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Bears build up what fish flush out

July 12, 2007 By Ewen Callaway This article courtesy of Nature News.

Overlooked pollutants might be having a bigger effect than we thought.

Efforts to control chemical pollution may overlook thousands of toxins that concentrate as they march up the food chain, say researchers. Compounds that do not accumulate in fish can still build up in marine birds and mammals — and possibly the people that eat them, they have found.

The finding puts up to a third of industrial chemicals — including some perfumes and pesticides — under suspicion. Regulations should be changed, says environmental toxicologist Frank Gobas of Simon Fraser University in Burnaby, British Columbia: "We have to re-evaluate how we are assessing these substances."

Some organic chemicals — such the pesticide DDT and PCBs, which have been used as coolants — are known to build up when one animal eats another, a process called biomagnification.

The most common measure of a chemical's propensity to bioaccumulate is how easily it passes through water. The less soluble a chemical, the harder it is for an animal with gills to expel it. The concentration of PCBs in fish, for example, can be 100 times that found in the algae at the base of the food chain.

But to study bioaccumulation in birds and mammals, you must also measure how easily a chemical passes through the air, say Gobas and his colleagues. Studying more than a dozen animals, including ducks, beluga whales and polar bears, they fingered several compounds that regulators have overlooked because they can escape into water, but not air.

Chemical traps

We have to re-evaluate how we are assessing these substances.
Frank Gobas, Simon Fraser University, Barnaby, British Columbia.
For instance, hexachlorocyclohexane, an ingredient of the pesticide lindane, has the same concentration in algae, shellfish and fish, but accumulates to high levels in polar bears. "These chemicals get stuck in our bodies because they cannot get out through exhalation," says Gobas.

The study, published in Science1, did not address the health effects of any of the chemicals.

A computer model of the diet of Inuit populations in northern Canada found that some chemicals could concentrate 2,000-fold compared with their levels at the base of the food chain. The Inuit, who hunt marine mammals such as Beluga whales, and are known to contain high levels of PCBs, are especially vulnerable to biomagnified pollutants, the researchers believe.

Substance abuse

Derek Muir, an environmental toxicologist at Environment Canada in Burlington, Ontario, supports incorporating the findings into chemical risk assessment. But he wonders whether regulators will act. "It's one thing to be aware of it, and another to actually rewrite regulations. That's a tougher challenge," he says.

Governments may be waiting for better data on how the chemicals are metabolized — something the new study did not address — before they act, he says. If an animal breaks down a pollutant before it has a chance to move up the food chain, no biomagnification occurs.

The use of the most toxic chemicals such as dioxin and DDT is covered by the Stockholm Convention on Persistent Organic Pollutants, signed by 131 countries in 2004. To address subtler threats, governments have begun assessing the toxicity, persistence and bioaccumulation of tens of thousands of industrial compounds.

A new Stockholm Convention is needed to convince governments they should look at biomagnification in mammals, Gobas says. Individual countries are unlikely to go it alone, he believes. "They are now so busy dealing with the existing criteria that their appetite for new criteria is limited."


  1. Kelly, B. C. et al. Science 317, 236-238 (2007).


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