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Heavy metals are commonly defined as those elements with a high (>5.0) relative density. There is, however, an ongoing debate regarding the exact definition and, in terms of health, it is better to think of these as metals or semi-metal compounds which have the potential to cause environmental or human toxicity. Heavy metal poisoning can be acute or chronic and may be caused by the following:

Lead.
Mercury.
Iron.
Cadmium.
Thallium.
Bismuth.
Arsenic (technically not a true metal but a semi-metal - ie non-metal with some metallic properties).
The metals may enter the body by:

Ingestion.
Inhalation.
Absorption through the skin or mucous membranes.
They are then stored in the soft tissues of the body. The heavy metals once absorbed, compete with other ions and bind to proteins, leading to impaired enzymatic activity resulting in damage to many organs throughout the body.

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