Attention-grabbing headlines flooded the newspapers in the UK at the weekend referring to study published in the journal Science that found that levels of PCBs and dioxins in farmed salmon, particularly in Europe, had reached higher levels than their wild counterparts.
In the article on dioxins below, Dr Andrew Wadge, director of Food Safety Policy at the Food Standards Agency (FSA) seeks to reassure the consumer.
Risk assessments for dioxins are carried out by comparing intakes from the diet (and other sources) with safety guideline levels established by independent expert scientific committees, he writes.
"The recent findings of dioxins in salmon reported by American researchers is in line with earlier work by the FSA." But, he asks, the question that needs to be answered is why have the American researchers advised against eating farmed salmon, except on a very occasional basis, whereas the FSA has advised that salmon can continue to be eaten regularly as part of a healthy balanced diet.
"The answer lies in the different approaches to setting safety guideline levels used by the US Environmental Protection Agency (EPA) compared with the other bodies, such as the US Food and Drug Administration, the World Health Organisation, the Scientific Committee on Food that advised the European Commission and the UK's Committee on Toxicity.
In its risk assessment the EPA does not take into account the mechanism by which compounds cause cancer."
Dioxins are known to cause a wide variety of toxic effects, including cancer, in laboratory animals when exposed to high levels over a lifetime.
They have also been found to cause reproductive toxicity in laboratory animals. The EPA risk assessment approach for dioxins, which has been evolving since 1991, but has not been finalised, assumes that any level of exposure to dioxins carries with it some risk, albeit small, of getting cancer.
According to Dr Wadge, the WHO assessment took into account the mechanism by which dioxins cause cancer and they have concluded that dioxins are 'non-genotoxic' carcinogens, for which it is possible to establish a threshold of toxicity.
In 2001, experts advising the WHO concluded that provided exposure to dioxins is kept below this threshold then there will be no adverse effect upon health.
The safety guideline value was then set below the threshold to allow for some uncertainties in the precise nature of the effects and extrapolation from animals to man.
The FSA director stresses that the levels of dioxins in salmon are such that normal exposure via the diet would not lead to intakes above the safety guideline value.
"There are clearly established benefits from eating oily fish such as salmon because it contains factors that are protective against cardiovascular disease.
For this reason, independent experts agree that the benefits of eating one portion a week of oily fish such as salmon far outweigh any possible risks," he added.
Selecting farm salmon from supermarkets all over the world and wild from the waters of North America to make a total of 700 samples, the scientists involved in the US study used only PCBs, dioxins, dieldrin and toxaphene to calculate consumption safety guidelines, deeming these four toxins to most strongly impact human health.
Breaking it down by region, the researchers found levels of all 14 toxins were significantly elevated in both European and North American farm-raised salmon when compared with wild Pacific salmon. Levels of only six toxins were significantly elevated in South American farm-raised salmon.
Europe seems to have fared particularly badly, with the study finding toxin levels in European farm-raised salmon significantly higher than in North American or South American equivalents.
"We think it's important for people who eat salmon to know that farmed salmon have higher levels of toxins than wild salmon from the open ocean," said IU school of public and environmental affairs professor Ronald Hites, who led the study.
Environmental campaigners in the UK are calling on the government to respond to the findings.
"The FSA should be trying to encourage British farmed salmon to be as healthy as possible. They should recognise this point, rather than accept the status quo," Mary Taylor, a spokesperson for the environmental campaigners Friends of the Earth told FoodNavigator.com.