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 metal" also refers to a form of rock and roll music, but such references are unlikely to be found in an MSDS. Additional Info: 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 is instead 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. Here's an example of cadmium bioaccumulation that is threatening wildlife in Alaska. Lead poisoning is another example of heavy metal poisoning. Lead is neurotoxic, so individuals whose body is still developing (such as children or developing fetuses) are most at risk. While some aspects of lead poisoning are reversible in adults, in children this can interfere with normal development, cause irreparable brain damage, or kill. Note: In 2014, researchers at the US National Institutes of Health discovered that mice exposed to low doses of arsenic in drinking water developed lung cancer at levels that are similar to possible human doses. It is a generally accepted principle that no level of arsenic exposure is safe. The US EPA has only recently phased out its use as an herbicide in most applications, although exceptions still apply for a few select industries. Heavy metal exposure occurs through three primary routes: (1) Inhalation. Common examples include workers scraping or sanding lead paint and workers in factories where heavy metals are melted and processed. In the days before leaded gasoline was banned, those living alongside heavily traveled roads faced significant exposure through tailpipe emissions. (2) Ingestion. The leading cause of lead poisoning in children is eating old paint chips. A major source of elevated mercury levels in humans comes from eating contaminated fish. And people can drink arsenic from wells contaminated by arsenic-containing pesticides. (3) Skin absorption. Day to day contact with heavy metals can cause poisoning. Dermal exposure is a serious concern for workers in fields where the irrigation water contains naturally-occuring arsenic (such as Asian rice paddies). See: Environ. Sci. Technol., DOI: 10.1021/es903667. MSDS Relevance: If you are occupationally exposed to heavy metals, your workplace should have written protocols in place to minimize your exposure. A variety of controls such as eliminating or minimizing the use of such materials, the use of fume hoods, respirator requirements, medical monitoring and more may be required. Presumably, employers should be taking care to ensure that no worker is exposed to potentially harmful levels of any substance. However, this is not always the case, so be vigilant. Be sure to read the MSDS for any heavy-metal containing material very closely. It should describe the symptoms of poisoning and chronic effects of exposure. Heavy metals are fetotoxic, so women of child-bearing age should avoid any exposure to heavy metals. Note: the U.S. Supreme Court has declared it illegalunder the Pregnancy Discrimination Act for employers to bar women from jobs that may expose them to lead or other toxins! See: This OSHA Interpretation Letter on the ruling or this EEOC PDA fact sheet for more information. See also: Alopecia, Nephrotoxin, Reproductive Toxin.