Contaminants of major concern include: aflatoxins; PCBs; toxic metals (lead, cadmium, mercury, tin); organochlorine pesticides (aldrin plus dieldrin, DDT complex, hexachlorocyclohexane, lindane, heptachlor and heptachlor epoxide, hexachlorobenzene, endrin and endosulfan); organophosphorous pesticides (diazinon, malathion, parathion methyl and fenitrothion).
The UK Working Party on Pesticide Residues reported that in 1991 there were chemical residues in 37% of the bread, milk and potatoes sampled; in 30% of the cereals; in 29% of the fruit and vegetables. Typically, 1 or 2% of foods sampled exceed their maximum residue level (MRL). The USA National Academy of Sciences found that the worst foods for residues are, in descending order, tomatoes, beef, potatoes, oranges, lettuces, applies, peaches, pork, wheat, soybeans, beans, carrots, chicken, corn and grapes. A consumer magazine survey found that 10-25% of "post-harvest" pesticides applied to the skin during storage had seeped into the flesh. Washing apples and potatoes left residues largely unaffected (many will not dissolve, others are inside the skin). Peeling removed about 85% of apple residues and 75-90% of those in potatoes, but it also removes up to a fifth of the product as well as important fibre and nutrients. Cooking often increases residues.