Origin Four elements are particularly considered; 
the Lead (Pb) , the Cadmium (Cd) , the nickel (Ni) and Arsenic (As) , and have different sources of emissions.

The Lead is found in the metallurgy of non-ferrous metals, but is also emitted by vehicles still run on leaded fuel (removal Super leaded to 01/01/2000). In urban areas, it has fallen sharply since the arrival of unleaded gasoline. It is also found around certain industrial sites.
 It is the same for Cadmium ,  Nickel and Arsenic emitted mainly around activity sites linked to metallurgy, foundries and certain combustion facilities (incinerators, oil complexes, etc.)   Pollution Heavy metals are dangerous for the environment because they are not degradable, moreover they are enriched during mineral and biological processes, and will eventually accumulate in nature.
Heavy metals can also be absorbed directly through the food chain causing chronic or acute effects.Effects :  The Lead is known to inhibit the production of hemoglobin and change the composition of the blood. It also acts on the central nervous system. His chronic poisoning is the poisoning
The Cadmium enters the body through the digestive tract or respiratory disorders and can cause breathing and urinary tract. It is responsible for hepato-digestive, blood, renal, bone and sometimes fatal disorders.
The Nickel causes inflammation of the nasal mucous membranes of the respiratory organs and is known to be carcinogenic.
The Arsenic is a cellular poison, it reaches the digestive organs and the liver.Regulationsconcerning
lead Regulatory TextNature of the limitValue
( m g / m3)Data concernedDecree 06/05/1998
Goal0.5Annual averageLimit value2Annual average