PRINCIPLES OF TOXICOLOGY

334 PROPERTIES AND EFFECTS OF METALS
genotoxic damage associated with nickel carcinogenicity. Arsenic likely acts as a non-genotoxic
indirect carcinogen via induction of oncogene expression and inhibition of DNA repair.
14.5 SOURCES OF METAL EXPOSURE
Natural Sources
Metals are naturally occurring elements in the earth’s crust, and thus exposure from natural sources is
inevitable. Metals are found in various concentrations in soils, sediments, surface and groundwaters,
and air. They move through the environment as part of the natural biogeochemical pathways, being
deposited on surfaces from the air, taken up from the soil and water by plants, passed up the food chain
when ingested and bioaccumulated, released back to the environment through excretion and decay of
dead organisms, and often going through various transformations and speciation changes in the
process. The importance of metals in these cycles is in large part due to their inherent persistence.
Table 14.4 lists the typical levels of some metals found in the environment. It is important to
remember that these levels may vary considerably, depending on whether the site of interest is rural,
urban, or near a concentrated source of the metal, such as an ore deposit or hazardous waste facility.
Thus, the levels reported in the table are only approximations or ranges for comparative purposes, and
should not be seen as reflective of levels to be expected in all areas. A great deal of literature is available
concerning the geographic distribution of naturally occurring metals in the United States.
Anthropogenic Sources
In addition to natural biogeochemical cycles, humans play a large role in the mobilization,
transformation, and transportation of metals in the environment. Mining, dredging, construction,
and manufacturing all remove metals from the locations in which they naturally occur, and may
incorporate them into the human economic sphere, thus increasing the potential for human
exposure. Metals often find their way back into the environment through the use and disposal of
products containing them, through the disposal of manufacturing wastes, or through the discharge
of mine tailings or dredge material.
TABLE 14.4 Typical Levels of Selected Metals Observed in the Environment
Metal Air (ng/m 3 )
Drinking Water
(µg/L)
Rivers and Lakes
(µg/L) Soil (mg/kg)
Aluminum