar to those in fat tissue. This indicates that the proximate source of these contaminants for breast milk is the fat stores of the mother.25 The average milk fat concentration in people from industrialized countries is about 2 nanograms per kilogram of 2,3,7,8-tetrachlorodibenzodioxin. Other dioxins are present at higher concentrations and in most instances more than one half is octachlorodibenzodioxin. Furans are generally found at lower concentrations. Levels in milk fat decline significantly as a function of the number of offspring borne if the mother breast feeds her children. The World Health Organization26 reports that the average background concentrations of dioxins and furans in human milk fat in industrialized countries range from 5 to 53 nanograms toxic equivalents per kilogram of milk fat.
Recent improvements in analytical sensitivity enable the detection of dioxins and furans in all samples of blood from members of the general population at the parts-per-quadrillion level.27 The blood of people exposed both occupationally and through transformer fires shows higher levels of contamination. In some instances these levels reach tens or even hundreds of parts-per-trillion of the dioxins or furans to which they have been exposed. Initial blood concentrations of total furans in the Taiwanese accidentally exposed to PCBs in the "Yu-Cheng" incident (see Effects on Humans) were similarly elevated.
14 Astle et al., 1987; Czuczuwa and Hites, 1986.
17 Norstrom et al., 1982, 1986; Norstrom and Simon 1983; Stalling et al., 1983, 1985, 1986; ICTC 1986; Braune and Norstrom 1989; Fox et al., 1988; Kubiak et al., 1989; Elliott et al., 1988, 1989; Ryan et al., 1986.
18 Government of Canada, 1988-89.
20 IJC, 1983; Mineau et al., 1984.
21 Elliott et al., 1988, 1989.
22 Norstrom et al., (submitted).
23 Baumann, P.C. and D.M. Whittle, 1988.
25 Jensen, 1987; WHO, 1989 (in press).
27 Jensen, 1987; Schecter and Ryan, 1989; Ryan and Norstrom (in press).
