Heavy Metals in Science

What are heavy metals?

Lead is an example of a heavy metal, a dense metal capable of causing environmental damage.
Lead is an example of a heavy metal, a dense metal capable of causing environmental damage. David Madison / Getty Images

In science, a heavy metal is a metallic element which is toxic and has a high density, specific gravity or atomic weight. However, the term means something slightly different in common usage, referring to any metal capable of causing health problems or environmental damage.

Examples of Heavy Metals

Examples of heavy metals include lead, mercury and cadmium. Less commonly, any metal with a potential negative health effect or environmental impact may be termed a heavy metal, such as cobalt, chromium, lithium and even iron.

Dispute over "Heavy Metal" Term

According to the International Union of Pure and Applied Chemistry or IUPAC, the term "heavy metal" may be a "meaningless term" because there is no standardized definition for a heavy metal. Some light metals or metalloids are toxic, while some high-density metals are not. For example, cadmium generally is considered a heavy metal, with an atomic number of 48 and specific gravity of 8.65, while gold typically is not toxic, even though it has an atomic number of 79 and specific gravity of 18.88. For a given metal, the toxicity varies widely depending on the allotrope or oxidation state of the metal. Hexavalent chromium is deadly; trivalent chromium is nutritionally significant in many organisms, including humans.

Certain metals, such as copper, cobalt, chromium, iron, zinc, manganese, magnesium, selenium, and molybenum, may be dense and/or toxic, yet are required micronutrients for humans or other organisms.

The essential heavy metals may be needed to support key enzymes, act as cofactors, or act in oxidation-reduction reactions. While necessary for health and nutrition, excess exposure to the elements can cause cellular damage and disease. Specifically, excess metal ions can interact with DNA, proteins, and cellular components, altering the cell cycle, leading to carcinogenesis, or causing cell death.

Heavy Metals of Significance to Public Health

Exactly how dangerous a metal is depends on several factors, including the dose and means of exposure. Metals affect species differently. Within a single species, age, gender, and genetic predisposition all play a role in toxicity. However, certain heavy metals are of grave concern because they can damage multiple organ systems, even at low exposure levels. These metals include:

  • arsenic
  • cadmium
  • chromium
  • lead
  • mercury

In addition to being toxic, these elemental metals are also known or probable carcinogens. These metals are common in the environment, occurring in air, food, and water. They occur naturally in water and soil. Additionally, they are released into the environment from industrial processes.

References:

"Heavy Metals Toxicity and the Environment", P.B. Tchounwou, C.G. Yedjou, A.J. Patlolla, D.J. Sutton, Molecular, Clinical and Environmental Toxicology Volume 101 of the series Experientia Supplementum pp 133-164.

"Heavy metals" a meaningless term? (IUPAC Technical Report) John H. Duffus, Pure Appl. Chem., 2002, Vol. 74, No. 5, pp. 793-807