Mercury is a bioaccumulative, persistent and toxic heavy metal that threatens human and ecosystem health.

Mercury is present in the natural environment in small quantities, however, since the industrial age, humans have elevated the levels of mercury in the environment to several times their natural levels. Mercury is released when coal is burned to produce electricity, through the manufacture, breakage, or disposal of mercury-containing products (such as mercury switches, thermometers, thermostats and compact fluorescent bulbs) and through industrial processes such as mining and chemical manufacturing.

Mercury has been identified by the International Joint Commission (IJC) as one of 11 Critical Pollutants in the Great Lakes. The IJC put mercury on its Priorities for Consideration for 2003 to 2005.

The Government of Canada has put mercury on its Toxic Substances List (Schedule 1 of the Canadian Environmental Protection Act, 1999

This older thermostat contains four mercury switches. See the silvery liquid in the bulbs? Each of these switches contains about 2.5 grams of mercury.