Mercury and fish consumption

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 1.2 Mb]). 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.

Correspondence between hair mercury concentrations and health effects observed

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

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.

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.

Process for determining fish consumption recommendations

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.

Health Canada

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:

For adults

• 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.

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.

Recommendations regarding the number of fish mealsThis table show color-coded recommendations regarding the number of fish meals to be consumed per month based on mercury levels.

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

Natural lakes

		Lake whitefish 500 mm (20 in.)
		Speckled trout 300 mm (12 in.)
		Landlocked salmon 500 mm (20 in.)
		Pike 800 mm (32 in.)
		Lake trout 600 mm (24 in.)