Tuna: toxic or misunderstood?
By Danielle Nerman

OTTAWA — When Missy Hecker found out that eating too much tuna could be potentially harmful to one’s health, she reacted immediately. “I took him right off the tuna,” says Hecker, a Toronto homemaker.

Hecker worried that her son's consumption of three to four cans of tuna per week was starting to affect his behavior.

Hecker is referring to her seven-year-old son Max, who for the past two years, had been eating canned tuna three to four times a week.

Hecker had read somewhere that tuna contains mercury. Over-exposure to this toxic metal can damage a child’s developing nervous system and cause irreversible intellectual impairment.

And since Max had already been diagnosed with Attention Deficit Hyperactivity Disorder (ADHD), Hecker was suspicious that her son’s irrational behaviour was being caused by something else.

“It was like a little light went on,” says Hecker. “I thought, ‘maybe…just maybe all the tuna he’s been eating is causing him to act this way,’” she adds.

Curious to find out, Hecker took Max to get his blood tested for mercury exposure.

It took five weeks for Hecker to get the results. During the wait, both Hecker and her husband noticed a difference in their son’s behaviour.

“Before, he was very hyper…his mind never stopped. Never, ever, ever,” says Hecker. “But once I stopped feeding him the tuna, he just…started acting more normal.”

When Hecker finally got Max’s tests back, they showed he had elevated levels of mercury in his blood. And though the levels weren't toxic, they were four times above normal.

Goodbye sushi?

So, does this mean everyone has to give up sushi and tuna sandwiches?

Each year, Canadian sushi restaurants serve millions of pounds of fresh bluefin and bigeye tuna to their customers.

Yes…and no.

Carole Saindon, a media relations officer at Health Canada, says it is important that people get their facts straight before they start cutting every indulgence out of their diets.

“Health Canada has set guidelines which advise Canadians to limit – not stop – their consumption of fresh and frozen tuna to one meal per week,” says Saindon.“And because mercury can strongly affect fetal and child development, we recommend that pregnant women and children eat it no more than once a month.”

Saindon says people also need to realize that these guidelines are ultra-conservative.

“Recommendations made by Health Canada are always way below what is considered ‘dangerous,’ ” says Saindon. “Otherwise we would ban it altogether. We would never take the risk of allowing tuna to be sold in Canada if we thought it could hurt someone.”

Furthermore, advice given by Health Canada is based on the potential exposure to mercury that can occur from eating tuna over a lifetime.

“The amount of mercury in each serving of tuna is so tiny, that you would have to eat it five times a week for your entire life to feel the effects,” says Saindon.

Thee average levels of mercury in fresh and frozen tuna are typically between 0.5 and 1.5 parts per million (ppm).

Saindon says this measurement is incredibly small.

“One part per million is equivalent to one minute in two years, one bad apple in 2,000 barrels of apples, and one inch in sixteen miles,” says Saindon. “So really, you would have to eat an awful lot of tuna, for a very long period time to get poisoned.”

So, perhaps that is the reason why Max – a child who had been eating tuna four times a week for two years – was more susceptible to the effects of mercury than other members of the population.

Mercury in nature

However, tuna isn't the only way in which humans are exposed to mercury. Traces of the toxic metal can be found on almost every inch of our earth.

Mercury is an element that exists naturally in the earth’s crust. For billions of years, rocks, soil and volcanic eruptions have been releasing the metal into the environment. It exists in three forms: elemental, inorganic and organic.

Elemental, or pure mercury, is the silvery liquid found in thermometers.

Inorganic mercury is formed when mercury combines with non-carbon substances. In the 1800’s, an inorganic type of mercury, called mercury nitrate, was used in the felt hat industry. Constant exposure to the chemical eventually poisoned the hatters and caused them to act peculiar. This is where the term, “mad as a hatter” came from.

Human activity only releases elemental and inorganic mercury into the environment. Typically, organic mercury is the by-product of elemental and inorganic mercury entering and reacting with the environment.

Luke Tripp is an environmental consultant for Specialists in Energy, Nuclear and Environmental Sciences (SENES) in Ottawa. He says that up until the Industrial Revolution, Mother Nature released the primary source of mercury into the earth’s oceans and atmosphere.

But in the last hundred years, human activity has greatly increased the amount of mercury in the global environment. Tripp says every year, coal-fired power plants, commercial mining and hospital incinerators worldwide release thousands of tonnes of mercury into the atmosphere.

“Many scientists worry that we may actually be reaching critical levels,” says Tripp. “But, because there's no population on earth that hasn't been affected by mercury pollution, we have nothing to measure against.”

Moving through the food chain

To make things even more difficult, mercury changes from a liquid to a vapour at room temperature. This enables large amounts of it to bypass industry smokestack filters.

Mercury vapours can stay airborne anywhere from 6 to 18 months.

“This gives it a lot of time to circle the globe,” says Tripp. “It’s possible that mercury emitted from a coal plant in Russia will end up polluting a lake or river in Canada.”

Whether or not it was produced locally or came from another continent, all atmospheric mercury eventually falls back to earth. Rainfall deposits the mercury into rivers, lakes and oceans.

Fresh tuna has higher levels of mercury than canned because it's caught at a much bigger size. This means it's had more time to accumulate mercury through the food chain.

There, bacteria transform elemental and inorganic mercury into methyl mercury – an extremely toxic blend of organic mercury.

Through the process of bioaccumulation, methyl mercury rapidly builds up in the food chain.

Bioaccumulation begins at the bottom of the food chain. Plants ingest methyl mercury as they feed, and pass it on to the small fish that eat them.

However, because these small fish generally don’t eat just one plant, every time they feed they end up ingesting three to four times the amount of mercury that’s in each plant. This process repeats itself up the food chain; causing mercury levels to get higher as the fish and animals get bigger.

Fresh vs. canned

"Tuna are big and predatory fish. They can eat at least a hundred fish a week,” says Tripp. “So, as you can imagine, one big tuna can accumulate a lot of mercury.”

However, Tripp is talking about the Bluefin and Bigeye species of tuna, typically seen in supermarkets and sushi bars. These fresh and frozen species are not used in the canned varieties.

Canned tuna is made up of younger and smaller species, such as Yellowfin, Skipjack and Albacore.

It is for this reason that Health Canada’s consumption guideline of one meal per month for pregnant women and children and one meal per week for the rest of the population does not apply to canned tuna.

“Because the tuna used in canned is caught so much smaller and younger than fresh and frozen species, it has less time to accumulate large amounts of mercury,” says Glenn McGregor, manager of Product Inspection for the Fish, Seafood and Production Division of the Canadian Food Inspection Agency (CFIA).

Health Canada recommends that Canadians limit their consumption of fresh and frozen tuna to one meal per week.

“So most canned tuna contains levels of mercury that are well below the 0.5 ppm guideline set by Health Canada for commercial fish,” he adds.

Fresh and frozen tuna typically contain levels of mercury that exceed Health Canada’s guideline level of 0.5 ppm. Applying this guideline to fresh and frozen tuna would virtually eliminate the species from the Canadian diet.

As a result, Health Canada does not enforce the 0.5 ppm guideline level on fresh and frozen tuna. Instead, they advise Canadians to limit their consumption of the species to avoid unnecessary exposure to hazardous levels or mercury.

However, since canned tuna does not fall under the fresh and frozen consumption guideline, it continues to be enforced under the 0.5 ppm guideline level for mercury.

“Every year, CFIA does about 60 to 80 tests on shipments of imported canned tuna,” says McGregor.

“We randomly pick one in every twenty shipments and from each shipment, we test 5 to 13 cans,” he adds.

If a can exceeds the guideline levels of 0.5 ppm, CFIA will start monitoring the packer’s shipments. If the problem continues, the packer is put under import alert. This means anything the packer brings into Canada will be held in detention until it can be tested by one of CFIA’s inspectors.

“The packer’s shipments of canned tuna must pass four inspections before their name can come off import alert,” says McGregor.

McGregor says overall, Health Canada and the CFIA are very careful in ensuring that Canadians are protected from mercury exposure.

The rest is up to Canadians, who need to read between the lines and use their common sense before making the decision that eating tuna is a ‘fishy’ business.