Cytogenetic damage related to low levels of methyl mercury contamination in the Brazilian Amazon

The mercury rejected in the water system, from mining operations and lixivi ation of soils after deforestation, is considered to be the main contributo rs to the contamination of the ecosystem in the Amazon Basin. The objective s of the present study were to examine cytogenetic functions in peripheral lymphocytes within a population living on the banks of the Tapajos River wi th respect to methylmercury (MeHg) contamination, using hair mercury as a b iological indicator of exposure. Our investigation shows a clear relation b etween methylmercury contamination and cytogenetic damage in lymphocytes at levels well below 50 micrograms/gram, the level at which initial clinical signs and symptoms of mercury poisoning occur. The first apparent biologica l effect with increasing MeHg hair level was the impairment of lymphocyte p roliferation measured as mitotic index (MI). The relation between mercury c oncentration in hair and MI suggests that this parameter, an indicator of c hanges in lymphocytes and their ability to respond to culture conditions, m ay be an early marker of cytotoxicity and genotoxicity in humans and should be taken into account in the preliminary evaluation of the risks to popula tions exposed it? vivo. This is the first report showing clear cytotoxic ef fects of long-term exposure to MeHg. Although the results strongly suggest that, under the conditions examined here, MeHg is both a spindle poison and a clastogen, the biological significance of these observations are as yet unknown. A long-term follow-up of these subjects should be undertaken.