The state attorney general's office, which filed a lawsuit seeking the labels in 2004, is appealing a decision in which a San Francisco Superior Court judge ruled in 2006 that the warnings are not required.
State lawyers argued in the lawsuit that mercury warnings were required under Proposition 65, a state law passed by voters in 1987. The measure mandates consumer warnings on products that cause reproductive harm or cancer.
But Superior Court Judge Robert Dondero said the Proposition 65 requirement was pre-empted by federal law. Dondero also found that mercury in tuna is naturally occurring and is not at levels high enough to require health warnings.
Lawyers for the state and the tuna companies will argue the case before a three-judge appeals panel at the State Building. The panel will then have 90 days to issue a written ruling.
Some environmental and medical groups, including the San Francisco Medical Society, say excessive exposure to mercury poses serious health risks, particularly to pregnant women and children.
Industry and restaurant groups, including the Center for Consumer Freedom, say moderate amounts of tuna bring important health benefits by providing protein and omega-3 fatty acids.
The three companies sued by the state are Tri-Union Seafoods LLC, maker of Chicken of the Sea; Del Monte Corp., maker of Starkist; and Bumble Bee Seafoods LLC, maker of Bumble Bee tuna.
