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Sunday, 7 October, 2001, 10:13 GMT 11:13 UK
Great Lakes heal themselves
Lake Michigan and Chicago skyline
The lakes are releasing pollutants into the atmosphere
The world's biggest freshwater system, the Great Lakes between Canada and the United States, is cleaning itself of pollutants, according to environmental researchers.

The lakes contain about 20% of the world's fresh surface water and have been contaminated for decades, but scientists say pollutants are evaporating in significant quantities from the surface.

The Lakes are like giant lungs that have been sucking in pollutants for the past 50 years and now it is as if they are starting to exhale

Dr Keith Puckett
While environmentalists say this is a positive development, it could be creating problems elsewhere.

A joint study from the Canadian government's environmental agency and American scientists says that significant quantities of chemicals, including PCBs (polychlorinated biphenyls) and pesticides, washed into the system are now being released from the Great Lakes, evaporating off the surface in a process called outgassing.

In essence, "the Lakes are like giant lungs that have been sucking in pollutants for the past 50 years and now it is as if they are starting to exhale," said Dr Keith Puckett, one of scientists who worked on the project.

Polluted food chain

For several years, people living near the Great Lakes have been advised to limit their fish consumption as PCBs, which have been linked to cancer, have become concentrated in the food chain.

Great Lakes map
But the study finds that there has been a net decrease of PCBs of 10 metric tonnes.

"So, in the longer term, we should see continued reductions in the levels of these compounds in fish. This would also decrease the risk of those people who rely on these fish for a large portion of their diet," said Dr Puckett.

PCBs are hard to break down and can travel thousands of kilometres in the air, so, if the Lakes are releasing them back into the atmosphere, this could create problems elsewhere.

"We need to go back and look at those existing substances seriously because the major health problems might well exist there, and we've just assumed they'll just settle out in the environment and may be we can forget them," said Ken Olgilvie of Toronto-based Pollution Probe.

He says the study shows how difficult it becomes to remove the toxins from the environment. In areas like the Arctic, where fish is a staple food, PCBs are found in high concentrations in the breast milk of nursing mothers and in animals like polar bears and seals.

Researchers now plan to carry out tests in Arctic waters to see whether they too are cleaning themselves of the pollutants.

See also:

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