Metallic elements with high atomic weights usually cited as public health hazards because they accumulate in the food chain and are toxic, e.g. mercury, chromium, cadmium, arsenic, and lead.

To be slightly more precise, metallic elements with a specific gravity of at least 5.0. All heavy metals are toxic in high enough concentrations, but some (zinc and copper, for example) are micronutrients, essential to plant and animal life.