Mercury is a naturally occurring element that's found in soil and rocks as well as bodies of water. We absorb trace amounts of it from dental fillings, air and water pollution and from the food we eat. Fish tend to be our largest source of mercury.

High levels of mercury can damage our nervous systems and can inhibit brain development in young children. It's not clear what the long-term effects of extremely low levels of mercury are.

Mercury binds tightly to the proteins in fish tissue. Most fish will contain trace amounts of mercury, depending on the level of mercury in their environment and their place in the food chain.

The bigger the fish and the higher up the food chain it is, the more mercury it will tend to contain. Large predatory fish species tend to have higher levels than non-predatory fish or species farther down the food chain.

The health benefits of eating tuna have been widely established; it is a relatively inexpensive source of high-quality protein, low in saturated fat and contains omega-3 fatty acids, touted for their heart-protective benefits.

But tuna also contains mercury, a dangerous contaminant that can affect the heart, brain and immune system.

Health Canada has established a guideline level of 0.5 parts per million (ppm) for mercury in commercial fish.

The Canadian Food Inspection Agency tests canned tuna before it gets to store shelves to ensure it meets the 0.5 ppm guideline.

The CFIA actually allows mercury levels up to 0.54, due to its system of rounding to one decimal point, so only eight per cent of the tuna tested should not have been for sale.

Health Canada suggests that "as a precaution":

    Pregnant or breastfeeding women can eat up to four servings of canned albacore tuna per week.
    Children between the ages of one and four years can eat up to one serving per week.
    Children between the ages of five and 11 years can eat up to two servings per week.
One serving of tuna is 75 gm, 2½ oz, 125 mL, or ½ cup.

Other countries, such as the United States, have issued advisories to consumers about the potential dangers of canned tuna — including special advice to pregnant and nursing women, because too much mercury can cause neurological damage, especially in children and fetuses.

Safe consumption advice ranges from no more than one can of albacore tuna a week in some jurisdictions to none at all in others.