The safe upper limit of mercury consumption is about 0.1 microgram per kg of body weight per day, according to the U.S. Environmental Protection Agency (EPA) [source: PBS]. Both the U.S. Food and Drug Administration and the World Health Organization think the safe dose is higher than that, more like 0.3 micrograms/kilogram/day [source: MercuryAnswers]. So if you weigh 150 pounds (68 kilograms), that's somewhere between 6.8 and 20 micrograms per day, or 48 to 140 micrograms per week, as a safe amount.
For most types of fish, you're going to end up well below that limit as long as you don't eat it every day. Smaller fish typically contain very little mercury -- canned tuna contains about 4.8 micrograms per ounce, scallops have about 1.4 micrograms per ounce and flounder has only about 1.1 microgram per ounce [source: EUROCBC]. You could eat 4 ounces of flounder every day of the week and have no problems at all (plus, your heart would probably thank you for the omega-3 fatty acids). That's not true of all fish, though.
While you'd be hard-pressed to find a piece of seafood that doesn't contain some amount of methylmercury, the larger fish contain the most [source: PBS]. And if we look at why we've got mercury in our fish in the first place, it becomes clear why large fish are have more of it than smaller ones. Coal-fired power plants emit mercury into the air as part of the electricity-producing process. These plants account for 40 percent of all the mercury released into our environment [source: GE]. From there, it goes something like the acid-rain dilemma.
When it rains, all that airborne mercury falls into bodies of water, where bacteria convert it into methylmercury. Methylmercury is ideal for absorption into tiny organisms. When smaller fish eat those organisms, they ingest that methylmercury. And then the mercury moves up the food chain. The bigger the fish, the more methylmercury-containing organisms they eat, until something like a swordfish is sporting 28.4 micrograms per ounce, and a tilefish can have up to 41 micrograms per ounce [source: EUROCBC]. Just one 4-ounce serving of tilefish per week is nearing the EPA's top dose for a 150-pound person.
So we know we need to be careful about how much fish we eat. Children and pregnant women need to be particularly vigilant. The question is, are we actually bringing even more mercury into our homes in our effort to be a "greener" society?