Are the dioxins the most dangerous chemicals in our environment?
Exposure to dioxins has decreased, and this decrease over the twenty or thirty years is reflected in concentrations in humans. This is shown especially clearly in the concentrations in human milk. Because the concentrations are very low, picograms (millionth of millionth of grams) per one gram fat (10–12 g/g), a large sample of fat is needed for analysis, even with the highly sophisticated and sensitive technique called mass spectrometry used to analyse the samples. A recent study analysed population levels in surgical patients, both cancer patients and those undergoing an appendicitis operation. Also placentas have been analysed for their dioxin levels. It is very hard to analyse these compounds in healthy people, because it is ethically questionable whether one should take fat samples of several grams simply for research purposes, or blood samples of some 200 ml (a full glassful) to ensure that the sample will be big enough. These kinds of studies are carefully considered and debated in ethics committees to ensure that they are worth the effort.
Breast milk seems to be a good substitute. The concentrations per gram fat seem to be similar to those in serum or fat tissue. Breast milk can be collected in small lots daily over several weeks so that the collection does not influence the baby's nutrition. Therefore it is possible to obtain enough fat for the analysis fairly easily. This is why human milk is popular for monitoring dioxin concentrations e.g. in international comparison studies.
In most industrialised countries, the concentrations were between 30 and 40 pg/g in 1987. By 2007 many of them had decreased to 10 to 20 pg/g. Swedish archived samples have indicated that the concentrations were even higher in the 1970s, probably close to 100 pg/g. Therefore the concentrations have decreased by at least 80%.
Surprisingly dioxin concentrations are low in most eastern European countries, being lowest in Albania. This probably reflects the difference in chemical risks between different countries. The socialist legacy means that there are some very badly polluted local "hotspots" and there were very high levels of sulphur dioxide and other traditional air pollutants in towns, but in general the chemical pollution was not as bad as in most Western countries.
Effects of dioxins in humans
Dioxins are generally regarded as super-poisons. One horrific example was the poisoning of the Ukrainian President, at the time when he was a presidential candidate in 2004. Within a few weeks, acneiform lesions developed on his face and his skin turned greyish. This is a typical chloracne. The reason is that the ducts of the sebaceous glands are closed by overgrowth of the epithelium, and the fat-like secretions start accumulating as lumps in the skin. Chloracne is well known in the chemical industries. After the Seveso accident in 1976, many inhabitants, especially badly exposed children, were diagnosed with chloracne.
Chloracne is the sole immediate sign of poisoning which has been repeatedly demonstrated in humans. In the Ukrainian case there was also severe abdominal pain, back pain, greatly enlarged liver and inflammation of pancreas. In this and another poisoning case in Vienna in 1998 the TCDD concentrations were huge, over 100,000 pg/g in fat. These concentrations are almost 100,000 times higher than TCDD concentrations in most people, and almost 10,000 times higher than all dioxins together (TEQs). These levels indicate that acute toxicity in man is not quite as high as in the most sensitive laboratory animals.
In chemical accidents and food contamination cases, there have usually been many chemicals involved, and therefore there has been uncertainty on the true cause of many consequences such as cancer. In Japan, a food poisoning incident occurred in 1968, when PCB oils leaked from cooling tubes to food oil, this accident was called Yusho poisoning. A similar accident took place in Taiwan in 1979. In both cases, the most serious consequences have been generally attributed to the dioxin-like dibenzofuranes, although the main chemical was PCB.
The most dramatic consequences occur during pregnancy. Miscarriages and a number of different birth defects were noted in children born to both Yusho and Yu-cheng mothers. Many different skin lesions were typical. The developmental effects evoked by dioxins are now considered as their most important and relevant risk. At very high industrial doses, there may be also a moderate risk of cancer, although here it is difficult to differentiate the contribution of other chemicals. Another possible, though in man so far unproven, risk is compromised immune functions possibly leading to increased sensitivity toward infections. Recently diabetes has been suspected to associate with high dioxin exposures, but this hypothesis requires much more firm evidence before it can be generally accepted.
In animal experiments, dioxins cause a variety of developmental effects. In animals, cancer is also a clear result of high doses, and in some species liver damage is seen. Dioxin toxicity differs extensively from species to species. In some animals such as the guinea-pig, dioxins are highly toxic, hence their reputation as super-poisons. But a hamster can resist more than a thousand-fold higher dose than a guinea pig. In fact one could not talk of a super-poison for a hamster. Also different dioxin congeners have different potencies. TCDD is the most toxic of this group of chemicals.
How dangerous are the dioxins to us?
The main difficulty in appreciating the risks of dioxins is that on one hand they are very toxic and harmful, but on the other hand the amounts to which people are exposed are very, very small. The daily intake is about 50 picograms (in some countries somewhat higher). If we eat about 2 kilograms of food every day, the average dioxin concentration is 25 pg/kg food (0.000,000,000,025 grams/kg). There is another way to consider this value. In order to obtain 5 grams (one teaspoonful) of dioxins, you would need 200,000,000,000 kilograms of food. If one truckload is 20 tons, this would mean 10 million truckloads that you would need. On the road, with one truck every 100 metres, this would make a line of trucks winding 25 times around the globe. Thus we are thus talking of very small amounts, and it is obvious that these are difficult to comprehend in a reasonable and responsible way, not only by lay people but also by the authorities.
Thus, it is not important whether dioxins are supertoxic or not, or whether a hamster or a guinea pig is killed by this or that dose. The essential point is: are the amounts of dioxins in our food such that they might evoke the effects which occur most sensitively. During recent years, most researchers have realised that the essential properties are developmental effects. If a female rat is given a dose of dioxin in mid-pregnancy, with a dose as low as 30 ng (30,000 pg) per kilogram of maternal weight, one can detect some effects on the teeth of the offspring. It is possible to calculate that during the 1970s, intakes in many countries were quite close to levels which would lead to similar concentrations in the mother as are found in rats after 30 ng/kg dioxin.
At the present time it does seem that we have some kind of safety margin. This means that the doses of dioxins we obtain from our food are at such a level that only five- to tenfold increases in the doses would cause a mild developmental effect on tooth enamel.
Benefit and risk
The problem of increasing the safety margins to hundred-fold or thousand-fold levels is that an important source of dioxins in many countries is fish. We have plenty of evidence that fish are a very healthy food. Fish proteins are of good quality. There is an abundance of some minerals which are otherwise in short supply, and fish meat is a good source of important vitamins like vitamin D. In addition, there is plenty of recent evidence that the fatty acids of marine food promote health, and also are beneficial in development of children, especially of their nervous systems.
Therefore it would be advisable to replace hard animal fats with fish oils. These oils are thought to be important in preventing cardiac deaths. Since cardiac mortality is so common, even if there were some deleterious effects from dioxins present in fish oils, their disadvantages would be overwhelmed by the health benefits of fish oils. This was clearly seen in a recent study on Finnish fishermen eating ample Baltic fish. Their risk of cardiac death was about 27 % lower and risk of cancer death 10 % lower than in the main population.
The most important sources of dioxins are different in different countries. The intake possibly causing the most sensitive slight adverse effects is probably 5 to 10 times higher than the present intake. Fish in general and Baltic Sea fish in particular contain relatively high amounts of dioxins. Nonetheless the benefits of fish consumption are so great that it is clearly advantageous to eat fish rather than to avoid this tasty food simply for fear of dioxin poisoning.
Notes and references
- Different dioxins are compared by giving each congener a Toxicity Equivalence Factor, TEF. TCDD has been given TEF of 1. Seventeen other dioxins have been given a TEF value from 0.0001 to 1. When the amount of dioxin congener is multiplied by its TEF, the result is the amount of TCDD that would be equivalent to it in terms of toxicity. When all dioxins are expressed as TCDD equivalents, their amounts can be simply added, and a TEQ (TCDD equivalent quantity) value expressing the toxicity of the whole mixture is obtained.
One level up: Is man defiled by what goes to his mouth?
Previous chapter: Where do the dioxins come from?
Next chapter: Are the PCB compounds really super-poisons?