Dioxin

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Image:TCDD structure2.png Dioxin is the popular name for the family of chlorinated organic compounds consisting of polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzodioxins (PCDDs). PCDD/Fs have been shown to bioaccumulate in humans and wildlife due to their lipophilic properties.

The basic structure of PCDD/Fs comprises of two benzene rings joined by either a single (furan) or a double oxygen bridge (dioxin). Chlorine atoms are attached to the basic structure at any of 8 different places on the molecule, numbered from 1 to 10. There are 210 different PCDD/F congeners, comprising of 75 PCDDs and 135 PCDFs. The toxicity of PCDD/Fs is dependent on the number and position of the chlorine atoms, with only congeners that have chlorines in the 2,3,7,and 8 positions have any observable toxicity. Out of the 210 PCDD/F compounds in total, only 17 congeners (7 PCDDs and 10 PCDFs) have chlorine atoms in the relevant positions to be considered toxic by the NATO/CCMS international toxic equivalent (I-TEQ) scheme.

2,3,7,8-Tetrachloro dibenzene-para-dioxin is the most toxic of the congeners. By convention it is assigned a toxicity rating or toxic equivalence factor (TEF) of 1.0 with the remaining PCDD/Fs being assigned lower relative values. 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.

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.

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Sources of dioxin

The 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. [1]

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 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 breast feeding increases the child's body burden. 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 EPA data the major sources of dioxin are:

  • Trash burn barrels;
  • Land application of sewage sludge
  • Coal fired utilities
  • Residential wood burning
  • Metal smelting
  • Diesel trucks

These sources together account for nearly 80% of dioxin emissions.

Dioxins are also in smoke from typical cigarettes, those with chlorine-bleached paper and residues of many chlorine pesticides. Dioxin in cigarette smoke was noted as "understudied" by the EPA in its "Re-Evaluating Dioxin" (1995). In that same document, the EPA acknowledged that dioxin is "anthropogenic" (man-made, "not likely in nature"). Dioxin cannot come from the tobacco or any natural plant. Since then, the USA classified dioxin as a Known Human Carcinogen, and the USA signed the Stockholm Convention on POPs to globally phase out dioxin and 11 other of the worst industrial pollutants. Nevertheless, chlorine tobacco pesticides and chlorine-bleached cigarette papers remain legal, with no warning required to consumers.

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. Chemical Engineering, December 2002 has a detailed article on this issue. 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 are present in minuscule amounts in a wide range of materials used by humans — including practically all substances manufactured using plastics, resins or bleaches. Such materials include tampons, and a wide variety of food packaging substances. The use of these materials means that all modern humans receive (at least) a very small daily dose of dioxin—however, it is disputed whether such exceptionally tiny exposures have any clinical relevance. It is even controversially discussed if dioxins might have a non-linear dose-response curve with beneficial health effects in a certain lower dose range, a phenomenon called hormesis.

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Health effects

Dioxins build up in living tissue (bioaccumulate) over time, so even small exposures may accumulate to dangerous levels.

Excessive exposure to dioxin may cause a severe form of persistent acne, known as chloracne. This is the only known direct result of dioxin exposure at levels below the lethal dose. Other possible effects may be

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Studies of dioxins effects in Vietnam

US 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. The most recent study, paid for by the National Academy of Sciences, was released in an April 2003 report.

The Center for Disease Control found that dioxin levels in Vietnam veterans [4] 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.

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Dioxin exposure incidents

  • In 1963 a dioxin cloud escapes after an explosion in a Philips-Duphar plant (now Solvay Group) near Amsterdam. Four people died of dioxin poisoning, and 50 more suffer severe health problems. In the 1960s Philips-Duphar produced 2250 tonnes of 'Agent Orange' for the US Army.
  • In 1976 large amounts of dioxin were released in an industrial accident at Seveso, although no human fatalities or birth defects occurred.
  • In the 1960s, parts of the Spolana chemical plant in Neratovice, Czechoslovakia, were heavily contaminated by dioxins, when the herbicide 2,4,5-T (also a component of Agent Orange) was produced there. Workers in this factory were exposed to high concentrations of dioxins at that time. Dozens of them fell seriously ill. A possibly large amount of dioxins was flushed from the factory into the Labe river during the 2002 European flood. No direct consequences of this incident have thus far been recorded.
  • In May 1999, there was a dioxin crisis in Belgium: quantities of dioxin had entered the food chain through contaminated animal feed. 7,000,000 chickens and 60,000 pigs had to be slaughtered. The scandal that followed caused a landslide in the elections one month later.
  • In a 2001 case study [5], physicians reported clinical changes in a 30 year old woman who had been exposed to a massive dosage (144,000 pg/g blood fat) of dioxin equal to 16,000 times the normal body level; the highest dose of dioxin ever recorded in a human. She suffered from chloracne, nausea, vomiting, epigastric pain, loss of appetite, leukocytosis, anemia, amenorrhoea and thrombocytopenia. However, other notable laboratory tests, such as immune function tests, were relatively normal. The same study also covered a second subject who had received a dosage equivalent to 2,900 times the normal level, who apparently suffered no notable negative effects other than chloracne. These patients were provided with olestra to accelerate dioxin elimination.
  • In 2004, a notable individual case of dioxin poisoning, Ukrainian politician Viktor Yushchenko was exposed to the second-largest measured dose of dioxins, according to the reports of the physicians responsible for diagnosing him. This is the first known case of a single high dose of TCDD dioxin poisoning. Although experts suggest his face could clear up after several years, it is highly unlikely. One news report explained, that during the cold war, the KGB used dioxin to induce suicides. Dioxin was described as a mind-altering substance, that caused depression and would definitely cause the person to commit suicide, sooner or later.
  • In [Early 2000's], residents of the city of New Plymouth New Zealand report many illnesses of people living around and working at the Dow Chemical plant. This plant ceased production of 245T in 1987. Staff volunteering to have blood tests analysed are denied access to professional anlyses of their blood samples.
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Incineration and dioxin emissions

Modern waste incinerators are equipped with pollution control equipment which reduces dioxin emissions to insignificant levels. Incineration of municipal solid waste, medical waste, sewage sludge, and hazardous waste together produce less than 3% of all dioxin emissions. When the original EPA inventory of dioxin sources was done in 1987, incineration represented over 80% of known dioxin sources. As a result, EPA implemented new emissions requirements. These regulations have been very successful, dramatically reducing dioxin emissions and making modern waste-to-energy systems one of the cleanest sources of electricity.

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External links

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Health effects

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