No specific definition; however some are defined by molecular weight or density; some are mixed in terms with trace elements. They can be extracted from the ground as an ore and once extracted from nature, they enter a cycle or incorporated in some products. However, when the life of that product is over, they may end up in a landfill or some other place in the environment but never as an ore or in their previous state. They can be widely dispersed, mostly by human activities. When in the environment, they can enter the human body like other chemicals. What makes them different is that they can accumulate in some certain tissues. Another major characteristic is that they are toxic in very low levels (ppb or ppm) What distinguishes is that they have tendency to form ionic bonds. They are of very low toxicity: class b: very toxic (e. g. mercury, lead, silver and gold).