RHODIUM METAL FUME

OSHA comments from the January 19, 1989 Final Rule on Air Contaminants Project extracted from 54FR2332 et. seq. This rule was remanded by the U.S. Circuit Court of Appeals and the limits are not currently in force.

CAS: 7440-16-6; Chemical Formula: Rh

The current OSHA PEL for rhodium metal fume and insoluble salts is 0.1 mg/m³ as Rh; the current PEL for soluble rhodium compounds is 0.001 mg/m³ as Rh. Rhodium is a silvery white, hard, ductile, and malleable metal. The ACGIH recommends a 1 mg/m³ TLV for rhodium metal and insoluble salts and a 0.01 mg/m³ TLV for soluble rhodium salts. The current OSHA PELs for rhodium compounds (i.e., the 1968 ACGIH TLVs) were based on the then-existing TLVs for platinum because of concern that exposure to rhodium might be associated with respiratory sensitization effects. This concern was prevalent because rhodium belongs to the platinum family of metals and because the toxicologic data on rhodium that were formerly available were “meager” (ACGIH 1966/Ex. 1-13).

The ACGIH’s decision to increase the TLVs for rhodium compounds was based primarily on a personal communication to the TLV Committee (Johnson, Matthey and Co., Ltd. 1981b, as cited in ACGIH 1986/Ex. 1-3, p. 512). This communication indicated that, in a major precious metals refinery, “procedures which were abandoned for the refining of platinum because of cases of sensitization have been carried out for a year with analogous rhodium compounds without any problems” (ACGIH 1986/Ex. 1-3, p. 512). In addition, the ACGIH noted that none of the substances in the platinum group was known to produce respiratory effects similar to those of platinum. The ACGIH reported that rhodium exhibited “slight” carcinogenic activity in mice (ACGIH 1986/Ex. 1-3). After considering all of this evidence, the ACGIH judged the previous TLVs to be inappropriate and increased them tenfold.

NIOSH (Ex. 8-47, Table N1) concurs that OSHA should retain its PELs for these substances. No other comments on rhodium were received. OSHA concludes that the evidence adduced by the ACGIH is not sufficient to meet the standard of proof the Agency must achieve before it can raise an exposure limit. This conclusion is based on that fact that the ACGIH relied heavily on a personal communication when making its decision, and no exposure or other data are available to support the ACGIH’s action. Thus OSHA is unable to adequately evaluate the toxicologic evidence pertaining to the rhodium compounds and retains the existing PELs for rhodium metal fume and insoluble salts (0.1 mg/m³ TWA) and rhodium soluble salts (0.001 mg/m³ TWA).