CHAPTER 5. Mercury

Abstract

Mercury is a metal that exists in liquid form and easily evaporates in air at room temperature. Mercury creates various compounds which are classified into inorganic mercury and organic mercury compounds. Different mercury compounds have different toxicological profiles. Timing of exposure in the life stage causes different health effects.In classical poisoning by mercury vapor inhalation, the patients develop tremor, behavioral and personality changes, loss of memory and insomnia. Acute poisoning after ingestion of mercuric salts is characterized by dysfunction of the intestinal tract and renal failure. In the case of methylmercury poisoning the central nervous system is the critical organ. There are two types of poisoning depending upon timing of exposure; fetal exposure and adult exposure. In the typical poisoning cases by adult exposure, ataxia, concentric constriction of the visual field, impairment of hearing, and extrapyramidal tract symptoms appear. Vulnerability of the fetus to methylmercury has been shown in the disaster of Minamata Disease. Fetal Minamata Disease patients showed mental retardation, cerebellar ataxia, primitive reflex, dysarthria, seizure and pyramidal signs. These symptoms are long-lasting.As for the risk of fetal methylmercury exposure, WHO claimed in 1990 that “ a 5% risk may be associated with the peak mercury level of 10-20 micro g/g of maternal hair.” Therefore, effects of fetal methylmercury exposure under general environment have been investigated among various populations. These studies have shown small and subclinical developmental effects. Numerous studies on experimental animals demonstrated vulnerability of the fetus to methylmercury exposure and consequent postnatal effects.Past exposure to mercury vapor that ceased a couple of decades ago caused deteriorating psychomotor performances and neurological impairments among elderly ex-workers.Although the studies concerning effects of mercury vapor exposure during prenatal period have been extremely scarce in both human cases and animal experiments, recently postnatal behavioral effects have been reported in animal experiments. The results of these animal experiments showed that 1) small amount of mercury was transferred to the fetus by in utero mercury vapor exposure; 2) postnatal behavioral changes were observed with considerably low concentration of mercury in the offspring brain after mercury vapor exposure.In conclusion age specific toxicities of mercury vapor and methylmercury have been demonstrated by experimental and epidemiological studies. Vulnerability to mercury at fetal period should be elucidated further in terms of mechanism, and dose-effects and dose-response relationships. In addition investigations to examine the hypothesis that mercury accelerates aging are needed.