Mercury in Fish

EXPOSURE TO VARIOUS FORMS OF mercury can cause a number of mercury-related diseases. Mercury can exist in three different forms, namely mercury vapour (elemental), methyl or phenylmercuric (organic) and both mercurous and mercuric salts (inorganic). Exposure to any form of mercury either from a single, very high dose or on a repeated basis can lead to the disease of acute and chronic mercury poisoning.

Three main symptoms manifested by mercury poisoning include gum problems, mood and mental changes and disturbances in nervous system. The effects on the nervous system are normally observed as psychomotor impairments, especially tremor, and an impairment in nerve conduction velocity.

Other health effects often associated with poisoning by mercury and mercury-containing compounds are skin allergy, loss of appetite, discolouration of the eyes lens and severe irritation of the skin and eyes.

These signs and symptoms are also manifested in poisoning caused by mercury exposure from fish. Lately, there have been many debates about the presence of mercury in fish and dental amalgam fillings. A lot of controversial allegations have ensued from this issue.

There are several relevant questions concerning mercury being put forward by the public. Generally, these questions are about whether mercury originating from any source could reach toxic levels and adversely affect health.

There has been growing concern and heated arguments about potential adverse effects from products containing traces of mercury, especially those present in dental amalgam fillings and fish.

The World Health Organisation (WHO) has come out with some figures depicting the general sources of mercury and how much the body is exposed in micrograms per day (ug/day). These sources come mainly from air, 0.040 ug/day, fish, 2.34 ug/day, non-fish foods, 0.25 ug/day, drinking-water 0.0035 ug/day and mercury vapour from dental amalgams 3.17 ug/day.

Referring to the figure by the WHO, it seems to point strongly in the direction of dental amalgams as the main mercury source in non-occupational exposure of mercury.

As far as mercury in dental amalgams is concerned, reports on its toxic effects are still rather inconclusive. However, fish remains the biggest source of mercury in individuals and in sub-populations consuming fish which is highly mercury contaminated.

Mercury often occurs naturally in the environment. Mercury vapour in particular is easily disposed and transported in the atmosphere. It is deposited on land and water and then released again to the atmosphere. Trace amounts of soluble mercury in water can undergo chemical changes initiated by bacteria to become methylmercury. This is known to be more toxic than any other forms of mercury.

Aquatic organisms, particularly aquatic insects, accumulate mercury in high concentrations. Fish also take up the metal and retain it in tissues principally as methylmercury, although most of the environmental mercury they're exposed to is inorganic mercury. Knowing how the food chain works, larger predator fish or even man for that matter, will be exposed to high levels of methylmercury from their prey.

Methylmercury is an organic mercury which one mainly gets from eating fish. Mercury is bound to cell wall or cell membrane, depending on the concentration of mercury and binding sites available in the cells.

These effects are often irreversible. Mercury binds tightly to the protein in fish tissue. Cooking does not substantially reduce the methylmercury content of the fish. Nearly all fish contain trace amounts of methylmercury, some more than others.

Some food experts have established a level of one part per million (ppm) as a limit to provide an adequate safety margin for mercury content. This figure is 10 times lower than the lowest levels of mercury associated with adverse effects. At the same time, this does not mean that it is safe to regularly and frequently eat fish that contain one ppm methylmercury.

Fish still remains an important source of high-quality protein, minerals and vitamins. Given the increasing consumption of fish, a number of specialists from the FDA in the United States say that eating a variety of fish in the normal pattern of consumption does not put any one in danger of mercury poisoning. They have identified a number of fish which are known to accumulate higher concentrations of mercury.

Among the species identified are shark and swordfish. Pregnant women and women of childbearing age are advised to limit their consumption of such fish.

The limit for human consumption has been established at one ppm. This level was arrived at after taking into consideration the types of fish that people eat, the levels of mercury present in each species and the amount of fish that are normally consumed.

Is there any safe level for mercury?
Although no safe level of mercury has been determined for occupational exposure monitoring, urinary excretion of up to 200 microgram per liter (ug/l) and blood concentration of 50 microgram per deciliter (ug/dl) are acceptable. Early effects of mercury toxicity have been found when blood level exceeds 3 ug/dl. Mercury levels in contaminated freshwater and saltwater are generally low.

This level is estimated to be around 0.03 to 0.04 ug/l. The concentrations vary accordingly from relatively unpolluted areas to polluted areas. In some cases, it was estimated that the sediment levels in unpolluted areas range from 0.05 microgram per kilogram (ug/kg) to quite high concentrations near industrial sources of contamination.

Occupational exposure to mercury vapour in the air increases the incidence of certain less severe toxic effects. These usually present only in particularly sensitive individuals. As far as mercury level in children and foetuses is concerned, there are less information about which mercury levels one should regard as safe.

A general rule to minimise the exposure to any form of mercury poisoning is to stay away from sources of contamination.

However, this is not as easy as it may appear to be. Despite this, cessation of the exposure remains the single most important measure. Additional research is needed to address the question of whether the minimal exposure of mercury in fish could pose any significant health risk to people.

The writer is a pharmacist at the National Poison Centre, Universiti Sains Malaysia, Penang.