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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