Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans - PSL1

Environmental Risk Assessment

The dioxins and furans substituted in the 2, 3, 7 and 8 positions are highly toxic substances. They are pervasive in the environment and bioaccumulate readily in the food chain. When wildlife consumes fish from the Great Lakes contaminated with dioxins and furans their intake could exceed the no-observed-adverse-effect- level. A growing body of evidence indicates that dioxins and furans or related compounds are already adversely affecting some wildlife populations.

As a result of these considerations, the Minister of the Environment has concluded that polychlorinated dibenzodioxins and polychlorinated dibenzofurans, occurring in complex mixtures of environmental contaminants, have immediate and long-term harmful effects on the environment.

They are considered toxic as defined under Section 11(a) of the Canadian Environmental Protection Act.

Dioxins and furans are highly persistent compounds that bioaccumulate in the food chain. From the Atlantic to the Pacific and throughout the Far North, dioxins and furans have been found in all compartments of the ecosystem, including air, water, soil, sediments, and foods. All animals and humans in Canada have been, and continue to be exposed to these substances.

The Canadian environment is subjected to four major sources of dioxins and furans: commercial chemicals containing dioxins, incineration activities, releases from pulp and paper mills that use chlorine for the bleaching process, and both accidental fires and spills involving PCBs (which contain principally furan contaminants). In addition, transborder contamination from the United States and long-range aerial transport from other areas contribute to the environmental load. While the chemical sources are, for the most part, subject to stringent regulations, dioxins and furans continue to be released in large quantities from incinerators and pulp and paper mills that use chlorine in the bleaching process. The use and storage of PCBs is now strictly controlled, but there remains the potential for releases through accidental spills and uncontrolled fires.

A review of the scientific literature shows that dioxins and furans tested in the laboratory are highly toxic to a wide variety of mammals (including humans), birds and fish. These compounds are both extremely toxic in acute exposure and some dioxins and furans which have been tested are capable of causing birth defects and cancer, and adversely affecting reproduction and the immune system, following repeated exposures. Several act together with other substances to cause cancer. Most sources release these substances as complex mixtures that are highly toxic to a variety of organisms.

The calculated⁷⁰ total body intake of 2,3,7,8-tetrachlorodibenzodioxin in daily food that produces a semi-chronic no-effect level varies for mammals, birds and fish. In mammals, it is estimated to be 550 to 5200 nanograms per kilogram of body weight; in birds, 2100 nanograms per kilogram of body weight; and in fish, 760 nanograms per kilogram of body weight. The threshold dose at which there is no effect in chronic studies of carcinogenicity and reproduction in rodents is approximately 1 nanogram per kilogram body weight per day.

A diet of the more highly contaminated fish from the Great Lakes could conceivably reach a cumulative dose in toxic equivalents exceeding these no-effect levels for sensitive wildlife species such as mink. Reproductive failure and anomalies in fish-eating birds on the Great Lakes and on the West Coast of Canada are ongoing concerns. They correlate strongly with dioxin and furan levels in eggs and adult tissues. Studies of fish-eating, colonially-nesting birds attribute low reproductive success to two major factors: embryotoxic chemicals in eggs, and aberrant behaviour caused by pollutants, resulting in poor nest incubation and care of nestlings. Congenital malformations have been reported in Great Lakes terns, gulls and double-crested cormorants. Such effects on reproduction and development are characteristic of exposure to dioxins and some of the other organochlorine contaminants that co-occur with dioxins and furans.

If dioxins and furans continue to be released into the environment, exposures to the more persistent and toxic ones will unnecessarily be prolonged. A growing body of evidence indicates that dioxins and furans in mixtures with other contaminants adversely affect some wildlife populations in Canada.

As a result of these considerations, the Minister of Environment has concluded that polychlorinated dibenzodioxins and polychlorinated dibenzofurans, occurring in complex mixtures of environmental contaminants, have immediate and long-term harmful effects on the environment. They are considered "toxic" as defined under Section 11(a) of the Canadian Environmental Protection Act.

⁷⁰ Kenaga et al., 1983.