Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans - PSL1

Sources of Environmental Contamination

The most significant dioxin sources are the wood preservative pentachlorophenol, municipal incinerators, and pulp and paper mills using chlorine for the bleaching process. Polychlorinated biphenyls (PCBs) are the most significant potential source of furans.

Pentachlorophenol as a wood protection agent is being phased out of use and replaced by alternatives. Substantial quantities of dioxins and furans continue to be emitted from municipal incinerators employing old technology and from pulp mills using chlorine for the bleaching process. The technology to limit these releases is available. PCBs are subject to strict storage, handling, and inventory controls, and await the establishment of suitable facilities for their destruction.

Dioxins and furans may enter the environment from four broad categories of sources: chemical products, combustion, natural and industrial.

Most of these sources produce complex mixtures containing both dioxins and furans. For instance, the herbicide 2,4-D contains a mixture of dichloro-, trichloro- and tetrachloro-dioxins, while the wood preservative pentachloro-phenol contains hexachloro-, heptachloro-, and octachloro-dioxins and furans. Incinerators produce a wide range of tetrachloro-, pentachloro-, hexachloro-, heptachloro-, and octachloro-dioxins and furans. The dioxin and furan contaminants most often associated with pulp and paper mills are predominantly 2,3,7,8-tetrachlorodibenzofuran and 2,3,7,8- tetrachlorodibenzodioxin. Complex mixtures of furans are found in PCBs.

Chemical sources

A wide variety of commercial chemicals contain dioxins or furans as impurities. None of these is currently manufactured in Canada, and the use of chemicals containing these contaminants has decreased considerably over the past ten years (see Table 5), a trend that is expected to continue.

Pentachlorophenol (include tetrachlorophenol) is still widely used in Canada to preserve and protect wood. Although in 1981 this compound was one of the largest potential sources of dioxins to the environment, the dioxin content and the amounts used have since been reduced.

Consequently, today its potential as a source is only one tenth of that noted earlier by the National Research Council of Canada (NRCC),⁹ and this is likely to decrease further as pentachlorophenol is replaced by alternative wood protection chemicals or processes in the near future.¹⁰

The second largest chemical source for dioxins is the herbicide 2,4-D. The dioxins contained in 2,4-D are not substituted in 2, 3, 7 and 8 positions, and are considered to be relatively benign. Their concentration in 2,4-D is now strictly controlled. The chemical known as 2,4,5-T, and mixtures such as Agent Orange, both of which contained 2,3,7,8-tetrachlorodibenzodioxin, are no longer registered or used in Canada. The same is true for the closely related 2,4,5-trichlorophenol. Hexachlorophene, which is believed to contain 2,3,7,8-tetrachlorodibenzodioxin, is still used for a number of registered health care products.

Table 5 Canadian Sales, in tonnes, for dioxin-containing chemical

	19801	1987
Pentachlorophenol	2300	1340
Sodium pentachlorophenate and tetrachlorophenate	1200	402
2,4,5-T	50	0
2,4-D	3800	4546
Hexachlorophene	7	7

¹approximate values for me period from 1978 to 1981 (See NRCC, 1981a)

The most significant potential source of furans is PCBs. The total amount of furans contained in PCBs in storage or in use in Canada is estimated to be 75 kilograms.¹¹ While the use and storage of PCBs is now strictly controlled, the potential for releases through accidental spills, or fires in equipment containing PCBs, remains. Existing stocks of PCBs await the establishment of suitable destruction facilities, which are now starting to become available.

Combustion sources

Dioxins and furans are released by numerous combustion sources. In Canada, municipal incinerators are the major concern. Environment Canada's National Incinerator Testing and Evaluation Program (NITEP) has demonstrated that emissions from even the oldest of incinerators can be controlled to acceptable levels, provided correct management practices and modern control technologies are applied. Newer incinerators can be controlled even more successfully. Environment Canada's current estimates for total dioxin and furan emissions from all Canadian incinerators range widely, depending on the assumptions used, from between 20 grams per year if all incinerators were operating with Best Available Practicable Technology to 6000 grams per year if all were old and poorly managed incinerators.

A variety of other combustion sources have been reported to give rise to dioxins and furans but their contribution to total exposures is uncertain. Burning wood in stoves and fireplaces and barbecuing produce dioxins and furans in small amounts. Cigarette smoke also contains a variety of dioxins.¹²

Natural sources

There may be two natural sources of dioxins and furans: forest fires and volcanoes. There are no data specific to these sources, although sediments that are 300 to 1000 years old have low concentrations of the less toxic dioxins and furans.¹³

Industrial sources

Several industries release dioxins and furans in their effluents. Of significance to Canada are the pulp and paper mills that use chlorine in the bleaching process. Effluents and products from these mills contain concentrations of dioxins and furans at the parts-per-trillion level. Environment Canada estimates of the total amount of dioxins and furans released, based on the number of mills, the estimated annual bleached-pulp production, and the estimated effluent discharges, are approximately 100 to 150 grams per year of 2,3,7,8-tetrachlorodibenzodioxin and 2000 to 3000 grams per year of 2,3,7,8-tetrachlorodibenzofuran. Dioxin and furan production from pulp and paper mills can be reduced by changing the processing system and reducing the demand for bleached pulp.

Considering only the total quantity of dioxins and furans emitted from a source does not necessarily reflect the true toxic potential. The use of International Toxicity Equivalency Factors enables us to compare the toxic potential of the different sources of these chemicals. For example, the toxic potential of both pentachlorophenol and incinerator emissions would be reduced significantly by applying these factors while the toxic potential of pulp mill effluents would hardly change at all. PCBs would remain the most significant potential source of furans.

⁹ NRCC, 1981a.

¹⁰ Government of Canada, 1989.

¹¹ Sheffield, 1985.

¹²  Muto and Takizawa, 1989.

¹³  Jansson et al., 1987.