【英语时差8,16】吃三文鱼有风险？（在线收听）

A study has found that salmon raised in sea farms contain higher levels of pollutants than wild salmon. But the levels are still well within legal limits. The study took two years. It found higher levels of P-C-B's, dioxin and other chemicals in farm-raised salmon. The levels were an average of ten times higher than in salmon caught in the open sea. The researchers say people can safely eat four to eight servings of wild salmon a month. The United States Food and Drug Administration, however, disagrees with this advice. Agency officials say they find no health concern. Their advice to people is not to change their eating of farmed or wild salmon. Federal and industry officials say the level of dangerous chemicals in salmon has decreased by ninety-percent since the nineteen-seventies. They say any threat is small compared to the good that eating salmon can do. Salmon is high in omega-three fatty acids. These can help prevent heart disease. Salmon is also among fish that are lower in levels of mercury pollution.