Children and women of child-bearing age should be careful about how much fish they eat and what types because of the health risks from mercury contamination, according to an international report released Thursday.

Mercury is a dangerous contaminant that can affect the heart, brain and immune system.

The Madison Declaration on Mercury Pollution summarizes the latest scientific knowledge about:

• Health risks and toxic effects of methylmercury, the form most people are exposed to from eating marine fish.
• Effects on wildlife.
• Socioeconomic consequences of mercury pollution.
• Recovery of fisheries contaminated with mercury.

Methylmercury now poses a public health problem in most regions of the world, and high levels of mercury have been found in people worldwide at all levels of society, the paper concluded.

When a fetus is exposed to the pollutant, it can later affect that child's development — enough to justify the warning to be careful about what species of fish are eaten during pregnancy, the researchers said in Thursday's issue of the journal Ambio, published by the Royal Swedish Academy of Sciences.

"Exposure during pregnacy will affect children's development later on in life," said Donna Mergler, an environmental health professor at the Université du Québec à Montréal, who contributed to the report.

The exposure to mercury in the womb does not cause sickness as does exposure from major mercury disasters, but it does lead to a shift in children's intellectual capacities, said Mergler. Some children may have learning difficulties and fewer will be super bright. 

New evidence suggests that exposure to methylmercury may increase the risk of cardiovascular diseases, particularly in men, the report said.

Be selective with fish

The authors suggest that consumers look for fish with high levels of healthy omega-3 fatty acids and low levels of methylmercury.

Generally, larger predatory fish such as shark and albacore tuna tend to accumulate more of the pollutant than species further down the food chain, like perch.

Canada needs to develop a fish-eating culture in which people avoid predators higher in mercury and take a look at salmon, sole, whitefish and tilapia, Mergler told CBC Newsworld.

On average, three times more mercury is falling from the sky today than before the Industrial Revolution 200 years ago, the researchers said. The pollutant is released by burning coal and humans are also exposed to it in gold-mining products and consumer goods that contain mercury, such as batteries.

The concentration of methylmercury in fish could decline as pollution is curbed, but the rate of decline could vary depending on the ecosystem.

The researchers said they hope the documents will help spread the best science on the environmental problem to policymakers worldwide. It was based on the findings of the Eighth International Conference on Mercury as a Global Pollutant that was held last August in the U.S.