Heavy metals are metal elements that are categorized by their high density relative to other metals. Many, but not all, heavy metals are considered toxic. Heavy metals are typically considered heavy when they have a density ratio of 5:1 when compared with water.
Examples of the heavy metal group can vary by the organization or standard defining them. For instance, some lower density metals that are toxic may be categorized as heavy metals simply because they are poisonous. An example of a low-density metal that is sometimes classified as a heavy metal is beryllium. There are also heavy metals that are not toxic. They are considered heavy metals solely on the basis of their relatively high specific gravity. Examples of heavy metals that are not toxic but have a high density include gold and platinum.
While there are heavy metals that may not be both toxic and heavy, many of them are. There are many different types of heavy metals that have a specific gravity greater than 5 and are inherently toxic.
Perhaps the two most notorious heavy metals are lead and mercury. Historically, lead pipes and lead paints were used in abundance. This eventually resulted in overexposure to humans, which caused lead poisoning with symptoms including cognitive and physical disabilities. Mercury poisoning is another type of heavy metal toxin that has caused disabilities and death in humans and other organisms. Excessive mercury exposure can result from consuming fish with high amounts of mercury and dental inserts that contain mercury.