A huge ships' graveyard in Mauritania may be the source of high levels of PCBs off the African coast. Credit: JS Callahan/tropicalpix/Alamy

Researchers cruising off the western coast of Africa have confirmed the presence of mysteriously high levels of airborne toxic polychlorinated biphenyls (PCBs).

The production and use of these chemicals is now largely banned by national laws and under the 2001 Stockholm Convention on Persistent Organic Pollutants — because of worries over their potential for causing cancer and other health problems. The reason the levels detected off the West African coast are a surprise — and a cause for concern — is that this region has not previously figured highly in thinking about the pollutants.

"We found high levels of PCBs in a region of the world where we wouldn't expect to find them, " says Rosalinda Gioia, an atmospheric pollution researcher at Lancaster University, UK, and the lead author of the report of the high levels in Environmental Science & Technology 1.

"In the global inventories of PCBs, Africa does not really represent a place where PCBs were sold or used," she says. "Of course, the long-term exposure to these levels of PCBs could lead to adverse effects to human health; however, a detailed toxicological study is needed in order to determine the effects."

Gioia says that action must now be taken to find and deal with the major West African sources of the pollutants. Illegal dumping and poor disposal of toxic waste — including a large ships' graveyard in Mauritania — are among the chief suspects.

Pollution problems

Although PCBs have a low immediate toxicity, they are very persistent in the environment and accumulate in tissues over time.

Many African countries are developing inventories of their pollutants as part of their responsibilities under the Stockholm Convention, says Jana Klánová, who works on African pollution issues at the Research Centre for Toxic Compounds in the Environment at Masaryk University in Brno, in the Czech Republic. The next step is to identify priority problems in each country and draw up plans to deal with them.

"Although most researchers in Africa are concerned with organochlorine pesticides — especially DDT which is still used for malaria control — PCB contamination should not be overlooked," Klánová told Nature. "According to our long-term measurements, the atmospheric levels of PCBs at background sites in Africa are lower than in Europe, but the urban, and especially industrial, sites have the same PCB levels in Africa as they do in Europe."

PCB plume

Gioia's team first noticed something odd off West Africa on a 2001 research cruise, when they observed a spike in the concentration of seven of the most important PCBs — the so-called ICES set. They noticed a similar plume of high levels of PCBs during a 2005 cruise.

In 2007, aboard the German research vessel Polarstern, Gioia again observed the PCB plume some 400 kilometres off the West African coast. The spike in levels was most pronounced when the wind was blowing from the African coast out to sea. This time, rather than just report the concentrations of the pollutants, she and her colleagues tried to determine their sources.

One possible source of PCBs is biomass burning, which is thought to release PCBs previously deposited in the soil. However, burning also produces another type of chemical called polynuclear aromatic hydrocarbons (PAHs). In the samples taken on the Polarstern cruise, PAHs were at their lowest when PCBs were highest, which argued against burning as a source.

Gioia says that one likely source of the pollution is the dumping of old electrical waste such as capacitors and other PCB-containing products. Developing countries take large amounts of waste from Western nations and break them down or dispose of them, often with little regard for safety or pollution.

Another probable source of the PCBs is one of the world's largest ships' graveyards. Just south of Nouadhibou in Mauritania, hundreds of wrecks have been grounded in shallow water where they are scavenged or simply left to decay. This break-up probably releases PCBs, Gioia says.

The precise sources of the PCB plume should be pinned down and used to complete the pollutant inventories of African countries, she says. These nations must then be helped to safely dispose of the potentially dangerous chemicals.