Mercury is a naturally occurring heavy metal that exists in three forms: elemental, inorganic and organic. The most common form of organic mercury is methylmercury. Use of mercury in consumer products has been limited. It has been phased out of most products including thermometers and gauges. It is still used in some medical devices and dental fillings, as well as in button-cell batteries (commonly used in small electronics and hearing aids) and, more recently, in compact fluorescent bulbs.

Mercury is found throughout the environment, including in remote Arctic regions, due to its persistence, mobility and tendency to accumulate in colder climates. Exposure of the general population is primarily to methylmercury and occurs through the consumption of fish and seafood. To a much lesser extent, the general population may be exposed to inorganic mercury through dental fillings.

Chronic exposure to high levels of methylmercury may cause numbness and tingling in the extremities, blurred vision, deafness, lack of muscle coordination and intellectual impairment, as well as adverse effects on the cardiovascular, gastrointestinal and reproductive systems. Prenatal exposure may interfere with development of the central nervous system and can cause neurological and developmental delays.

Health Canada has established a total mercury (that is, to all forms of mercury combined) blood guidance value of 20 micrograms per litre (µg/L) for the general adult population. A revised guidance value of 8 µg/L for children, pregnant women and women of childbearing age has recently been recommended.