Rain washes mercury out of the air and returns it to rivers, lakes, oceans, and the soil. This cycle of vaporization and washing-out has probably taken place since the earth formed. Because plants and animals evolved in the earth's environment, all contain trace quantities of mercury. Mercury and the Environment Plants absorb mercury from the soil and air during normal growth. In some cases, plants concentrate mercury to small metal droplets.
Some bacteria convert inorganic mercury to organic mercury compounds.
Fish and animals may consume mercury containing bacteria. Some animals and vegetables convert organic mercury back to inorganic compounds. This constant cycling of mercury from one form to another has gone on for eons without any recognizable toxic effect on the world's food supply. Our use of mercury has probably not significantly increased the mercury concentration in the oceans. Isolated events tragically demonstrated the potential dangers.