Heavy metals are generally defined as environmentally stable elements of high specific gravity and atomic weight. They have such characteristics as luster, ductility, malleability, and high electric and thermal conductivity. Whether based on their physical or chemical properties, the distinction between heavy metals and non-metals is not sharp. For example, arsenic , germanium, selenium, tellurium, and antimony possess chemical properties of both metals and non-metals. Defined as metalloids, they are often loosely classified as heavy metals. The category "heavy metal" is, therefore, somewhat arbitrary and highly non-specific because it can refer to approximately 80 of the 103 elements in the periodic table. The term "trace element" is commonly used to describe substances which cannot be precisely defined but most frequently occur in the environment in concentrations of a few parts per million (ppm) or less. Only a relatively small number of heavy metals such as cadmium , copper , iron, cobalt, zinc, mercury , vanadium, lead , nickel , chromium, manganese, molybdenum, silver, and tin as well as the metalloids arsenic and selenium are associated with environmental, plant, animal, or human health problems.
While the chemical forms of heavy metals can be changed, they are not subject to chemical/biological destruction. Therefore, after release into the environment they are persistent contaminants. Natural processes such as bedrock and soil weathering , wind and water erosion , volcanic activity, sea salt spray, and forest fires release heavy metals into the environment. While the origins of anthropogenic releases of heavy metals are lost in antiquity, they probably began as our prehistoric ancestors learned to recover metals such as gold, silver, copper, and tin from their ores and to produce bronze. The modern age of heavy metal pollution has its beginning with the Industrial Revolution. The rapid development of industry, intensive agriculture, transportation , and ur