PRINCIPLES OF TOXICOLOGY

342 PROPERTIES AND EFFECTS OF METALS
alkyl mercury compounds, or ingestion of the treated grain, may also result in toxic endpoints. The
most sensitive endpoint for oral exposure to alkyl mercury compounds is the developing nervous
system. In addition, mercury may adversely affect a wide range of other organ systems after exposure
to high levels. Affected systems include the immune, respiratory, cardiovascular, gastrointestinal,
hematologic, and reproductive systems.
The inhalation reference concentration (RfC) for elemental mercury is 3 × 10 –4 mg/m 3 . An oral
RfD of 3 × 10 –4 mg/kg daily for inorganic mercury and 1 × 10 –4 mg/kg per day for organic mercury
has also been established. These values are under review by USEPA at present. Mercury is not classified
as a carcinogen by any route of exposure.
Nickel
Nickel is a hard, silver-white, malleable, ductile metallic element used extensively in alloys and for
plating because of its oxidation resistance. Nickel, combined with other elements, occurs naturally in
the earth’s crust. Nickel released to the atmosphere typically exists in particulate form or adsorbed to
particulate matter. Primary removal mechanisms of atmospheric nickel include gravitational settling
and precipitation. Nickel released to soil may be adsorbed to soil surfaces depending on the soil
conditions. Nickel released to aquatic systems generally exists in particulate forms that settle out in
areas of active sedimentation. However, nickel also may exist in soluble form under appropriate
conditions.
Nickel salts exhibit significant solubility in water. Nickel occurs naturally in drinking water at an
average concentration of about 2 µg/L. Adult daily intake of nickel from water is about 2 µg/day. About
170 µg of nickel is consumed in food per day. Available information indicates that nickel does not pose
a toxicity problem following ingestion because the absorption from food or water is low.
The most prevalent effect of nickel exposure is nickel dermatitis in nickel-sensitive individuals.
Nickel dermatitis typically exhibits two components: (1) a simple dermatitis localized in the contact
area and (2) chronic eczema or neurodermatitis without apparent connection to such contact. Nickel
sensitivity, once acquired, may be persistent. Toxicological information of concern to industrially
exposed humans is primarily confined to two potential categories of effects: (1) dermatoses, contact
and atopic dermatitis, and allergic sensitization; and (2) cancers of the lung and nasal sinuses. Cancers
of the lung and nasal sinuses in nickel workers have been described for more than 50 years in association
with nickel refining processes (calcination, smelting, roasting, and electrolysis) and from nickel plating
and polishing operations (e.g., electrolysis and grinding). Noncarcinogenic respiratory effects, such
as bronchitis and emphysema, have also been seen in occupational exposures. These effects occurred
at concentrations much higher than those found in the environment.
The current daily oral RfD for nickel is 2 × 10 –4 mg/kg. An inhalation RfD has not been established
and is currently under review. USEPA has classified nickel refinery dust and nickel subsulfide as class
A carcinogens by the inhalation route. Nickel is not presently considered to be a carcinogen by the
oral route.
Zinc
Zinc is found in natural form in the air, in soil and water, and occurs in the environment primarily in
the +2 oxidation state. It is a bluish-white metal that can combine with a number of other elements.
Most zinc ore is zinc sulfide. Zinc is an essential trace element in the diet. In water, zinc partitions to
sediments or suspended solids through sorption. Zinc is likely to sorb strongly onto soil, and the
mobility of zinc in soil depends on the solubility of the speciated forms of the compound and on soil
properties such as pH, redox potential, and cation exchange potential. In air, zinc is present primarily
as small dust particles.
Humans are exposed to small amounts of zinc in food and drinking water each day. Levels in air
are generally low and fairly constant. Occupational exposure to zinc occurs in a number of mining and
industrial activities, such as the manufacture of zinc-containing alloys, paints, and pesticides.