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Dioxins and PCBs
Hazard Identification What are dioxins and PCBs? The term dioxins refers to a group of compounds with similar chemical and physical properties and structures. Dioxins are colourless, odourless organic compounds containing carbon, hydrogen, oxygen and chlorine. There are many different dioxins, of which 17 are known to be toxic to humans. The most toxic known dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), and significant concentrations of this compound can be measured in parts per trillion (PPT). Dioxins are ubiquitous environmental contaminants, having been found in soil, surface water, sediment, plants, and animal tissue worldwide. They are highly persistent in the environment with half-lives ranging from months to years. They have low water-solubility and low volatility, meaning that they remain in soil and sediments that serve as environmental reservoirs from which the dioxins may be released over many years. PCBs, or polychlorinated biphenyls, are chlorinated aromatic hydrocarbons produced by the direct chlorination of biphenyls. There are about 209 related PCBs, known as congeners of PCBs, of which 20 reportedly have toxicological effects. Some of the PCBs have toxicological properties similar to those of dioxins and are therefore often referred to as 'dioxin-like PCBs'. Like dioxins, PCBs are widespread environmental contaminants and are very persistent in soil and sediments. It has been suggested that highly contaminated bottom sediments in sewers and receiving streams may represent a reservoir for the continued release of PCBs into the environment. Occurrence in foods Dioxins and PCBs enter the food chain through a variety of routes. Grazing animals and growing vegetables may be exposed directly, or indirectly, to these contaminants in the soil. Leafy vegetables, pasture and roughage can also become contaminated through airborne transport of dioxins and PCBs. Dioxins in surface waters and sediments are accumulated by aquatic organisms and bio-accumulated through the food chain. The concentration of dioxins in fish may be hundreds to thousands of times higher than the concentrations found in surrounding water and sediments. Because dioxins are not very soluble in water, they tend to accumulate in the fatty tissues of animals and fish. Theoretically, the longer the lifespan of the animal, the longer the time it has to accumulate dioxins and PCBs. Foods that are high in animal fat, such as milk, meat, fish, eggs and related products are the main source of dioxins and PCBs and contribute about 80% of the overall human exposure, although almost all foods will contain these contaminants at some (generally very low) level owing to their ubiquitous nature. The main contributors to the average daily human intake of dioxins and PCBs have been found to be milk and dairy products, contributing between 16 and 39%; meat and meat products, contributing between 6 and 32%; and fish and fish products, contributing between 11 and 63%. Other foods, mainly vegetables and cereals, contributed 6-26% in the countries for which data was available (Codex Alimentarius Commission, 2001). Human milk can contain elevated levels of dioxins, some of which can pass to the infant during lactation. However, the intake of babies from their mothers is limited to a relatively short period of their lives. It is estimated that the average dietary intake of dioxins and dioxin-like PCBs has fallen amongst adults in the UK from 1.8 picograms World Health Organization toxic equivalents (WHO-TEQ)/kg* of bodyweight per day in 1997 to 0.9 picograms WHO-TEQ/kg bodyweight per day in 2001. Similar decreases have been reported in other countries. In November 2001, the Independent Committee on Toxicity recommended a TDI (tolerable daily intake) of 2 picograms WHO-TEQ/kg of bodyweight per day. *Note: the TEQ is a weighted toxicity value designed to take into account the variable toxicity of different dioxins and dioxin-like PCBs in comparison with the most toxic dioxins, and give a comparable overall measure of dioxin and PCB levels. Hazard Characterisation Effects on health Humans accumulate dioxins in fatty tissue mostly by eating dioxin-contaminated foods. The toxicity of dioxins is related to the amount accumulated in the body during the lifetime. Dioxins and PCBs have a broad range of toxic and biochemical effects, and some are classified as human carcinogens. In animal testing, dioxins have been implicated in causing damage to the immune and reproductive systems, developmental effects and neuro-behavioural effects. Despite the variety of adverse effects observed in animals exposed to dioxins, documented adverse health effects in humans have generally been limited to highly exposed populations in industrial environments, or following accidental chemical contamination. The most commonly observed adverse health effect in humans following acute over-exposure to dioxins and PCBs is the skin disease chloracne, a particularly severe and prolonged acne-like skin disorder. The accidental contamination of edible rice bran oil with PCBs in Japan in 1968 led to a poisoning epidemic amongst those who consumed the oil. The poisoning caused chloracne, liver disturbances, abdominal pain, headaches, skin discolouration, and the birth of abnormally small babies to mothers who had consumed the oil. A more recent example of dioxin contamination happened in Belgium in 1999, when PCB-contaminated feeds were fed to farm animals. The contamination was discovered as a result of the direct biological effects of dioxins observed in poultry. Sources Dioxins are often man-made contaminants and are formed as unwanted by-products of industrial chemical processes, such as the manufacture of paints, steel, pesticides and other synthetic chemicals, wood pulp and paper bleaching, and also in emissions from vehicle exhausts and incineration. Dioxins are also produced naturally during volcanic eruptions and forest fires. Most industrial releases of dioxins are strictly controlled under pollution prevention and control regulations. Currently, the major environmental source of dioxins is incineration. PCBs have been used in manufacturing industry since the early 1930s, mainly as cooling and insulating fluids in electrical equipment. The manufacture and general use of PCBs was banned in the 1970s because of environmental and health concerns. However, some PCBs remain in use, sealed inside older electrical equipment, although the use of this equipment must be phased out, and the PCBs removed and destroyed. Stability in foods Dioxins and PCBs are highly stable with reportedly long half-lives. In animals, they accumulate in fat and in the liver and are only very slowly metabolised by oxidation or reductive dechlorination and conjugation. They are therefore likely to persist in animal tissues, especially fatty tissue, for long periods. They are not generally affected significantly by food processing such as heat treatments, or fermentation. Control Options There is very little scope for the removal of dioxins and PCBs from foods once they have entered the food chain. It is generally agreed that the best means for preventing dioxins and PCBs from entering the food chain is to control their release into the environment. The overall goal of European policy is to reduce the contamination levels of dioxins and PCBs in the environment, and in food and feed. The EU has prohibited the use of most PCBs from 1978 and for certain applications from 1986. A deadline of 2010 has been set for removing all PCB-containing equipment from service. Dioxins, on the other hand, cannot be banned owing to their formation as unwanted by-products of many industrial processes. The amounts of dioxins and PCBs ingested in food are similar in the EU and the USA. Intakes are falling and have reduced by 85% since 1982, demonstrating some international success in controlling environmental contamination by these compounds. Product use While studies suggest that there is no cause for alarm from potential health issues concerning dioxins in the diet, choosing leaner cuts of meat, removing the skin from chicken or trimming the fat off meat may help to minimise any potential exposure of consumers to dioxins in food. Similarly, drinking reduced- or low-fat milk may also help to reduce exposure slightly, as may the washing of fruit and vegetables to remove any airborne dioxin-contaminated dust particles that might have been deposited on produce in fields. Legislation EU New EU regulations on contaminant levels in foods have recently been introduced (March 2007). These new regulations will require tougher safety controls in food manufacturing plants. The regulations aim to ensure a harmonised approach to the enforcement of permitted contaminant levels across the EU. Regulation (EC) 1881/2006 sets maximum levels for certain contaminants, including dioxins and dioxin-like PCBs in foods.
Methods of sampling for dioxins Regulation (EC) 1883/2006 lays down the methods for sampling and analysis for the official control of levels of dioxins and dioxin-like PCBs in certain foodstuffs. USA There are no tolerances or other administrative levels for dioxins in food or feed in the USA and the FDA considers all detectable levels to be of concern. Action levels have been set for PCBs in red meat and fish. Temporary tolerances have also been set for animal feeds and paper packaging. These are published in the Federal Register. Sources of further information Published Codex Alimentarius Commission, 2001 Institute of Food Science and Technology, UK (IFST) On the web EPA Dioxin Homepage JECFA evaluation of the safety of some dioxins and PCBs Food contaminants, dioxins and PCBs: USDA Dioxin resources page OurFood Database: Dioxinfacts.org: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||