**2.6 Airborne sources/origins**

Metals can be found in the air due to stack or duct emissions of air, gas, or vapor streams, as well as fugitive emissions including dust from warehouses or garbage dumps. Metals emitted from the air are usually discharged as particles in the gas stream. Following high-temperature processing, several metals, such as Pb, Cd, and As, can also volatilize. Natural air currents can also disperse stack emissions over a large area until they are removed from the gas stream by dry and wet precipitation processes.

Agricultural lands near smelting sites have been discovered to have very high levels of Cd, Pb, and Zn. Airborne emissions of Pb from the combustion of fuel including tetraethyl lead are yet another major cause of soil pollution; this contributes significantly to the Pb concentration in urban areas. Tires, lubricating fluids are two sources of Cd and Zn that can be introduced into soils near highways [23].
