Mercury is a heavy metal, chemical symbol Hg, that is liquid at room temperature and was formerly called quicksilver. In food safety it is a priority contaminant because inorganic mercury released by industry and natural sources is converted by aquatic microbes into methylmercury, which accumulates up the food chain and concentrates in long-lived predators. Large predatory fish such as tuna, swordfish, and shark carry the highest levels. Regulators including the Codex Alimentarius, the FDA, and the EU set maximum limits in fish and issue consumption advisories for pregnant women and young children, for whom neurodevelopmental effects are the main concern. Laboratories test fish tissue with cold-vapor atomic absorption or fluorescence methods to enforce these limits.