ESTIMATES OF THE CHROMIUM(VI) REDUCING CAPACITY IN HUMAN-BODY COMPARTMENTS AS A MECHANISM FOR ATTENUATING ITS POTENTIAL TOXICITY AND CARCINOGENICITY

Estimates of the overall reducing capacity of hexavalent chromium(VI) in some human body compartments were made by relating the specific red ucing activity of body fluids, cell populations or organs to their ave rage volume, number, or weight, Although these data do not have absolu te precision or universal applicability, they provide a rationale for predicting and interpreting the health effects of chromium(VI), The av ailable evidence strongly indicates that chromium(VI) reduction in bod y fluids and long-lived non-target cells is expected to greatly attenu ate its potential toxicity and genotoxicity, to imprint a threshold ch aracter to the carcinogenesis process, and to restrict the possible ta rgets of its activity, For example, the chromium(VI) sequestering capa city of whole blood (187-234 mg per individual) and the reducing capac ity of red blood cells (at least 93-128 mg) explain why this metal is not a systemic toxicant, except at very high doses, and also explain i ts lack of carcinogenicity at a distance from the portal of entry into the organism, Reduction by fluids in the digestive tract, e,g, by sal iva (0,7-2.1, mg/day) and gastric juice (at least 84-88 mg/day), and s equestration by intestinal bacteria (11-24 mg eliminated daily with fe ces) account for the poor intestinal absorption of chromium(VI), The c hromium(VI) escaping reduction in the digestive tract will be detoxifi ed in the blood of the portal vein system and then in the liver, havin g an overall reducing capacity of 3300 mg, These processes give reason s for the poor oral toxicity of chromium(VI) and its lack of carcinoge nicity when introduced by the oral route or swallowed following reflux from the respiratory tract. In terminal airways chromium(VI) is reduc ed in the epithelial lining fluid (0.9-1.8 mg) and in pulmonary alveol ar macrophages (136 mg), The peripheral lung parenchyma has an overall reducing capacity of 260 mg chromium(VI), with a slightly higher spec ific activity as compared to the bronchial tree. Therefore, even in th e respiratory tract, which is the only consistent target of chromium(V I) carcinogenicity in humans (lung and sinonasal cavities), there are barriers hampering its carcinogenicity, These hurdles could be only ov erwhelmed under conditions of massive exposure by inhalation, as it oc curred in certain work environments prior to the implementation of sui table industrial hygiene measures.