Bacteria make methylmercury, which accumulates and concentrates in the food chain and becomes available to people. Methylmercury is absorbed fully when ingested and crosses the placenta and blood brain barrier. Its biological half-life is 70 days. There have been 5 major instances of high-dose prenatal exposure to mercury which scientists have studied: grain contamination in Sweden, 1953; water pollution in Minimata, Japan, 1958; water pollution in Nugota, Japan, 1965; grain contamination in Iraq, 1971; and grain contamination in New Mexico, USA, 1972. The studies in Iraq demonstrated a dose-response relationship which indicated that foetal exposure in the range achieved by regularly eating fish might adversely affect brain development. Consequently, five studies of low-dose effects on the foetus were designed to further investigate exposure through contaminated fish consumption: Canada, 1983; New Zealand, 1984; Peru, 1995; Seychelles, 1995; and Faroe Islands, 1996.
A US bill was introduced in 2002 to phase out mercury thermometers except by prescription and coordinate a plan to manage surplus mercury. Nine states had already banned mercury thermometers.