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Published: 12 Aug, 2020

Old sins cause new problems

FEATURE Dioxins in herring do not decrease at the expected rate, despite the fact that many sources of air pollution have reduced emissions. Earlier chlorophenol use may be part of the explanation.

Text: Kristina Viklund

Dioxin concentrations in the Baltic Sea have long been so high that the sale of fish as food for both humans and animals is outlawed in the EU. In humans, dioxins are suspected to affect reproduction and the immune system, and also appear to cause cancer. They bind to organic matter and fat, and can therefore accumulate in animals with high fat content, such as herring. 

This is a well-known and widely discussed environmental problem for the Baltic Sea. When the dioxin problem was discovered in the 1970s, measures were put in place, and levels in fish have dropped since then. In recent years, however, the declining rate of these toxic levels has been less pronounced, and the composition of dioxins in herring has changed. What sources are most important today? Why has the composition changed and why do toxic levels in fish not decrease faster?

Formed during combustion

Dioxin is not just one substance, but a collective name for hundreds of different substances. The various dioxins, and their related dibenzofurans and biphenyls (PCBs) have very similar basic structures, with various numbers of chlorine atoms bound to them. How the chlorine atoms are placed in the molecules determines whether they are classified as dioxin or dioxin-like substances and importantly, their toxicity. Different sources have different composition and the composition of the dioxin mixture tells you where the dioxins come from.

The dioxins are formed as pollutants in various processes, usually under high temperatures, for example in waste incineration and metal production. Chlorophenols, which were previously used to prevent fungal infections in the timber industry, also contain high levels of dioxins. The use of chlorophenols in Sweden has been prohibited for over 30 years, but the dioxins remain in the soil or in nearby sediments in a large number of places where chlorophenol was previously used.

Researchers track the sources

For many years, EcoChange researchers have focused on dioxins and closely related substances, and have developed methods for tracing the sources of the dioxins found in the marine environment. Developments in the environment have also been closely monitored, and concentrations in the physical environment have decreased as emissions to the air have decreased. But the dioxin mixtures found in the physical environment are not the same as those found in herring, and the decline seen in the physical environment is not as clear when it comes to the levels in herring in different parts of the Baltic Sea.

Unclear sources

The dioxins are persistent, which means that they hardly change when transported by air, water and bound to particles. But even though the substances are stable, the different forms of dioxins are absorbed to different degrees and metabolized differently in the fish. This means that the composition of dioxins changes when they enter the herring, making it more difficult to determine the source of the dioxins in the fish. The results from herring are therefore more difficult to interpret, but by linking the results from concentrations and composition in herring with the previously known patterns for different dioxin sources, the researchers have been able to draw some conclusions about the sources of the dioxins in the herring.

Leakage is ever important

Dioxin emissions from various forms of combustion are decreasing, but still account for a large proportion of dioxins in herring. Sweden accounts for about a third of the deposition of dioxins from air, so measures in other countries are therefore also important in reducing levels. In recent years, dioxins derived from chlorophenols of various kinds appear to constitute an increasing proportion of the dioxins in the herring. Leakage from sediments or directly from contaminated areas on land seems to be an increasingly important contribution to the dioxins in the fish.

To some extent, this may be a result of remediation measures for air emissions given priority over leakage from coastal contaminated soils and sediments. Changing levels may explain why the current trend is slowing down compared to twenty years ago. If the growth of the herring decreases, a fish of a certain length will be older, and thereby have stored more dioxins in its body. Changes in composition may have several physical and ecological explanations linked to various possible sources. Factors such as increased runoff with increased land leakage, altered food choices in marine organisms, new species, altered food webs and other ecological factors affect uptake and transport in the food chain, and may result in a changed composition in fish. 

The article has earlier been published in the annual report EcoChange 2019. The text is based on the following scientific publication:

Assefa, Anteneh; Tysklind, Mats; Bignert, Anders; et al.
2019. Sources of polychlorinated dibenzo-p-dioxins and dibenzofurans to Baltic Sea herring. Chemosphere, Elsevier 2019, Vol. 218 : 493-500