Illegal levels of cancer-causing dioxins showed up in 8 percent of food and feed samples taken in Europe between 1999 and 2008, a report from Europe's food safety agency said on Wednesday.
Animal and fish liver products had the highest dioxin levels in food while fish oil showed the strongest concentrations in animal feed, the Italy-based European Food Safety Authority (EFSA) said.
"Long-term exposure to high levels of dioxins has been shown to cause a range of effects, including cancer," EFSA said.
"Their persistence and the fact that they accumulate in the food chain, notably in animal fat, therefore continues to cause some safety concerns," it added.
The report used more than 7,000 samples collected in 21 European countries. The high rate of illegal dioxin levels in the samples resulted partly from "targeted sampling during specific contamination episodes," EFSA said.
A lack of information on which samples resulted from targeted versus random tests made it hard to identify a clear trend in dioxin levels in food and feed, EFSA said.
Dioxins are toxic substances formed by burning - for example in waste incinerators or forest fires - and in some industrial processes.
Airborne dioxins are deposited onto plants and in soils and water, and enter the food chain when ingested by livestock and fish.
In recent years, health scares in Europe linked to dioxin contamination in food have included Italian buffalo mozzarella cheese and Irish pork.
EFSA said continuous random testing of "a sufficient number of samples" was needed to allow a more accurate assessment of the levels of dioxins in food and feed.