A metal with a specific gravity greater than about 5.0, especially one that is poisonous, such as lead or mercury.
effects are:-
Living organisms require trace amounts of some heavy metals, including iron, cobalt, copper, manganese, molybdenum, vanadium, strontium, and zinc, but excessive levels can be detrimental to the organism. Other heavy metals such as mercury, lead and cadmium (with one exception for the latter) have no known vital or beneficial effect on organisms, and their accumulation over time in the bodies of mammals can cause serious illness. The pathway for toxic effects on humans is normally:

for the entry of heavy metals into the atmosphere as industrial stack gas
to enter the soil as a soil contaminant
to enter groundwater as a water pollutant
to be deposited in ocean bottoms or bay mud, which materials at a later time be dredged to the surface
In medical usage, the definition is considerably looser, and heavy metal poisoning can include excessive amounts of iron, manganese, aluminium, or beryllium (the seventh-lightest metal) as well as the true heavy metals.

Heavy metals in a (haz-mat) setting are for the most part classified in Misc. on the UN model hazard class but, they are sometimes labeled as a poison when being transported.