Rhodium

Rhodium (Rh), a rare metal belonging to the platinum group elements, occurs naturally at low concentrations in the Earth’s crust. It is widely used in the glass, chemical, and electronic industries, but its principal application is as an active catalyst material in automobile converters to reduce noxious gas emis- sions to a less harmful form. As a result of its use in catalytic converters, automotive emissions of carbon monoxide, unburned hydrocarbons, and nitrogen oxides have been greatly reduced. However, while catalytic converters have improved general air quality, they have also become a primary anthropogenic source of Rh in the environment. In fact, a clear link has been established between the increasing use of automobile catalysts and more elevated environmental Rh concen- trations. Several studies have demonstrated that Rh is emitted in small quantities as a result of chemical, physical, and thermal stresses on the converters due to mechanical abrasion and high temperatures. Rh contamination, which begins in airborne particulate matter, roadside dust, soil, sludge, and water, subse- quently leads through diverse pathways to the bio- accumulation of this element in living organisms. In exposed subjects, Rh salts have been associated with immunological disorders such as asthma and contact dermatitis. The limited data regarding Rh toxicity and its effects, the general belief that environmental Rh levels are too low to pose a serious threat to human health, and the idea that Rh is only released in its metallic, relatively inert form in biological reactions have prevented an adequate assessment of the risk related to environmen- tal exposure to this metal. Recently, however, reported increases in environmental levels and more informa- tion on Rh concentrations in the finest fractions of particulate matter have stimulated considerable inter- est and debate concerning the potential impact of this metal on human health. Moreover, the findings of a number of innovative studies that demonstrate the cytotoxic and genotoxic effects of Rh on cellular sys- tems and the induction of immunological alterations in animal models have provided the impetus for further investigation into the effects of Rh on human health and for a reassessment of the risk derived from expo- sure to the metal.