The nomenclature has been proposed to define those metals denser than iron, with known environmental effects and also for health but afterward the list was being extended leaving the density factor and keeping the others.
At least, heavy metals are mercury, lead, cadmium, nickel, silver, antimony, cobalt, copper, zinc, selenium or more although not every one posses risks until a concrete level of contamination and characteristics. For instance copper, an intensely used metal in almost every circuit board, is perhaps a heavy metal when it's fine enough to form airborne particles or contaminate water. Others when they are being heated or mixed with another elements.
To keep up reference, heavy metals pose a risk for health when there's not protection to avoid the usual methods of interaction with the organism. Touching them, by inhalation or also having contaminated sources of food are the most usual but not the only ones.
Arsenic, chromium, mercury and cadmium are very dangerous, with the possibility of creating fatality. Lead is also another one that fits in the consideration of poison that unfortunately was very used for a long time even in water pipes.