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Polychlorinated dibenzodioxinsPolychlorinated dibenzodioxins (PCDDs) are a group of halogenated organic compounds which are significant because they act as environmental pollutants. The term "polychlorinated dibenzodioxins" can be abbreviated as PCDDs, but they are commonly referred to as dioxins for simplicity in current technical literature because a dioxin skeletal structure is at the center of the chemical structure of a PCDD molecule. Typically, the p-dioxin skeleton is at the core of a PCDD molecule, giving the molecule a dibenzo-p-dioxin ring system. Members of the PCDD family have been shown to bioaccumulate in humans and wildlife due to their lipophilic properties, and are known teratogens, mutagens, and suspected human carcinogens. Additionally sometimes with the word "dioxins", a similar but unrelated compound type, the polychlorinated dibenzofurans (PCDFs) of like environmental importance, are also implied. Additional recommended knowledge
Chemical structure of dibenzo-p-dioxins
The structure of dibenzo-p-dioxin comprises two benzene rings joined by two oxygen bridges. This makes the compound an aromatic diether. In PCDDs, chlorine atoms are attached to this structure at any of 8 different places on the molecule, at positions 1.-4. and 6.-9. There are 75 different types of PCDD congeners (that is: related dioxin compounds). The toxicity of PCDDs depends on the number and positions of the chlorine atoms. Congeners that have chlorines in the 2., 3., 7., and 8. positions have been found to be significantly toxic. In fact, 7 congeners have chlorine atoms in the relevant positions which were considered toxic by the NATO Committee on the Challenges to Modern Society (NATO/CCMS) international toxic equivalent (I-TEQ) scheme. Historical perspective
Concentrations of dioxins in nature prior to industrialization, due to natural combustion and geological processes, were generally about three times lower than today.[1][2] The first intentional synthesis of chlorinated dibenzodioxin dates back to 1872. Today, concentrations of dioxins are found in all humans, with higher levels commonly found in persons living in more industrialized countries. The most toxic dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), became well known as a contaminant of Agent Orange, a herbicide used in the Vietnam War.[3] Later, dioxins were found in Times Beach, Missouri[4] and Love Canal, New York[5] and Seveso, Italy.[6] More recently, dioxin has been in the news with the poisoning of President Viktor Yushchenko of Ukraine in 2004.[7] Sources of dioxinThe United States Environmental Protection Agency Dioxin Reassessment Report is possibly the most comprehensive review of dioxin, but other countries now have substantial research. Australia, New Zealand and the United Kingdom all have substantial research into body burdens and sources. Tolerable daily, monthly or annual intakes have been set by the World Health Organization and a number of governments. Dioxin enters the general population almost exclusively from ingestion of food, specifically through the consumption of fish, meat, and dairy products since dioxins are fat-soluble and readily climb the food chain [8].
Occupational exposure is an issue for some in the chemical industry, or in the application of chemicals, notably herbicides. Inhalation has been a problem for people living near substantial point sources where emissions are not adequately controlled. In many developed nations there are now emissions regulations which have alleviated some concerns, although the lack of constant sampling of dioxin emissions causes concern about the understatement of emissions. In Belgium, through the introduction of a process called AMESA, constant sampling showed that periodic sampling understated emissions by a factor of 30 to 50 times. Few facilities have constant sampling. Most controversial is the United States Environmental Protection Agency (US EPA) assessment's (draft) finding that any reference dose that were to be set would be far below current average intakes. Children are passed substantial body burdens by their mothers, and breastfeeding increases the child's body burden[citation needed]. Children's body burdens are often many times above the amount implied by tolerable intakes which are based on body weight. Breast fed children usually have substantially higher dioxin body burdens than non breast fed children until they are about 8 to 10 years old. The WHO still recommends breast feeding for its other benefits. Dioxins are produced in small concentrations when organic material is burned in the presence of chlorine, whether the chlorine is present as chloride ions or as organochlorine compounds, so they are widely produced in many contexts. According to the most recent US EPA data, the major sources of dioxin are:
These sources together account for nearly 80% of dioxin emissions. When the original US EPA inventory of dioxin sources was done in 1987, incineration represented over 80% of known dioxin sources. As a result, US EPA implemented new emissions requirements. These regulations have been very successful in reducing dioxin stack emissions from incinerators. Incineration of municipal solid waste, medical waste, sewage sludge, and hazardous waste together now produce less than 3% of all dioxin emissions. In incineration, dioxins can also reform in the atmosphere above the stack as the exhaust gases cool through a temperature window of 600 to 200°C. The most common method of reducing dioxins reforming or forming de novo is through rapid (30 millisecond) quenching of the exhaust gases through that 400°C window [10]. Incinerator emissions of dioxins have been reduced by over 90% as a result of new emissions control requirements. Incineration is now a very minor contributor to dioxin emissions.
Dioxins are also generated in reactions that do not involve burning — such as bleaching fibers for paper or textiles, and in the manufacture of chlorinated phenols, particularly when reaction temperature is not well controlled. Affected compounds include the wood preservative pentachlorophenol, and also herbicides such as 2,4-dichlorophenoxyacetic acid (or 2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Higher levels of chlorination require higher reaction temperatures and greater dioxin production. See Agent Orange for more on contamination problems in the 1960s. Dioxins may also be formed during the photochemical breakdown of the common antimicrobial compound triclosan [11]. Dioxins are also in typical cigarette smoke[citation needed]. Dioxin in cigarette smoke was noted as "understudied" by the US EPA in its "Re-Evaluating Dioxin" (1995). In that same document, the US EPA acknowledged that dioxin in cigarettes is "anthropogenic" (man-made, "not likely in nature"). Nevertheless, the use of chlorine-containing tobacco pesticides and chlorine-bleached cigarette papers remains legal[citation needed]. Dioxins are present in minuscule amounts in a wide range of materials used by humans — including practically all substances manufactured using plastics, resins, or bleaches.[citation needed] Such materials include tampons, and a wide variety of food packaging substances[citation needed]. The use of these materials means that all Western humans receive at least a very small daily dose of dioxin[citation needed]—however, it is disputed whether such exceptionally tiny exposures have any clinical relevance[citation needed]. It is even controversially discussed whether dioxins might have a non-linear dose-response curve with beneficial health effects in a certain lower dose range, a phenomenon called hormesis[citation needed]. Dietary sources of dioxin in the United States have been analyzed by the EPA and scientists from other organizations. ToxicityDioxins are absorbed primarily through dietary intake of fat, as this is where they accumulate in animals and humans. In humans, the highly chlorinated dioxins are stored in fatty tissues and are neither readily metabolized nor excreted. The estimated elimination half-life for highly clorinated dioxins (4-8 chlorine atoms) in humans ranges from 7.8 to 132 years [12]. The persistence of a particular dioxin congener in an animal is thought to be a consequence of its structure. It is believed that dioxins with few chlorines, which thus contain hydrogen atoms on adjacent pairs of carbons, can more readily be oxidized by cytochromes P450.[citation needed] The oxidized dioxins can then be more readily excreted rather than stored for long time.[citation needed]
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic of the congeners. Other dioxin congeners (or mixtures thereof) are given a toxicity rating from 0 to 1, where TCDD = 1. This toxicity rating is called the Toxic Equivalence Factor, or TEF. TEFs are consensus values and, because of the strong species dependence for toxicity, are listed separately for mammals, fish, and birds. TEFs for mammalian species are generally applicable to human risk calculations. The TEFs have been developed from detailed assessment of literature data to facilitate both risk assessment and regulatory control [13]. Many other compounds may also have dioxin-like properties, particularly non-ortho PCBs, some of which can have TEFs as high as 0.1. The total dioxin toxic equivalence (TEQ) value expresses the toxicity as if the mixture were pure TCDD. The TEQ approach and current TEFs have been adopted internationally as the most appropriate way to estimate the potential health risks of mixture of dioxins. Recent data suggest that this type of linear scaling factor may not be the most appropriate treatment for complex mixtures of dioxins; further research into non-linear toxicity models is required to substantiate this hypothesis. Dioxins and other persistent organic pollutants (POPs) are subject to the Stockholm Convention. The treaty obliges signatories to take measures to eliminate where possible, and minimize where not possible to eliminate, all sources of dioxin. Health effects in humansDioxins build up primarily in fatty tissues over time (bioaccumulate), so even small exposures may eventually reach dangerous levels. In 1994, EPA reported that dioxin is a probable carcinogen, but notes that non-cancer effects (reproduction and sexual development, immune system) may pose an even greater threat to human health. TCDD, the most toxic of the dibenzodioxins, has a half-life of approximately 8 years in humans, but at high concentrations, the elimination rate is enhanced by metabolism [14]. The health effects of dioxins are mediated by their action on a cellular receptor, the aryl hydrocarbon receptor (AhR) [15]. Dioxins also accumulate in food chains in a fashion similar to other chlorinated compounds (bioaccumulate). This means that even small concentrations in contaminated water can be concentrated up a food chain to dangerous levels due to the long biological half life and low solubility of dioxins. Exposure to high levels of dioxin in humans causes a severe form of persistent acne, known as chloracne [16]. Other effects in humans may include:
Recent studies have shown that exposure to dioxin changes the ratio of male to female births among a population such that more females are born than males.[24] Health effects in other animalsWhile it has been difficult to prove that dioxins cause specific health effects in humans due to the lack of controlled dose experiments, studies in animals have shown that dioxin causes a wide variety of toxic effects. In particular, TCDD has been shown to be teratogenic, mutagenic, carcinogenic, immunotoxic, and hepatotoxic. Furthermore, alterations in multiple endocrine and growth factor systems have been reported. The most sensitive effects, observed in multiple species, appear to be developmental, including effects on the developing immune, nervous, and reproductive systems [25]. These effects are caused at body burdens close to those reported in humans. Among the animals for which TCDD toxicity has been studied, there is strong evidence for the following effects:
Studies of dioxin's effects in VietnamUS veterans' groups and Vietnamese groups, including the Vietnamese government, have convened scientific studies to explore their belief that dioxins were responsible for a host of disorders, including tens of thousands of birth defects in children, amongst Vietnam veterans as well as an estimated one million Vietnamese, through their exposure to Agent Orange during the Vietnam War, which was found to be highly contaminated with TCDD. Several exposure studies showed that some US Vietnam Veterans who were exposed to Agent Orange had serum TCDD levels up to 600 ppt (parts per trillion) many years after they left Vietnam, compared to general population levels of approximately 1 to 2 ppt of TCDD. In Vietnam, TCDD levels up to 1,000,000 ppt have been found in soil and sediments from Agent Orange contaminated areas 3 to 4 decades after spraying. In addition, elevated levels have been measured in food and wildlife in Vietnam [38]. The most recent study, paid for by the National Academy of Sciences, was released in an April 2003 report. This report is currently (March 2007) being revised for release again later in 2007. The Centers for Disease Control and Prevention found that dioxin levels in Vietnam veterans [39] were in no way atypical when compared against the rest of the population. The only exception existed for those who directly handled Agent Orange. These were members of Operation Ranch Hand. Long-term studies of the members of Ranch Hand have thus far uncovered a possibility of elevated risks of diabetes. Dioxin exposure incidents
References
Categories: IARC Group 1 carcinogens | IARC Group 3 carcinogens | Immunotoxins |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Polychlorinated_dibenzodioxins". A list of authors is available in Wikipedia. |