There is both concern and controversy over mercury in fish, but an occasional tuna fish sandwich for an adult who is not going to become pregnant is not the main worry.
The biggest concern is the risk to a fetus. Mercury builds up in muscle cells of fish and in those who ingest it, and can remain to damage the nervous system of a fetus even months after ingestion, but what levels are dangerous has been debated.
Recently, after an eighteen-month study, the National Academy of Sciences put the boundary of concern at levels only one-fifth as high as other proposed standards: 0.1 micrograms per kilogram of body weight a day.
Translated to consumption of fish, to be on the safe side, a potentially pregnant woman might want to eat no more than one tuna fish sandwich a week. But the safe limit for other adults is probably much higher.
The riskiest fish are sharks and swordfish.
Just behind them are the large species of tuna. But canned tuna uses much smaller species, with a smaller range of mercury accumulation, and so canned tuna is safer than tuna steaks.