The intricate journey of inorganic chromium(III) compounds through the environment and their interaction with living systems reveals a complex profile, distinct from other chromium valency states. These ubiquitous substances are found naturally in rocks and soil, forming a complete cycle through plants, animals, and humans before returning to the earth. Beyond natural occurrences, human activities contribute to their atmospheric presence, particularly through combustion processes, where chromium exists primarily as fine particulate matter. In soil, chromium(III) predominantly exists as insoluble oxides, exhibiting limited mobility due to its rapid and strong adsorption to soil surfaces like iron and manganese oxides.
Exposure pathways for the general population are primarily through the daily diet, though other sources include chromium-containing supplements, ambient air, chromium-tanned leather products, cosmetics with chromium pigments, and stainless steel items. Within the human body, the absorption of chromium is significantly influenced by its oxidation state. While chromium(VI) readily penetrates cell membranes, chromium(III) does not, leading to different distribution patterns once absorbed. After oral exposure, particularly to chromium(III) compounds, much of the chromium is recovered in faeces due to its poor absorption rate, with animal studies indicating urine as the major route of elimination for absorbed chromium.
Upon absorption, chromium(VI) is rapidly reduced to chromium(III) within cells, subsequently binding to macromolecules. Animal studies show accumulation mainly in the liver, kidneys, spleen, and bone marrow, with distribution varying by speciation. In humans, the highest concentrations are observed in hilar lymph nodes and lungs, followed by the spleen, liver, and kidneys, with tissue levels tending to decline with age.
The assessment of health effects clearly differentiates between chromium(III) and chromium(VI). Unlike chromium(VI), which has been classified as carcinogenic to humans, metallic chromium and chromium(III) are not classifiable as to their carcinogenicity. This distinction is critical in understanding the potential risks posed by different chromium compounds.
For human exposure to insoluble chromium(III) oxide, the primary concern revolves around persistent local irritation and inflammation stemming from the accumulation of inhaled particles in the lungs when clearance mechanisms are overwhelmed. With soluble chromium(III) salts, exemplified by basic chromium sulfate, the key endpoints of concern include local respiratory toxicity and skin sensitization. While tolerable concentrations for these effects have been established, they generally exceed typical ambient air concentrations of chromium(III), even near point sources.
Regarding skin sensitization, soluble chromium(III) salts are unlikely to induce sensitization in individuals not previously sensitized, even through products like chromium-tanned leather. However, in already sensitized individuals, chromium allergy might be triggered by small amounts of chromium leached from leather products. Cases of skin sensitization among workers handling chromium(III) compounds are rare, and there is currently no clear evidence to suggest that exposure to chromium(III) compounds induces occupational asthma.
It is also recognized that chromium(III) plays an essential role for some microorganisms, being involved in specific metabolic processes such as glucose metabolism and enzyme activation. This highlights the dual nature of chromium(III), both as an environmental presence requiring careful assessment and as a vital element in certain biological functions. The comprehensive understanding of these compounds is crucial for managing their presence in the environment and protecting human health.
