Toxicokinetics - trdoer40178

according to epidemiological evidence. The USEPA has concluded that only

chromium (VI) is carcinogenic. As a result, chromium (VI) compounds are

classified as human carcinogens via inhalation (IRIS 1998). The USEPA points

out the uncertainty in the relevance of occupational exposure to chromate mists

and environmental exposures to chromium particulates (USEPA 1980).

Toxicokinetics

Gastrointestinal absorption of chromium (VI) occurs with greater efficiency

than absorption of chromium (III), though absorption of ingested chromium (VI)

is estimated to be less than 5percent (USEPA 1998). The absorption of

chromium by the lung is dependent upon many factors including the size,

oxidation state, solubility of the chromium particles, as well as the activity of

alveolar macrophages and the interaction of chromium with reducing agents in

the lung. Absorption also occurs through the skin with diffusion constants

reported to be 314 10-6 cm2/min (Mali 1963 as cited in USEPA 1998). Factors

influencing dermal absorption include the chromium salt employed, the valence

state (III or VI), anionic form, concentration, and pH (USEPA 1998).

Once absorbed, chromium (VI) crosses the red blood cell membrane where it

can bind to cellular compounds or undergo reduction to chromium (III). There

appears to be significant in vivo conversion of chromium (VI) to chromium (III).

Chromium (VI) is cleared slowly from blood and rapidly from tissues while the

opposite applies to chromium (III). Chromium is distributed primarily to the

liver, spleen, bone marrow, lung, and kidney.

Excretion primarily occurs through the urine (50 to 60 percent) with some

fecal elimination (about 8 percent) (USEPA 1998). The remainder is deposited

in various tissue compartments and has a long biological half-life. Chromium

(VI) is eliminated much faster than chromium (III). Adipose and muscle tissue

retain chromium for about 2 weeks, while liver and spleen tissue retain

chromium for about 1 year.

Ecological effects

USEPA (1980) summarizes studies on the acute effects of hexavalent

chromium on various marine species. The species represent a wide range of

taxonomic categories and trophic levels and include. The acute value for

polychaete worms ranged from 2,000 ug/l (Eisler and Hennekey 1977) to

7,500 ug/l (Reish and Carr 1978). Mollusks displayed relatively high acute

values which ranged from 22,000 ug/l for the brackish water clam (Olsen and

Harel 1973) to 105,000 ug/l for the mud snail (Eisler and Hennekey 1977). Acute

values for fish species ranged from 15,000 ug/l for Atlantic silverside to

91,000 ug/l for mummichog. USEPA (1980) indicates that the chronic value for

polychaetes from <13 to 37 ug/l and for mysids it is 132 ug/l. They also indicate

that the toxicity to macroalgae ranged from 1,000 to 5,000 ug/l.

Acute toxicity values for chromium (VI) are available for freshwater animal

species in 27 genera and range from 23.07 ug/l for a cladoceran to 1,870,000 ug/l

D26

Appendix D Toxicological Profiles