For the first time, the Washington State Department of Health has issued a statewide fish advisory: children younger than six and women of childbearing age should avoid fresh tuna, shark, swordfish, tilefish and king mackerel. The advisory is the result of a scientific review into the safe levels of mercury in food. In Washington, the limits are about six times more restrictive than those set by the US Food and Drug Administration for the whole country, and fish is the highest source of methylmercury, a toxin that can harm the human nervous system. The fish become contaminated with mercury that exists naturally or is disposed of as industrial or household waste.