What is a heavy metal? It depends on who you ask! According to Chemistry International Vol 23, No.6 November 2001 - "Heavy Metals" - A Meaningless Term
Over the past two decades, the term "heavy metals" has been used increasingly in various publications and in legislation related to chemical hazards and the safe use of chemicals. It is often used as a group name for metals and semimetals (metalloids) that have been associated with contamination and potential toxicity or ecotoxicity. At the same time, legal regulations often specify a list of heavy metals to which they apply. Such lists may differ from one set of regulations to the other, or the term may be used without specifying which heavy metals are covered. In other words, the term "heavy metals" has been used inconsistently. This practice has led to general confusion regarding the significance of the term. The inconsistent use of the term "heavy metals" reflects inconsistency in the scientific literature. It is, therefore, necessary to review the usage that has developed for the term, paying particular attention to its relationship to fundamental chemistry. Without care for the scientific fundamentals, confused thought is likely to prevent advances in scientific knowledge and to lead to bad legislation and to generally bad decision-making. Def 1.) A generalized definition in common use: One of 23 chemical elements that has a specific gravity (a measure of density) at least five times that of water. Def 2.) Definition according to OSHA - The term "Heavy Metals" is generally interpreted to include those metals from periodic table groups IIA through VIA. The semi-metallic elements boron, arsenic, selenium, and tellurium are often included in this classification. At trace levels, many of these elements are necessary to support life. However, at elevated levels they become toxic, may build up in biological systems, and become a significant health hazard. Def 3.) Definition used by many - The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic, highly toxic or poisonous at low concentrations. Examples of heavy metals include mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb). Heavy metals are dangerous because they tend to bioaccumulate. For example, marine organisms can consume a particularly dangerous form of mercury called methylmercury. When fish eat these organisms, the methylmercury is not excreted, but retained in bodily tissues. The older the fish and the more contaminated organisms it has consumed, the greater the amount of methylmercury in its tissues. When another fish eats the first fish, the accumulated methylmercury is passed up the food chain, eventually becoming hundreds or thousands of times its original concentration. Any organism at the top of the food chain (humans, polar bears etc.) faces a serious risk of mercury poisoning by eating such fish. More on heavy metals here.
