Hexachlorobenzene (HCB), formerly widely used as a fungicide, is one of the so-called dirty dozen – the first twelve toxic chemicals whose use was banned worldwide in 2004 under the Stockholm Convention on Persistent Organic Pollutants. However, due to its chemical structure, this compound is extremely stable and is only broken down very slowly in nature. “For this reason, HCB is still found in the environment,” says Michael Burkard, a postdoctoral researcher at Eawag’s Environmental Toxicology department.

Global grasshopper effect

HCB is not only persistent but also, under certain conditions, volatile. It enters the atmosphere via evaporation over tropical and temperate zones and is subsequently deposited with rain or snow. Burkard explains: “This is the grasshopper effect whereby HCB is gradually transported to polar regions. But because it’s cold at the poles, HCB remains and accumulates there – so the polar regions are the natural sinks.”

This is precisely where the summer feeding grounds of humpback whales lie – in the seas of the Arctic and Antarctic. Here, for around three months, these marine mammals (15 metres long and weighing up to 30 tons) build up the fat reserves which they live off for the rest of the year – and which they require for their long migration (covering several thousand kilometres) to and from their winter breeding grounds in the tropics. Burkard says: “When the fat reserves are mobilised, fat-soluble substances, including HCB, are released. Various pollutants accumulate in fasting whales – on average, concentrations are increased two- to threefold in these creatures before they return to their summer feeding grounds.”