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Questions and Answers on Dioxins and PCBs in Food
Updated BfR FAQ, 13. April 2012
Dioxins and polychlorinated biphenyls (PCBs) are environmental contaminants that, due to their lipophilic properties, accumulate especially in foods of animal origin with high fat content.
Dioxins are not produced for specific purposes but occur as by-products, chiefly during combustion processes. They can also be generated as a result of forest fires or volcano eruptions.
In contrast, PCBs have been used for various purposes - as non-flammable fluids in heat exchangers, transformers and electrical capacitors, for example, or as softeners in coating materials, sealing compounds and plastics. The sale of PCBs was prohibited in most countries in the 1980s.
Humans mainly ingest dioxins and PCBs through foods of animal origin.
- What are dioxins?
- How are dioxins formed?
- What are polychlorinated biphenyls (PCBs)?
- What are PCBs used for?
- Which effects have dioxins and DL-PCBs on health?
- Which foods are responsible for the uptake of dioxin?
- What are WHO-PCDD/F-TEQ, WHO-PCB-TEQ and WHO-PCDD/F-PCB-TEQ?
- What are the tolerable daily intakes for dioxins?
- Why different foods have different legal maximum levels for dioxins?
- Why do the maximum levels for dioxins in fish apply to fresh weight and other foods of animal origin are related to fat content?
- How high is dioxin intake in Germany?
- What happens if the maximum levels are exceeded?
- What happens if the tolerable daily intake (TDI) is exceeded?
The term "dioxins" refers to two categories of differently chlorinated compounds of 75 polychlorinated dibenzo-p-dioxins (PCDDs) and 135 polychlorinated dibenzofurans (PCDFs). Dioxins (PCDD/Fs) have similar chemical and physical properties. 17 congeners which are chlorinated in 2,3,7,8-position are particularly toxic as well as persistent; they accumulate in fatty tissue of animals and humans.
Dioxins are not produced for specific purposes but occur as by-products, chiefly during combustion processes when organic carbon compounds are burned in the presence of chlorine and temperatures of at least 300 degrees. They can also be generated as a result of forest fires and volcano eruptions. Dioxins bind to dust particles, and in this manner they spread throughout the environment. They are thus not produced for certain purposes (except in small quantities for scientific purposes).
The category of polychlorinated biphenyls (PCBs) comprises 209 substances that differ in terms of the number and position of the chlorine atoms on the biphenyl molecule and possess different (toxic) properties. Several PCB congeners have a molecular structure and biological effects that are similar to those of dioxins and are therefore called dioxin-like PCBs (DL-PCBs). The remaining PCB congeners have no dioxin-like properties, a different toxicological profile and are called non-dioxin-like PCBs (NDL-PCBs).
PCBs are used for various purposes: they serve as non-flammable fluids in heat exchangers, transformers and electrical capacitors, for example, or as softeners in coating materials, sealing compounds and plastics. The sale of PCBs was prohibited in most countries in the 1980s.
Dioxins and dioxin-like PCBs (DL-PCBs) are very persistent compounds. They accumulate in fatty tissue and are metabolised only very gradually. In long-term experiments with laboratory animals, disorders of the reproductive function, of the immune and nervous system as well as the hormone balance were observed. A very high susceptibility to dioxin was observed in male rats showing developmental effects on the im-mune and genital system after prenatal exposure (e.g. WHO 2002). Furthermore, effects on liver and thyroid gland were identified as susceptible endpoints. Currently, the importance of these observations for human is unresolved. It is assumed that some dioxins and DL-PCBs may increase the risk of cancer.
Acute short-term effects of high doses of dioxin have been described in humans following occupational, accidental or intentional exposure only. The most characteristic effect is a per-sistent skin dermatitis referred to as “chloracne”. Changes of parameters of blood chemistry (especially elevated levels of triglycerides, cholesterol and transaminases) indicate involve-ment of the liver and changes in lipid metabolism.
For the assessment of consumer health risk, it is necessary to consider not only the dioxin concentrations in food but also the commonly consumed amounts of food. For reasons of consumer protection, dioxin contamination should be absolutely minimised. In this regard, unnecessary and avoidable exposure is not acceptable.
Due to the presence of dioxins and PCBs in the natural environment, their entry into the food chain cannot be completely avoided. Farm animals primarily ingest these contaminants via soil particles - either directly, for example by pecking, or if soil particles stick to the feed; but there is also the possibility of the direct intake of these contaminants into the feed itself. Dioxins accumulate in the fatty tissue of animals. Consequently foodstuffs of animal origin have higher dioxin concentrations than plant-based foods. Humans therefore mainly ingest dioxins by foodstuffs like meat, fish, eggs and milk and the products made from these foods.
The system of toxic equivalents (TEQs) considers the different degrees of toxicity of individual compounds. TEFs are so-called toxic equivalency factors that assign a ranking to the various congeners. The toxicity of individual substances is rated in relation to the compound with the greatest toxicity, 2,3,7,8-TCDD, better known as “Seveso dioxin”. First, the concentrations of the individual compounds as toxic equivalents will be multiply with the relevant toxic equivalency factor (TEF) to give the TEQ of each specific compound. The sum of these TEQs provides the total concentration of toxic equivalents, related to the effect of the concentration of pure 2,3,7,8-TCDD.
WHO-PCDD/F-TEQ is the sum of toxicity equivalents of the 17 toxicologically most important dioxins and furans.
WHO-PCB-TEQ is the sum of toxicity equivalents of the 12 DL-PCBs which have been assigned TEFs like the dioxins to classify the toxicity of these PCB congeners relative to 2,3,7,8-TCDD.
The sum of WHO-PCDD/F-TEQ and WHO-PCB-TEQ is called the total dioxin equivalent (WHO-PCDD/F-PCB-TEQ). Section 5 of the annex of Regulation (EC) No 1881/2006 lists maximum levels for both WHO-PCDD/F-TEQ and WHO-PCDD/F-PCB-TEQ.
The tolerable daily intake (TDI) is the amount that can be taken daily over lifetime without any anticipated adverse effects on human health.
In 2000, the WHO calculated a TDI in the range from 1 to 4 picograms WHO-PCDD/F-PCB-TEQ per kg body weight. One picogram (pg) is equivalent to one trillionth of a gram (10-12).
In 2001, the Scientific Committee on Food (SCF) of the European Union (EU) calculated a tolerable weekly intake (TWI) of 14 pg WHO-PCDD/F-PCB-TEQ per kg body weight.
In 2001, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) calculated a provisional tolerable monthly intake (PTMI) of 70 pg WHO-PCDD/F-PCB-TEQ per kg body weight and month.
Maximum limits are laid down for selected foods (e.g. chicken eggs) in the EU. These specifications are essentially based on unavoidable environmental dioxin contamination of foods, so-called background presence. Thereby it is the aim to keep away foods with a particularly high contaminant load in the specific food category (e.g. chicken eggs) from the market. Therefore maximum levels are not primarily based on toxicological criteria, but follow the principle of minimizing the total exposure.
The maximum limits are listed in the section 5 of the annex of Regulation (EC) No 1881/2006, which define the maximum levels for specific contaminants in foodstuffs.
Why do the maximum levels for dioxins in fish apply to fresh weight and other foods of animal origin are related to fat content?
The maximum levels for dioxins in foodstuffs mainly apply to the fat content of foods because dioxins accumulate in the fatty tissue of the meat of the animals from which the foods are produced.
Fish and fish products are an exception to this; here the maximum levels apply to fresh weight. Because fish varies greatly in fat content, fresh weight is the preferred reference value for better comparability.
According to the results of analyses conducted between 2000 to 2003, the average daily intake of dioxins and PCBs (as WHO-PCDD/F-PCB-TEQ) through foodstuffs in Germany amounted to about 2 picograms WHO-PCDD/F-PCB-TEQ per kg body weight and day. Due to the decrease of the environmental contamination the current daily amount of dioxins from food is assumed to be 1-2 picograms WHO-PCDD/F-PCB-TEQ per kg body weight. The reduction in dioxin intake is also reflected in the decreasing dioxin concentration in breast milk. (BfR Information No. 011/2011, 23 March 2011 (28.8 KB)).
Foods with dioxin equivalent concentrations that exceed the applicable maximum EU limit may not be sold. Where controls establish concentrations that exceed these maximum limits, the products are taken off the market. If maximum levels in food are exceeded for a short time, this does not necessarily mean that the intake of these foods entails a health risk. In the case of dioxins and PCBs, maximum levels are not primarily based on toxicological criteria.
The TDI is the amount that can be taken in daily over a lifetime without any anticipated adverse effects on human health. Moderate short-term exceeding of the TDI does not automatically pose a health risk. With substances like dioxins and PCBs, it is not the daily intake but the total body burden that is of decisive relevance for effects on human health. Dioxins and PCBs accumulate in the body, and all of us ingest traces of dioxins and PCBs via various foods on a daily basis due to the aforementioned "background presence" of these substances. This daily intake should not exceed the TDI value in the long term in order to prevent harmful body burdens as we get older. However, moderate short-term exceeding of the TDI has little or no effect on the overall body burden and does not therefore constitute a risk to health. Nevertheless, levels of these contaminants above the TDI resulting from avoidable exposure (e.g. dioxin concentrations above the maximum limit in chicken eggs) are not acceptable for reasons of principle.