According to the organization, unpurified fish oil supplements can contain contaminants such as polychlorinated biphenyls (PCBs) and dioxins which ED health program director Dr John Balbus called "potent developmental and neurological toxins".
Thirty-seven of the 54 companies included in the survey used molecular distillation and steam deodorization to separate pollutants from omega-3 fatty acids - the method deemed to be the most scrupulous.
The survey does not show that products made by the remaining 17 producers and suppliers are necessarily unsafe, however, since participation was voluntary. Nine companies submitted incomplete responses to the organization's request for information on their practices and eight companies failed to respond.
The eight non-respondents were classified by ED as 'worst choice' for consumers, since it was not able to assess the purity of their products based on their declared practices.
Among a number of fish oil standards from US and foreign government agencies, those of the Environmental Protection Agency and the State of California (Proposition 65) were adopted as the baseline for the survey, as they bear the most relevance to human health.
The Council for Responsible Nutrition has also drawn up its safety guidelines for fish oil supplements supplied by its members, which are said to be as strict - and in some aspects more so - than the EPA's and California's.
"We are pleased, too, that so many companies are taking care to manufacture fish oil supplements that rate high in purity and quality," said CRN president Annette Dickinson.
The primary purpose of the survey was to make consumers aware of the potential risks of environmental contaminants and encourage them to choose products that have been properly purified and are derived only from well-managed, ecologically sound fisheries.
Fish oil is an excellent source of omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which have been shown in numerous studies to have a beneficial effect on cardiovascular health and cholesterol.
Last September consumer awareness of the health benefits of fatty acids was given a boost when the FDA issued a qualified heart health claim enabling manufacturers to label products containing omega-3 EPA and DHA as being heart healthy.
Sales of omega-3 supplements - preferred by many consumers who have an aversion to the taste of fish - reached $190 million in 2003.