HM are commonly defined as those having a specific density of more than 5g/cm3. The main threats to human health from HM are associated with exposure to lead, cadmium, mercury and arsenic (arsenic is a metalloid, but is usually classified as heavy metal). Heavy metals have been used in many different areas for thousands of years. Lead has been primarily used at least 5000 years ago in building materials, pigments for glazing ceramics and pipes for transporting water.