Cadmium (atomic weight 112.41) is a silver white metal (density 8.65 g/mL). The oxidation states are 0, +2. The main uses of cadmium were steel production, non-ferrous metal production, refining, cement manufacture, cadmium plating, battery manufacture, waste and combustion, and phosphate fertilizers. Nowadays, because of concerns about its environmental toxicity, the use of cadmium has drastically decreased. About two thirds of the cadmium in use today come from nickel-cadmium batteries, the rest from pigments, metal plating and the plastic industry. It is a lot like lead and mercury, in that it accumulates both in the environment and in the body, causing long-term damage to life.

Cadmuim toxicity can manifest in a variety of syndromes, as hypertension, renal dysfunction, bone defects, hepatic injuries, lung damage, and reproductive effects. The maximum acceptable cadmium in drinking water is 0.003 mg/L (WHO, 2008).

1.1.1 Chromium

Chromium (atomic weight 51.99) is a lustrous, brittle, hard silver-gray metal (density 7.14 g/ mL). It exists in different oxidation states: -2, 0, +2, +3, +6. Chromium is mainly used in steel production and in chrome plating. Its products are also used in leather tanning, printing, dye production, pigments, wood preservatives, and many others.

The respiratory and dermal toxicity of chromium are well-documented. Workers exposed to chromium have developed nasal irritation (at <0.01 mg/ m3, acute exposure), nasal ulcers, perforation of the nasal septum (at ~2 pg/ m3, subchronic or chronic exposure) and hypersensitivity reactions and "chrome holes" of the skin. Among the general population, contact dermatitis has been associated with the use of bleaches and detergents. Compounds of both Cr(VI) and Cr(III) have induced developmental effects in experimental animals that include neural tube defects, malformations, and fetal deaths. The speciation of chromium has become of relevant interest because of the association Cr(VI)-cancer. The different toxicity of the two forms Cr(VI) and Cr(III) are now under examination, even if at the moment the WHO Guidelines report the provisional value 0.05 mg/L referred to total chromium (WHO, 2008).