Where do the dioxins come from?
The name dioxin has become a metaphor of evil. Dioxins were the main impurities of herbicide Agent Orange used by American army to destroy the jungles of Vietnam in the beginning of 1970s, and the Vietnam veterans have blamed it for many of their ailments. A pressurized container in a chlorophenol production plant released its contents into the atmosphere in Seveso in 1976, and the chemical cloud which covered the town contained several kilograms of the most toxic dioxin, TCDD. Dioxins have been claimed to be responsible for the impaired reproduction of white tailed eagles and seals in the Baltic Sea, and salmon fry mortality, so called M74 syndrome.
Dioxins belong to the "dirty dozen" listed first by environmental organisations and subsequently also by the United Nations. These are substances that are very persistent in the environment, and bioaccumulate in the food chain and some of them are also very poisonous. Dioxins are not a single compound nor even a single group of compounds, but groups of compounds with similar actions. The most important groups are polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofuranes, which consist of 210 different chemicals. Seventeen of them are very toxic and therefore worth monitoring. The most toxic of the lot is 2,3,7,8-tetrachlorodibenzo-p-dioxin, often referred to as TCDD.
Origin of dioxins
Dioxins are formed in all burning processes, if chlorine is present. The chemical reaction is catalysed by metals, especially by copper. Incineration of municipal waste has been especially problematic, because the waste usually contains a lot of PVC plastic (vinyl plastics), and often there is also wood impregnated with chlorophenols. In these products, chlorine is already present in a precursor form to make dioxins, and the chlorophenols even contain some dioxins as impurities. After the problem came to light, the incineration techniques have undergone a thorough overhaul, and at present the new modern incinerators emit much less dioxins than are present in the waste they burn. In other words, instead of being a source, incineration is now a way of getting rid of dioxins.
Today the most important source of dioxins is the metal industries. PCB-oils and chlorophenols contain some dioxins as synthesis side products and impurities, and some of those may end up in the environment. Small amounts of dioxins have also been found in nature. A very special source was found to be certain clays, and this turned out to be important because clay (kaolin) is sometimes added to animal feed. Natural dioxins may originate from forest fires or other “natural” combustion processes long time ago.
Exposure to dioxins
Most of the human intake comes from our food, over 90%. Especially in crowded parts of Central Europe, municipal waste is to large extent incinerated, and the previous poor incineration techniques meant that large amounts of dioxins were released into the environment. They spread out with fly ash to fields and pastures, and cattle ate them along with grass. Therefore for inhabitants of Central Europe, meat and dairy products have been the most problematic. Lately the dioxin concentrations of organically produced hen eggs have given cause for concern, because free foraging hens collect dioxins from soil and soil organisms.
After the problem was appreciated, incineration technology was rapidly improved, and the emissions have decreased dramatically. This is reflected also in the dioxin concentrations present in foodstuffs, and the daily dioxin intake has declined from several hundred picograms (pg = 0.000,000,000,001 g) per day in the 1980s to 50–200 picograms per day (1–3 pg/kg/day) in late 1990s.
In Finland and to some extent in Sweden, dumpsites have been the most important means of municipal waste treatment. Even though dumpsite fires could be a highly important source of dioxins, this was really not a serious problem, because these regions do not have high population densities. Therefore agricultural products have been very pure, and the dioxin levels of milk and meat have been very low. Baltic fish, however, have been contaminated with dioxins, with concentrations being much higher in the older fish. This means a questionable world record in Finland: fish are responsible for two thirds or more of dioxin intake, with the role of other foodstuffs being very minor. Nonetheless, the total exposure has still been lower than in Central Europe, less than 1 pg/kg/day.
It is also interesting that human levels have decreased at almost the same rate, even though one might imagine that such a huge storage site as the Baltic Sea would maintain high human concentrations in the adjoining countries. It is still unclear, how much the levels in the Baltic herring are influenced by the stores in the sea, most of which are in sea bottom sediments, and how much by the on-going atmospheric fallout. The spectrum of the types of dioxin congeners detected in herring is quite similar to that found in meat and milk, and very different from those analysed in bottom sediments. The concentrations in herring seem to have been reduced to about one fifth of those seen in 1970s.
Contaminated soils have been a hot topic for decades. Famous incidents have been Times Beach, Missouri, where roads and horse arenas were sprayed with contaminated oils (1972-1976), and Love Canal, Niagara Falls, New York, where a school was built in 1955 on an abandoned dumpsite of toxic wastes which were leaking and caused health problems in the late 1970s. Both of these are examples of irresponsible activities. In Europe, the name of dioxin is forever associated with Seveso, Italy; this was an industrial accident that caused a toxic cloud with several kilograms of TCDD to spread over the town. This is also the site which has been most thoroughly monitored. The most badly contaminated parts of the town had practically all of the surface soil removed, and contaminated buildings and items were transported to a dump site. Two huge stadium-size special dumpsites were dug to a depth of 9 metres, lined with clay and thick plastic covering. Contaminated soil and scrap were layered into these dumpsites, again covered with plastic and clay to prevent rainwater from pouring through it, and clean surface soil layered on top of the hill to turn it into a park. There is a drainage system in the bottom, and the drain water is continuously monitored for dioxin levels.
In the Nordic countries and probably elsewhere in the boreal forestry regions, the most common sites of dioxin-contaminated soil are old sawmill sites. Pentachlorophenol and other chlorophenols were used for many decades to prevent blue-stain fungal growth in timber. Technical chlorophenol preparations contained many dioxin-like compounds as impurities. The water soluble chlorophenols have by and large been dissolved by rain waters, and their concentrations may be relatively low at sawmill sites. However, the non-soluble dioxins have remained in the surface soil.
Since dioxins are not water-soluble, they pose little risk to ground water; and in reality they do not cause any other problems as long as the contaminated area is left alone. The problem is that future land development is made impossible due to the existence of these high concentrations of dioxins. Often the old sawmill and thus the contaminated land may be on the lake close to the centre of a town, in fact the town may have grown up around the mill and it would often be the main employer of the region. Today city developers would like to convert the old factory site for a better use, often for luxury homes next to the lake or river. This is what causes the problems. If the soil was left alone it would not pose a risk to the community, but if people start growing potatoes and carrots in their new gardens, and children use it as a playground, then we can run into real problems.
In some cases, surface soil has been completely removed similar to the Seveso example on a smaller scale. Even though expensive, this seems to lead to completely satisfactory results. Human exposure from contaminated soil even among people very near to these areas has not been shown to cause nearly as much intake as that from food.
Dioxins are formed in all burning processes, especially waste incineration, if the conditions are not optimal. The emissions have decreased to a fraction of those in 1980s; the most important remaining source is the metal industries.
Notes and references
- See the chapter "The dirty dozen – not just an old movie?"
- Dioxins are present at extremely low concentrations, therefore also the units may be less well known. Microgram (μg) is one millionth of a gram, nanogram (ng) is one thousand millionth of a gram, picogram (pg) is one millionth of millionth of a gram (10–12 g). One thousand million is called a milliard in Europe, but a billion in America. One million million is called a billion in Europe, but a trillion in America. Therefore one should also be very careful with units when reading texts on dioxins, a wrong unit may easily lead to a million-fold error.
- Congener means a chemical derived from a similar basic structure: there are 35 congeners of chlorinated dibenzo-p-dioxins and 75 congeners of chlorinated dibenzofuranes differing within the group from each other only by the number and location of chlorine atoms in the molecule.
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