Aquatic Pollution and Coastal Zone Managememnt (2+1)

Unit 4 : Pesticide pollution

4.14.Heavy Metals
Mercury, cadmium, lead and tin are the heavy metals most frequently endangering human health; they have varying effects, but all enter the food chain through industrial waste and bioaccumulate in aquatic and marine environments. However the only heavy metal which seems likely to biomagnify is mercury.

Mercury
In freshwater, inorganic mercury is transformed by microorganisms into highly toxic methylated mercury. This compound rapidly accumulates in plants, invertebrates and fish and then biomagnifies along the food chain. Mercury has always been known to be harmful, but the full extent of this danger was brought home by the epidemic of poisoning at Minamata Bay in Japan in the 1950s. From 1952, the only local industry discharged mercury-containing effluent directly into the bay. One of the compounds concerned - methyl mercury - acts as a cumulative poison causing delirium, loss of motor control and irreversible brain damage.

The first case of "Minamata disease" - as it came to be known - appeared among fishermen in 1953; a link was made with consumption of seafood by 1956. A partial ban of fishing was in place by the next year, but discharge of mercury into the bay continued for another 11 years. Eventually 107 people died of Minamata disease and at least another 700 were left permanently disabled.

The World Health Organisation (WHO) recommends that the total consumption of methyl mercury should not exceed 0.2mg per week. Many countries have now introduced legal limits for mercury in saleable food (although sometimes these have been increased because too many fish were being excluded.)