The three modern "faces" of mercury are our perceptions of risk from exposure to methyl mercury in fish, mercury vapor from amalgam fillings, and ethyl mercury in thimerosal, a preservative in vaccines. This review discusses the toxicology and human exposure to these three mercury species. Methyl mercury, found in fish, was first linked to public health concerns in 1969 when high levels were found in fish from Lake St. Clair. The U.S. EPA reduced safe intake levels in 1997. Methyl mercury is absorbed in the gastrointestinal tract and accumulates in the brain and scalp hair. It is metabolized to inorganic mercury in the intestines and excreted in feces. Adverse effects include neurological damage, particularly in the developing brain, and potential cardiovascular effects. Ethyl mercury, in thimerosal, was used in vaccines but was removed from U.S. vaccines in 1999. Ethyl mercury is rapidly excreted in urine and feces, and its disposition in the body is similar to methyl mercury. However, it is less toxic than methyl mercury, with less severe neurological effects. Ethyl mercury is excreted more quickly than methyl mercury, and its conversion to inorganic mercury is faster. Mercury vapor from dental amalgam fillings, used since the 19th century, has been a subject of debate. Mercury vapor is released during chewing and can be inhaled. While concentrations in the oral cavity may exceed occupational health standards, the quantity is small due to the small volume of the cavity. The debate over the health risks of mercury from amalgam remains ongoing. The global cycling of mercury involves the conversion of inorganic mercury to methyl mercury by microorganisms in aquatic environments, leading to bioaccumulation in the food chain. Mercury is found in fish and can be ingested by humans, leading to neurological effects. The disposition of mercury in the body varies by species, with methyl mercury accumulating in the brain and scalp hair, while ethyl mercury is excreted more quickly. The toxic effects of methyl mercury are primarily on the central nervous system, with adverse effects on the developing brain being more severe. Ethyl mercury, while less toxic, can still cause neurological effects. The mechanisms of action for both mercury species are not fully understood, and there are gaps in knowledge regarding their toxicology and human exposure. The review highlights the need for further research to understand the health risks associated with these forms of mercury.The three modern "faces" of mercury are our perceptions of risk from exposure to methyl mercury in fish, mercury vapor from amalgam fillings, and ethyl mercury in thimerosal, a preservative in vaccines. This review discusses the toxicology and human exposure to these three mercury species. Methyl mercury, found in fish, was first linked to public health concerns in 1969 when high levels were found in fish from Lake St. Clair. The U.S. EPA reduced safe intake levels in 1997. Methyl mercury is absorbed in the gastrointestinal tract and accumulates in the brain and scalp hair. It is metabolized to inorganic mercury in the intestines and excreted in feces. Adverse effects include neurological damage, particularly in the developing brain, and potential cardiovascular effects. Ethyl mercury, in thimerosal, was used in vaccines but was removed from U.S. vaccines in 1999. Ethyl mercury is rapidly excreted in urine and feces, and its disposition in the body is similar to methyl mercury. However, it is less toxic than methyl mercury, with less severe neurological effects. Ethyl mercury is excreted more quickly than methyl mercury, and its conversion to inorganic mercury is faster. Mercury vapor from dental amalgam fillings, used since the 19th century, has been a subject of debate. Mercury vapor is released during chewing and can be inhaled. While concentrations in the oral cavity may exceed occupational health standards, the quantity is small due to the small volume of the cavity. The debate over the health risks of mercury from amalgam remains ongoing. The global cycling of mercury involves the conversion of inorganic mercury to methyl mercury by microorganisms in aquatic environments, leading to bioaccumulation in the food chain. Mercury is found in fish and can be ingested by humans, leading to neurological effects. The disposition of mercury in the body varies by species, with methyl mercury accumulating in the brain and scalp hair, while ethyl mercury is excreted more quickly. The toxic effects of methyl mercury are primarily on the central nervous system, with adverse effects on the developing brain being more severe. Ethyl mercury, while less toxic, can still cause neurological effects. The mechanisms of action for both mercury species are not fully understood, and there are gaps in knowledge regarding their toxicology and human exposure. The review highlights the need for further research to understand the health risks associated with these forms of mercury.