Heavy metals are emitted both in elemental and compound (organic and inorganic) forms.
Anthropogenic sources of emission are the various industrial point sources
including former and present mining sites, foundries and
smelters, combustion by-products and traffics (UNEP /
GPA, 2004). Cadmium is released as a by- product of
zinc (and occasionally lead) refining; lead is emitted during its mining and smelting activities
,from automobile
exhausts (by combustion of petroleum fuels treated with
tetraethyl lead antiknock) and from old lead paints;
mercury is emitted by the degassing of the earth’s crust.
Generally, metals are emitted during their mining and
processing activities (Lenntech, 2004).