The dirty dozen – not just an old movie?
The “dirty dozen” refers to a group of environmental chemicals that are very resistant in the environment, and are sometimes referred to erroneously as superpoisons. They include dioxins and furanes, polychlorinated biphenyls, and the insecticides aldrin, dieldrin, DDT, endrin, heptachlor, chlordane, mirex and toxaphene, and a fungicide called hexachlorobenzene.
The aim has been to ban the use of all of these because of their environmental effects. The ban of DDT started in Scandinavia (1970) and USA (1972), but this insecticide continued to be used in many other countries (e.g. Germany 1979, United Kingdom 1984, Canada 1990, Austria 1992), and was in fact produced in the U.S. for sale in foreign markets until 1985. In the Stockholm Convention of the United Nations signed in 2001, countries committed themselves to abolish both the production and use of these compounds. Now only DDT is in approved use for malaria control in tropical countries. In the Nordic Countries, the other pesticides have not been used for decades; some of them were never used here at all.
The crucial reason why substances are included in the dirty dozen is their persistence in the environment and accumulation in living organisms, i.e. belonging to POPs (persistent organic pollutants); it is not their toxicity as such. True, there are highly toxic compounds among them such as dioxins, but this group includes also very non-toxic substances to human beings, such as DDT and most of the PCB compounds. Accumulation within the organism is based on two factors, stability and lipid solubility. These molecules are so stable that chemical factors (acids, bases), physical factors (e.g. light), or even biological factors (e.g. microbial enzymes) are not able to break them down once they have gained access to the environment.
From water to fat
Another factor is that these compounds are very poorly water soluble but reasonably fat soluble. This means that they have a very high fat-water partition ratio; in other words they like to move from the water phase to the lipid or fat phase. In nature, fat is found mostly in living organisms, therefore a high lipid-water partition means in practice bioconcentration into living organisms.
These persistent substances are broken down or excreted from the organism very slowly, so their concentration can stay high for an entire lifetime. The next trophic level, that is organisms that consume the previous level as their food, will then receive the compound in their food continuously, and at each successive trophic level the concentrations multiply. In water, a typical chain of trophic levels is plant plankton, animal plankton, insects and other arthropods, small fish, large fish, and finally seals and fish-eating birds, especially birds of prey such as the white tailed eagle and osprey.
Thus species at the top of the food chain are the most sensitive to these poisons. These include seals and fish-hunting birds. Birds have encountered difficulties in breeding because of two reasons, hormonal disturbances and weakness of their egg shells due to poor mineralization. Seals have also suffered from breeding difficulties which have been caused by deformities in the uterine tubes and other structural deformities.
These compounds do not cause a real risk to humans, at least not at the present levels of these compounds in the environment, simply because humans are omnivorous. The amount of marine food even consumed by people who eat a lot of fish is only a fraction of that eaten by seals. The Baltic Sea is probably the most badly contaminated sea in the world, and when its environmental condition has shown signs of improvement at least to some extent, then seals again have started to reproduce. White tailed eagles have been fed with pig carcasses during the winters, and also their breeding success has improved. Finally the bird populations of the American Great Lakes have been recovering.
The problem is thus more acute for animals than for humans. It is however, necessary to monitor carefully some of these substances, since the safety margins between the tolerable daily intake and the actual intake are very low.
Persistent chemicals are a major problem especially for species at the top of the food chain. Human beings are top of the food chain, but because we eat so many different foods from many sources, it means we have a good buffer protecting us from chemicals enriched in one source.
Notes and references
- See the chapter "Where do the dioxins come from?".
- See the chapter "Are the PCB compounds really super-poisons?".
- See the chapter "If this kills insects, what effect is it having on me?".
- Elimination is a general name for all mechanisms that tend to make a substance disappear from an organism. It includes excretion into urine, via bile to faeces, in some cases to milk, escaping from the organism via lungs, and breakdown (metabolism) in the organism, typically by liver enzymes.
One level up: Is man defiled by what goes to his mouth?
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