Did You Know? Canned tuna is often the most common source of mercury in the diet. Mercury is a chemical often used in thermometers, light switches, as well as being put to use in industrial facilities, such as power plants, coal plants plants, and certain chemical manufacturers. When released into the environment, mercury can become a public health issue when it settles into our oceans and waterways. Large fish, such as tuna, can have mercury concentrations in their bodies that are 10,000 times higher than those of their surrounding habitat!!!