Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans - PSL1

Human Health 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. Some Canadian fisheries have been closed as a result of dioxin and furan contamination. Current estimates of general population exposures indicate that certain high-risk sub-groups may have exposures that approach or exceed the guideline for tolerable daily intake of dioxins and furans. Since some toxic dioxins and furans are very persistent, exposure will be unnecessarily prolonged if these substances continue to be released, with a resultant increase in the risk to the health of high-exposure sub-populations. Human exposure should be kept to a minimum, and efforts to control sources of dioxins and furans should continue to be vigorous.

As a result of these considerations, the Minister of National Health and Welfare has concluded that polychlorinated dibenzodioxins and polychlorinated dibenzofurans may enter the environment in quantities which constitute a danger to human health. They are considered "toxic" as defined under Section 11(c) of the Canadian Environmental Protection Act.

Animal studies confirm that most mammalian species respond in the same qualitative way to dioxins and furans. However, dose-effect relationships vary markedly between species for the various dioxin and furan compounds. The dioxins and furans substituted in the 2, 3, 7 and 8 positions produce several effects in animals that are significant for consideration of human health. Among these are their effects on the reproductive system of non-human primates and rats, their tendency to cause hyperplasia in rodents, their carcinogenic effects on livers in rats and their effects on the immune system in several species.

Studies of human populations indicate that short-term exposure to several milligrams of mixtures of dioxins and furans can lead to a variety of effects on skin, eyes, and sensory and behavioural processes. Most effects have been reported to be reversible, although a return to normal may take several years. High-level exposure of women to furans in contaminated rice-oil may have been responsible for reproductive anomalies and infant mortalities. There is no adequate demonstration that populations exposed to dioxins and Furans have suffered excess cancer. However, the evidence is conflicting and data are confounded by exposure to other chemicals, incomplete health records, inadequate case identification and small sample size.

A recent Federal-Ontario study has estimated the average total exposure of Canadian adults and children to dioxin and furan compounds from all possible pathways.⁷¹ These estimates are based on average Canadian intakes of air, water, soil and food with representative concentrations of dioxins and furans. Between 94 and 96 percent of the intake by non-smoking adults is estimated to be from food, with the remainder equally split between air and all other routes (Table 7).

Table 7 Estimates of Canadian intake of Dioxins an Furans (after Birmingham et al., 1989)

Substrate/Medium	Estimated Intake (picograms toxic equivalents per kilogram of body weight per day)
	Adult1	Child2	Infant3	Neonate4
Food:	0.495 to 2.06	1.185to 4.786	2.65to 10.76	165
Air:	0.04	0.07	0.1	0.04
Soil:	0.01	0.027 to 0.03	0.34 to 0.038	-
Water:	<0.01 to 0.05	<0.01 to 0.07	<0.002 to 0.11	-
Consumer7 Products:	<0.01	<0.01	<0.01	<0.01
Total Estimated Intake	0.56 to 2.1	1.3 to 5.0	3.1 to 11.0	165
Exposure Period:	53 years	14 years	2.5 years	0.5 years

1. Adult differ, slightly from quoted reference due to small difference, in calculations and an assumed body weight of 70 kilograms.
2. Child: Weighs 33 kilograms; Breathes 15m³ air: Intake 1 litre water, 0.02 gm soil, 113% of adult food (NH&W, 977).
3. Infant: Weighs 13 kilograms; Breathes l0 m ³ air: Intake 0.6 litre water, 0.1 gm soil, 100% of adult food (NH&W, 1977).
4. Neonate: Weighs 5 kilograms; Breathes l m³ air: Intake 750 millilitres, 3% fat breast milk containing 36.5 picograms toxic equivalents per gram of fat Ryan et al., 1985).
5. Lower end of all ranges assumes a reported Not Detectable = 0.
6. Upper end of all ranges assumes a reported Not Detectable = Limit of Detection.
7. Does not include intake from cigarette smoking.

The average daily intake of dioxins and furans over a lifetime can be calculated from Table 7 by adding total intakes during the exposure periods of neonates, infants, children, and adults, and then dividing the total by 70 years. This estimate of 2.0 to 4.2 picograms toxic equivalents per kilogram of body weight per day is based on assumptions that are likely to overestimate exposure in the interest of protecting human health.

Pharmacokinetic modelling of Canadian general population exposure to dioxins and furans has corroborated the above exposure assessment with an estimate of 1.86 picograms toxic equivalents per kilogram body weight per day. The model used the average concentration of dioxins and furans in Canadian fat tissue⁷² to estimate body burden, a half-life of 7.1 years, and a first-order kinetic model.⁷³

Data from a recent report⁷⁴ suggest tobacco smoking as another pathway of general population exposure to dioxins. The potential exposure of a person who smokes 20 cigarettes a day is 0.5 picograms toxic equivalents per kilogram of body weight per day (this assumes that a 70 kilogram person inhales 1 litre of smoke per cigarette⁷⁵ containing dioxins at the concentration of 1.8 nanograms toxic equivalents per cubic metre,⁷² and that uptake is 100%). This estimate is preliminary in that it does not consider sidestream smoke, the potential presence of furans, nor the differences between the experimental protocol and actual smoking. This pathway has not been included in the above estimates of general population exposure because of these uncertainties. Nonetheless, this calculation indicates that smoking may contribute significantly to dioxin and furan exposure.

Based on a traditional approach which uses the no-observed-adverse-effect-levels from animal tests and standard uncertainty factors, it is estimated that human intakes should be below 10 picograms per kilogram of body weight per day of toxic equivalents averaged over a lifetime. This estimate assumes that 2,3,7,8-substituted dioxins and furans are non-genotoxic carcinogens for which a threshold dose exists at approximately 1 nanogram toxic equivalents per kilogram of body weight per day. The no-observed-adverse-effect-level for reproduction in rodents is also approximately 1 nanogram of 2,3,7,8-tetrachlorodibenzodioxin per kilogram of body weight per day. Humans appear to be less sensitive to dioxins and furans than most laboratory species (especially rats and monkeys) and the effect threshold values in rodents were based on lifetime exposures. An uncertainty factor of 100 was applied to the no-observed-adverse-effect-level to obtain the 10 picogram toxic equivalents per kilogram of body weight per day value, to take account of variations in response between individuals and to account for the seriousness of the potential effects. In a recent review, Rozman⁷⁶ reached the same conclusion, deriving Acceptable Daily Intake estimates for 2,3,7,8- tetrachlorodibenzodioxin of 10 picograms per kilogram of body weight per day for tumour promotion; 14 picograms per kilogram of body weight per day for porphyria; and 21 picograms per kilogram of body weight per day for dermal effects.

Intake via breast milk during the first half year of life is estimated to be relatively high (Table 7). This estimate was based on levels of dioxins and furans in Canadian adipose tissue. Subsequent analyses of human milk reduce this estimate by approximately one half, to 70 picograms toxic equivalents per kilogram body weight per day.⁷⁷ Although estimated intakes for neonates would still exceed the permissible daily intake, breast feeding only occurs for a short part of the life span (i.e., less than 4%) and lower exposures throughout the remainder of the life span reduce lifetime exposure to below this guideline. The concentrations of dioxins and furans in target organs are not expected to be dramatically increased as a consequence of breast feeding because of the rapid increase in the amount of the infant's fatty tissue. The known benefits of breast feeding currently outweigh any potential risks that may be associated with dioxins and furans in breast milk.⁷⁷

The foregoing exposure estimates indicate that average Canadian lifetime intakes are currently below 10 picograms toxic equivalents per kilogram of body weight per day. However, these substances bioaccumulate in the food chain, and concern for the health of human populations living in some areas where shellfish are highly contaminated has already necessitated closure of the affected fisheries near some pulp and paper mills that use chlorine bleaching. Exposures are estimated to approach or exceed 10 picograms toxic equivalents per kilogram of body weight per day for persons consuming highly contaminated fish in quantities well in excess of the general population norm. Furthermore, there is considerable uncertainty associated with the estimates of exposure, as they do not consider other sources that may contribute significantly to exposure, such as cigarette smoking. Finally, some toxic dioxins and furans are very persistent, and their continued release into the environment will unnecessarily prolong exposure, with a resultant increase in the risk to the health of high-exposure sub-populations.

While most chemical sources are already subject to stringent regulations, these dioxins and furans continue to be released from incinerators employing old technology and pulp mills employing chlorine in the bleaching process. The use, handling and storage of PCBs is controlled. Existing stocks of PCBs await the establishment of suitable destruction facilities, which are starting to become available.

As a result of these considerations, the Minister of National Health and Welfare has concluded that polychlorinated dibenzodioxins and polychlorinated dibenzofurans may enter the environment in quantities which constitute a danger to human health. They are considered "toxic" as defined under Section 11(c) of the Canadian Environmental Protection Act.

⁷¹  Birmingham et al., 1989

⁷²  Ryan et al., 1985.

⁷³  OME, 1985.

⁷⁴ Muto and Takizawa, 1989.

⁷⁵ Rickert et al., 1986.

⁷⁶  Rozman, 1989.

⁷⁷ WHO, 1988.