6. Evaluation and conclusions
- 6.1 Human exposure to Dioxins
- 6.2 Observed health effects
- 6.3 Tolerable Daily Intake set by WHO for dioxins
- 6.4 Breastfeeding
6.1 Human exposure to Dioxins
6.1.1 In most industrialized countries, concentrations of dioxins in environmental samples, foods, human tissues and breast milk have decreased during the 1990s, mainly due to enforced environmental regulations.
In industrialized countries, the daily intake of dioxins (PCDDs and PCDFs) is in the order of 1 to 3 pg I-TEQ per kg body weight per day. If PCBs are included, the daily intake is up to 3 times higher, hereby exceeding the Tolerable Daily Intake (TDI) as put forward by the World Health Organization (WHO) (see 6.3).
Breast milk is less contaminated in developing countries (<10 pg/g milk fat) than in industrialized countries (10-35 pg/g milk fat). For breast-fed infants, the daily intake per kg of body weight is ten to hundred times greater than for adults, but has been reduced since the early 90´s by up to 50% in most industrialized countries.
Generally 2,3,7,8-TCDD accounts for only 10 to 20 % of the total dioxin TEQ-exposure and for less than 5% when dioxin-like PCBs are included. More...
6.1.2 Several factors determine the persistence of dioxins in the body, including dose, quantity of body fat, binding to liver proteins and rate of metabolic transformation and excretion.
Both humans and animals accumulate dioxins. Body burden is the most appropriate parameter to compare exposure and effects between species. Because of differences in the above factors, rodents need to ingest 100 to 200 times more dioxins than humans to reach a same body burden.
Because dioxins remain in the human body for a relatively long time, higher intakes for a short period will not result in significant changes to the long-term body burden. More...
6.2 Observed health effects
Dioxins can alter key biochemical and cellular functions by binding to the cellular Ah-receptor. The broad range of Ah-receptor binding affinities seen in human placenta samples suggests that the response to dioxins varies significantly from one person to another (see question 2.3).
A number of biochemical effects have been observed in experimental animals at body burdens comparable to those of the general human population. These effects may be harmful or not, and may or may not be due to interactions with the cellular Ah-receptors. To evaluate the risks for human populations, studies usually focus on the effects observed at the lowest doses. Toxic effects were observed on animals at body burdens in the range of 10 000 to 73 000 pg per kg body weight. A human daily intake can be calculated which would correspond to these animal body burdens.
The following non-cancer effects were observed:
- Mothers that were accidentally exposed to very contaminated rice oil, leading to an extremely high body burden of 2 to 3 million pg TEQ per kg body weight, gave birth to infants showing severe and persistent developmental and neurological effects.
- In workers exposed to high levels of dioxins in their workplace, health effects were observed at body burdens ranging from 28 000 to 400 000 pg per kg body weight. These effects include changes in the blood composition and increased cardiovascular diseases and diabetes.
- Some of the Seveso population, which was exposed to PCDDs and PCDFs at levels up to ten to hundred times higher than normal, suffered some temporary effects, such as a skin rash called chloracne and blood biochemistry changes. An increase in male cardiovascular deaths and a decrease in the boy to girl birth ratio were observed.
Humans may be as sensitive as animals to the carcinogenic effects of dioxins. Accidental exposure to TCDD at levels hundred to thousand times higher than those of the general population (ten to hundred times higher TEQ values in terms of total dioxins) increases the risk for all cancers combined by 40%.
Concerning the effects on birth weight, thyroid hormone effects and nervous system development, the interpretation of the results is complicated by simultaneous exposure to other chemicals. Some effects were observed at dioxin levels only slightly higher than the exposure of the general population. More...
6.3 Tolerable Daily Intake set by WHO for dioxins
6.3.1 The World Health Organization (WHO) recommends a Tolerable Daily Intake (TDI) of 1 to 4 pg WHO-TEQ/kg body weight per day. This figure is based on the lowest exposures at which adverse effects were observed in experimental animals. It includes an overall uncertainty factor of 10, in order to account for possible differences in susceptibility between humans and experimental animals and in between people. More...
6.3.2 The TDI represents a tolerable daily intake for a life-time exposure. Occasional exceeding of the TDI should have no health consequences, provided that the averaged intake over longer periods remains below it. In industrialized countries, some people exceed the TDI and may therefore show some subtle effects which have, however, not been proven to be harmful.
The upper limit of 4 pg WHO-TEQ/kg body weight per day is provisional: the ultimate goal is to reduce human intake levels below 1 pg WHO-TEQ/kg body weight per day. The World Health Organization (WHO) recommended that every effort should be made to limit emissions of dioxins and related compounds in order to reduce their presence in the food chain. Immediate efforts should specifically target exposure reductions of highly exposed sub-populations. More...
Breast-fed infants have higher intakes of dioxins but only during a small period of their life. Some studies found subtle effects in children of mothers exposed to dioxins, but these effects are, in all but one study, probably due to exposure through the placenta rather than through breast milk.
Breastfeeding has many beneficial effects. Therefore, the World Health Organization (WHO) promotes breastfeeding while recommending the reduction of dioxin emissions. Dioxin levels in human milk have decreased since the early 90's. More...