dioxin - Chemical structure, Sources of dioxin, Health effects, Studies of dioxins effects in Vietnam, Dioxin exposure incidents

A highly toxic contaminant of the chlorphenoxy group of herbicides whose level is currently regulated at 0·1 parts per million or less. It causes a severe form of skin eruption (chloracne), and in laboratory animals causes cancer and damages the fetuses of mothers exposed to it. Dioxin seems to be less toxic in humans than in animals. The jungle defoliant Agent Orange used in Vietnam contained high levels of dioxin as a contaminant. Many violent explosions have occurred during the manufacture of chlorophenoxy herbicides, causing the release of dioxin, including the Monsanto plant in West Virginia, USA (1949) and in Seveso, Italy (1976). In 1999 it was revealed that 1500 times the legal limit of dioxin had contaminated a batch of fat sold to animal feed manufacturers, prompting widespread withdrawal of meat products across Europe. Like PCBs, dioxins persist in body fat. The doses consumed by the Belgians were probably too low to cause cancer, according to scientists, but could affect neural and cognitive development, the immune system, and thyroid and steroid hormones, especially in the young.

Chemical structure

The basic structure of PCDD/Fs comprises two benzene rings joined by either a single (furan) or a double oxygen bridge (dioxin). There are 210 different types of PCDD/F congeners (herein, a congener means a related dioxin compound) comprising of 75 PCDDs and 135 PCDFs. Other dioxin congeners (or mixtures thereof) are given a toxicity rating from 0 to 1, where TCDD = 1.

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.

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. 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. 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. Breast fed children usually have substantially higher dioxin body burdens than non breast fed children until they are about 8 to 10 years old.

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:

Coal fired utilities Metal smelting Diesel trucks Land application of sewage sludge Burning treated wood Trash burn barrels

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 US EPA in its "Re-Evaluating Dioxin" (1995). 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.

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. 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. Higher levels of chlorination require higher reaction temperatures and greater dioxin production. Dioxins may also be formed during the photochemical breakdown of the common antimicrobial compound triclosan.

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. 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.

Dietary sources of dioxin in the United States have been analized by the EPA and other scientists.

And, "Levels of Dioxin in U.S. Food Supply (1995)" from May 2001 study by Arnold Schecter et.

Health effects

Dioxins build up in living tissue (bioaccumulate) over time, so even small exposures may accumulate to dangerous levels. TCDD, the moxt toxic of dioxins, has a half-life of approximately 7 years in humans, but at high concentrations, the elimination rate is enhanced by metabolism . The health effects of dioxins are mediated by their action on a cellular receptor, the Aryl Hydrocarbon Receptor (AhR).

Exposure to high levels of dioxin in humans causes a severe form of persistent acne, known as chloracne . Endometriosis Birth defects COPD Diabetes And at least in laboratory animals, increased rates of liver and thyroid cancer are observed

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 Center for Disease Control found that dioxin levels in Vietnam veterans were in no way atypical when compared against the rest of the population.

Dioxin exposure incidents

In 1963 a dioxin cloud escapes after an explosion in a Philips-Duphar plant (now Solvay Group) near Amsterdam. In 1976 large amounts of dioxin were released in an industrial accident at Seveso, although no immediate human fatalities or birth defects occurred. In 1978, dioxin was one of the contaminants that forced the evacuation of the Love Canal neighborhood of Niagara Falls, New York. Dioxin also caused the 1983 evacuation of Times Beach, Missouri. 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. In May 1999, there was a dioxin crisis in Belgium: quantities of dioxin had entered the food chain through contaminated animal feed. In a 2001 case study, 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. 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, and was diagnosed only after a toxicologist recognised the symptoms of chloracne while viewing television news coverage of his condition.

Incineration and dioxin emissions

It is claimed that modern waste incinerators--like Texas Industries' cement plant in Midlothian, Texas--are equipped with pollution control equipment which reduces dioxin emissions to insignificant levels. (However, the emissions from TXI's Midlothian plant have increased.) Incineration of municipal solid waste, medical waste, sewage sludge, and hazardous waste together produce less than 3% of all dioxin emissions. When the original US EPA inventory of dioxin sources was done in 1987, incineration represented over 80% of known dioxin sources. These regulations have been very successful in reducing dioxin stack emissions from incinerators.

However, there is debate over how "clean" this has made incineration, since the process of removing dioxin from stack emissions transfers dioxin residues to filter cake, slag and fly ash, toxic waste which still has to be disposed of safely. 44 (3) : 171-4 May/Jun89 Pesticide residues that are legal contaminants of tobacco Health effects of dioxins "Assessment of the Health Risks of Dioxins", a 1998 report by the World Health Organisation. Environment and Health 5:87 The risks of dioxin to human health (Review article) Re-evaluation of the health hazards posed by dioxins Ind. Review article that questiones the carcinogenity of dioxins. Synopsis on dioxins and PCBs (1999) Review article with some interesting tables and figures. Environmental Health Perspectives 112(13): 1265-1268 (2004) Dioxin Revisited: Developments Since the 1997 IARC Classification of Dioxin as a Human Carcinogen. Review article that provides evidence for the carcinogenity of dioxins. New Zealand Ministry of Health page on dioxins IARC monograph: "Polychlorinated Dibenzo-para-dioxins"