Mercury is a naturally occurring toxic metal that is widely distributed in the environment. It exists in several forms, mainly elemental mercury, inorganic mercury compounds, and organic mercury compounds such as methylmercury. Because mercury can persist in the environment, circulate through air, water and soil, and accumulate in living organisms, exposure represents an important example of the close inter-relationship between environmental conditions and human health. The health effects depend on the chemical form of mercury, dose, duration and route of exposure, and the age and health status of the exposed person. Human exposure to mercury occurs through several environmental and occupational pathways. Elemental mercury can enter the body mainly through inhalation of mercury vapour. Inorganic mercury compounds may be absorbed through ingestion or skin contact, while methylmercury is primarily acquired by eating contaminated fish and other aquatic foods. Mercury released into the atmosphere from natural processes and human activities can eventually enter soil and water. Microorganisms in aquatic ecosystems can convert inorganic mercury into methylmercury, which is then absorbed by small aquatic organisms and progressively concentrated as it moves up the food chain. Large predatory fish therefore often contain higher concentrations of methylmercury. This demonstrates how environmental pollution can be transferred through food systems and ultimately affect human health. The nervous system is one of the principal targets of mercury toxicity. Exposure to significant amounts of mercury can produce tremors, irritability, anxiety, mood changes, memory problems and difficulties with concentration. More severe exposure may cause impaired coordination, weakness, sensory disturbances, speech problems and impairment of vision and hearing. Chronic exposure can produce persistent neurological damage. Methylmercury is particularly dangerous because it readily crosses the blood-brain barrier and can interfere with normal development and functioning of the nervous system. Mercury is especially harmful to the developing nervous system of the fetus and young children. Pregnant women exposed to methylmercury may transfer it across the placenta to the developing fetus. Prenatal exposure can interfere with brain development and may result in impaired cognitive function, attention, language, memory, coordination and motor development. Children are also vulnerable because their nervous systems are still developing. Even when the mother does not show obvious symptoms, fetal exposure can have long-term developmental