For most people, the main source of mercury exposure is fish consumption. Methylmercury (or organic mercury) is readily absorbed by the human digestive system. It is then carried through the blood to all organs in the body. The highest concentrations are found in the liver, kidneys and brain. It takes from 50 to 70 days to eliminate half the methylmercury ingested. During pregnancy, methylmercury present in the mother's blood passes through the placenta and into the bloodstream of the unborn child.
Exposure revealed by the hair
Methylmercury is measured in the hair and blood. Hair analysis is an excellent way to determine a person's exposure to mercury throughout an entire year or a fishing season. Mercury is deposited at the roots of the hair, where it becomes permanently fixed. Because hair grows at a rate of about one centimetre a month, the mercury concentration measured for one centimetre of hair reflects the average concentration over that month (see poster Mercury in the blood and hair [PDF]). Therefore, a long strand of hair can be analyzed to obtain a record of a person's mercury exposure over several months or years.
As with any chemical, the health risk of mercury depends on its concentration in the organism. All people have a little mercury in their bodies, but the concentrations are generally very low and consequently do not pose any health risk. At higher doses, however, the nervous system, in particular, may be affected by methylmercury.
Most of the studies conducted to date have not reported any health effects associated with methylmercury in individuals who have been exposed over a long period of time and have hair mercury concentrations of less than 14 ppm. Developmental effects could be observed in children born to mothers who had more than 14 ppm of mercury in their hair during pregnancy. However, at levels close to this value, the impact on the child's health appears marginal. In adults, 50 ppm in the hair is the threshold at which symptoms could begin to appear in the most sensitive individuals. The earliest symptom commonly reported is a sensation of numbness in the fingers and toes.
|Hair mercury concentration (ppm)||Health effects observed|
|< 14||No significant effect on the child|
|15 to 50||No clinical effects
(subclinical effects not confirmed)
|50 to 200||Threshold for appearance of the earliest symptoms in adults (paresthesia)|
|200 to 1,000||Increase in frequency of neurological effects|
|> 1,000||Serious neurological effects leading to death|
Safe exposure in Eastern Canada
In Québec, even the highest fish mercury concentrations are not enough to cause symptoms of mercury intoxication, considering the usual low fish consumption. In the vast majority of cases, mercury concentrations measured in Québec fish consumers are well below levels that lead to health effects in adults or unborn children.
The latest data for Eastern Canada, including those collected in 2002 and 2003 by the Collaborative Mercury Research Network (COMERN) among anglers in Lac Saint-Pierre and the Abitibi region as well as among the Labrador Innu, shows that the average exposure is generally around 1 ppm in the hair (see the following table). This value is well below the threshold at which the earliest mercury-related effects would begin to appear in unborn children; according to the available studies, such effects would occur at 10 to 15 ppm in the mother’s hair.
|Population||Number of participants||Hair mercury concentration|
|Innus of Ekuanitshit||36||0.5||2.0|
|Anglers||Number of participants||Hair mercury concentration|
* Two studies are available for Montréal
Recommendations of public health authorities
Public health authorities are responsible for ensuring that fish consumers do not exceed the mercury exposure level deemed safe for their health. On the basis of the findings of recent epidemiological studies, they recommend a specific maximum number of fish meals per month according to fish mercury levels and catch locations.
A safe process
The following table illustrates the process used by public health authorities to establish their fish consumption recommendations, taking those of the World Health Organization (WHO) as an example.
|Step||Value adopted by the WHO|
|Determination of exposure threshold
|14 ppm in the hair|
|Calculation of daily intake
|1.5 µg Hg/kg/d|
|Application of a safety factor
|6.4 (from 4.5 to 10, depending on the institution)|
|Calculation of tolerable daily intake (TDI)
|0.23 µg Hg/kg/d|
|Number of meals per month||Depending on mercury level (Hg), body weight and portion consumed|
Public health authorities determine an exposure threshold at which the earliest symptoms of mercury intoxication would begin to appear, based on their analysis of recent epidemiological studies. In the example chosen, this threshold is 14 ppm, or micrograms (µg) of mercury (Hg) per gram (g) of hair, in the mother's hair.
Next, the daily intake of mercury is calculated; this is the amount the mother must assimilate on a regular basis to reach the exposure threshold. To reach 14 ppm, the daily intake is 1.5 µg of mercury per kilogram of body weight.
A safety factor is then applied to be absolutely certain to avoid any health risk. The WHO applied a safety factor of 6.4, which takes into account individual variabilities in the rate of mercury assimilation and in the efficiency of its transfer from the blood to the hair.
The tolerable daily intake (TDI) for mercury, that is, the daily intake that should not be exceeded, is then obtained by dividing the daily intake by the safety factor chosen. To protect the unborn child, the WHO recommends that the mother not exceed a daily intake of 0.23 µg of mercury per kilogram of body weight.
The number of fish meals per month that the mother can safely eat is based on the following criteria:
- a tolerable daily intake of 0.23 µg/Hg/kg of body weight
- a body weight of 60 kg
- a portion of 230 g of fish (uncooked)
For the majority of mothers, abiding by this consumption frequency will lead to a very safe exposure level of about 2 ppm in the hair, which is about 6.4 times lower than the 14-ppm exposure threshold at which the earliest symptoms would begin to appear.
The 14-ppm exposure threshold protects unborn children and is intended in particular for women who are pregnant or nursing, or planning to become pregnant, as well as for children, whose nervous systems are still developing. For other adults, the WHO considers that intakes of up to twice the TDI, or as much as 0.46 µg of mercury per kilogram of body weight, would not pose any risk of neurological effects.
Although its interpretation of the findings of the epidemiological studies differs slightly from the WHO's, Health Canada obtains essentially the same tolerable daily intakes for mercury by applying the same process:
- exposure threshold of 50 ppm, at which the earliest symptoms would begin to appear
- daily intake corresponding to 3 to 7 µg of mercury per kilogram of body weight
- safety factor of 10
- tolerable daily intake of 0.47 µg of mercury per kilogram of body weight
To protect the unborn child
- exposure threshold of 10 ppm in the mother's hair, at which the earliest symptoms would begin to appear in the unborn child
- daily intake of 1 µg of mercury per kilogram of body weight
- safety factor of 5
- tolerable daily intake of 0.20 µg of mercury per kilogram of body weight
Health Canada therefore recommends a tolerable daily intake of 0.20 µg/Hg/kg of body weight for women of childbearing age and for children, and 0.47 µg/Hg/kg of body weight for other adults.
Fish on the menu
Despite the presence of small quantities of methylmercury, fish is excellent for health. Among other things, it contains high-quality fatty acids, called omega-3, that are not found in red meat. These fats have a beneficial effect on the development of the unborn child and help prevent cardiovascular disease.
All it takes to avoid any mercury-related risk is to follow the consumption recommendations issued according to fish mercury levels and the tolerable daily intakes determined by Health Canada and the WHO.
A team effort
Because of the potential health risk which the marked but temporary increase in fish mercury levels caused by hydroelectric developments poses to fish consumers, Hydro-Québec works together with public health authorities in the regions where its projects are built.
While responsibility for monitoring mercury levels lies with Hydro-Québec, managing the health of recreational anglers and subsistence fishermen is the responsibility of Québec's regional health and social services agencies. As part of this team effort, Hydro-Québec supplies the data on fish mercury levels. The company is also involved in producing consumption guides based on the tolerable daily intakes recommended by the World Health Organization and Health Canada.
|Mercury level in fish
in ppm* (mg/kg)
|Maximum number of meals recommended per month|
|0.00 to 0.29||No restriction|
|0.30 to 0.49||8 meals per month|
|0.50 to 0.99||4 meals per month|
|1.00 to 1.99||2 meals per month|
|2.00 to 3.75||1 meal per month|
*parts per million
500 mm (20 in.)
300 mm (12 in.)
500 mm (20 in.)
800 mm (32 in.)
600 mm (24 in.)
The guides indicate the number of meals per month recommended for the different species of fish, based on fishing location. The recommendations are color-coded. For example, a green circle indicates that the mercury levels for that fish are low and it can be consumed without restriction. The red circle indicates that the fish mercury levels are high and a consumption frequency of no more than once per month is recommended.
These guides use maps to show, for each species of fish and each part of the region, the maximum number of meals per month that allows fish consumers to keep their mercury exposure below the values recommended by public health authorities. By following these recommendations, consumers can benefit from the great nutritional value of fish.
Specific recommendations apply to women who are pregnant or plan to become pregnant soon, so that their babies can also benefit from the nutrients in fish that promote brain development, while avoiding any mercury-related risk.
Guides adapted to local communities
The people of Gros-Mécatina, in Basse-Côte-Nord, took part in drawing up the consumption guide for their region by specifying what information they wished to have and how it should be presented. They did not want recipes describing healthy ways to prepare fish, but preferred to have information on the changes in fish populations in Robertson reservoir.
In the La Grande complex, fish mercury levels in the majority of modified areas have returned to levels representative of natural environments in the region. The time is therefore right to promote the health benefits provided by fish consumption.
The most recent communication tools
Two new food guides that present the nutritional value and health benefits of fish were published in 2013: the Northern Fish Nutrition Guide – Baie-James Region (in English and French) and the Guide alimentaire des poissons et fruits de mer de la Côte-Nord (Côte-Nord region fish and seafood guide, in French only). The guides were produced by the Environment department of the Direction – Gestion des actifs et conformité réglementaire, Hydro-Québec Production's regional management units La Grande Rivière and Manicouagan and Hydro-Québec's Direction – Santé et sécurité in close collaboration with the following public health agencies: the Institut national de santé publique du Québec (INSPQ), the CHU de Québec Research Center, the Cree Board of Health and Social Services of James Bay, the Centre régional de santé et de services sociaux de la Baie-James and the North Shore Health and Social Services Agency.
These guides emphasize the nutritional value and health benefits of fish and seafood in an effort to promote their consumption and help allay consumers' concerns about mercury. The guides also contain a wealth of information likely to interest fishing buffs, such as the favorite habitats of the main fish species, their most active periods, the most effective lures, the catch record, what they each taste like and, of course, many recipes to better savor the catch. They are distributed to all Cree households in the Baie-James region, the Innu and Montagnais councils in the Côte-Nord region, as well as to regional departments, municipalities, outfitters and fishing associations.
These guides can be consulted online or ordered by calling: 1 800 363-7443.