Mercury Contamination: A Human Tragedy

This is the first of a series of reports on the evaluation of health hazards in the environment by an international group of experts under the sponsorship of the World Health Organization and the United Nations Environment Programme, set up in 1973. The scope of the environmental health criteria reports is to compile, review and evaluate information on the biological effects of pollutants influencing human health, and to provide a scientific basis for decisions aimed at protection from the adverse consequences of exposure, both in the occupational and general environment. The more precise objectives are to assess information on the relationship between exposure and health, in order to provide guidelines for setting exposure limits; to identify potential pollutants by preparing preliminary reviews on health effects; to identify gaps in knowledge and thus to promote research, and finally and most important, to promote the harmonisation oftoxicological and epidemiological methods for the purpose of obtaining research results that are internationally comparable. Each criteria document, as it is styled, contains chapters on chemical and physical properties, analytical methods, sources of pollution, environmental transfer, distribution and transformation, metabolism, and experimental, epidemiological and clinical studies of effects. The evaluation which follows gives the opinion of the task group on relative contributions to the total dose from air, food, water and other exposures, doseeffect and dose-response relationships and guidelines on exposure. In addition the report gives a summary, followed by recommendations for future research. It is appropriate that the first of these criteria documents should be on mercury. A decade ago the epidemics of poisoning following the discharge of mercury in industrial effluents at Minamata and Niigata shook the world into a realisation that similar pollution was occurring in many other industrial centres. At that time we knew little about the complex behaviour of mercury in the ecosystem or of the metabolism of highly toxic methyl mercury in man. The study of predominantly fish-eating populations in Japan, Sweden and elsewhere, together with the observations made on the rural population of Iraq who had consumed bread made from wheat seed treated with methyl mercury, has given a fairly good but still incomplete picture of the longterm daily intake of methyl mercury in the diet associated with the earliest effects seen in an adult population. The levels quoted apply only to adults and relate to the probability of an effect in up to 5% of the population. Assumptions, approximations and some fairly sophisticated arithmetic have been used in arriving at estimates of total dose and of total body burden. However, precise assessment of neurological deficit is difficult enough in specialised centres, let alone in field studies. While the assessment of paraesthesiae must, for the present, remain subjective, the report gives no indication of criteria used in the assessment of the presence of ataxia, dysarthria or deafness. The crude nature of the clinical input data has not been stressed and this must be kept in mind when interpreting the evaluation made by the task group. The toxic properties of elemental mercury differ from those of its inorganic salts, and from the short chain alkyl mercurials which in turn differ from all other organic mercury compounds. The account of the clinical effects of these different categories is incomplete. In particular, renal disorder following immune complex deposition on the basement membrane of the kidney, and in general the role of hypersensitivity in influencing the toxic reactions to mercury, have not been considered. Occupational exposure to mercury occurs mainly to elemental mercury vapour. Although large numbers of workers have been exposed to mercury vapour for many years we still do not have precise measurements on exposure 266 levels, their relationship to concentrations in blood and urine and to tota. body burden. Information is also lackinE on the possible effects of exposure tc mercury vapour during pregnancy and these gaps in our knowledge are indicatec in the report. G. KAZANTZIq