Contact sensitivity to rhodium and iridium in consecutively patch tested subjects

Rhodium is primarily used as an alloying agent to harden platinum and palladium; it is a catalyst in nitrogen oxide combustion of ammonia. It is also found in electrical material because of its low electrical and contact resistance, in jewellery, in optical instruments and dental prostheses. Iridium is commonly used in electrochemical industries and for its chemical resistance in amalgams and/or in white gold used in dental practice. Rhodium and iridium have various applications in both occupational and non-occupational activities; their sensitizing capacity is not well known, although naturally occurring metals and their man-made compounds are the most important contact allergens in the general population. Both metals are considered allergologically safe, although there are a few reports of both occupational and non-occupational delayed sensitivity, as well as a case of contact urticaria with respiratory tract symptoms caused by occupational exposure. In our study, the frequency and relevance of positive reactions to rhodium and iridium (both 1% in water) in 720 consecutively patch tested patients were investigated. None of the 720 patch tested subjects showed positive or irritant reactions to iridium chloride, but 2 were found to have a positive patch test to rhodium chloride as well as other metals. The positive reactions were judged unrelevant; neither had metallic dental prostheses or amalgams. Mass spectrometry of rhodium chloride used for patch testing did not detect traces of nickel and/or cobalt; therefore we excluded a false-positive reaction to rhodium due to contaminants. Despite their low sensitization potential, we suggest to patch test rhodium and iridium salts in subjects who work in the jewellery and precious metals industries, or who are suffering from gingivostomatitis.