Geoscience Reference
In-Depth Information
indicating that occupational cadmium exposure is associated with lung cancer.
Cadmium exposure has also been linked to human prostate and renal cancer,
although this linkage is weaker than for lung cancer. Other target sites of cadmium
carcinogenesis in humans, such as the liver, pancreas, and stomach, are considered
equivocal. In animals, cadmium effectively induces cancers at multiple sites and
by various routes (Waalkes and Rehm
1994
; Waalkes
2000
,
2003
; Hertz-Picciotto
and Hu
1995
; Satarug et al.
2003
; McMurray and Tainer
2003
; Hayes
1997
).
Cigarette smoking is the major source of cadmium exposure, causing an average
blood cadmium level four to five times higher than for nonsmokers (Jarup et al.
1998
). In general, for the nonsmoking population, the major exposure pathway is
through food, via the addition of cadmium to agricultural soil from various
sources. Additional exposure to humans arises through the presence of cadmium in
ambient air and drinking water.
3.5.3 Chromium
Chromium toxicity is different for each of its species (especially Cr
VI
and Cr
III
). It
is, therefore, regulated separately for each oxidation state, unlike all other heavy
metals, which are regulated on the basis of their total concentration (Kimbrough
et al.
1999
). Chromium (III) occurs naturally in the environment and is an essential
nutrient. Chromium (VI) and chromium (0) are generally produced by industrial
processes. Chromium exists in small quantities throughout the environment.
Chromite ore (FeCr
2
O
4
) is the most important commercial ore and usually is
associated with ultramafic and serpentine rocks. Chromium also is associated with
other ore bodies (e.g., uranium and phosphorites) and may be found in tailings and
other wastes from these mining operations. Acid mine drainage can make chro-
mium available to the environment. In air, chromium compounds are present
mostly as fine dust particles, which eventually settle over land and water. Chro-
mium can attach strongly to soil, and only a small amount can dissolve in water
and move deeper in the soil to underground water.
Chromium and its compounds are used in refractories, drilling muds, electro-
plating cleaning agents, catalytic manufacture, and in the production of chromic
acid and specialty chemicals. The metal chromium, which is the chromium (0)
form, is used for making steel. Chromium (VI) and chromium (III) are used for
chrome plating, dyes and pigments, leather tanning, and wood preserving. Chro-
mium enters the air, water, and soil mostly in the chromium (III) and chromium
(VI) forms. The key to these uses is that under typical environmental and bio-
logical conditions of pH and oxidation-reduction potential, the most stable form of
chromium is the trivalent oxide. This form has very low solubility and low
reactivity, resulting in low mobility in the environment and low toxicity in living
organisms. However, the stable and generally nontoxic trivalent form of chromium
can be transformed (oxidized) in the environment to chromate (CrO
4
) and
dichromate (Cr
2
O
7
) anions, which are mobile and toxic. Hexavalent chromium
