The sense of smell is vulnerable to damage from xenobiotic agents, reflecting, in part, the direct exposure of its receptors to the outside environment. Industrial vapors and dusts, particularly those of heavy metals, are known to adversely alter smell function. In this study, we evaluated the olfactory function of 43 professional welders who worked in poorly ventilated confined spaces for 1-2 years on the San Francisco/Oakland Bay Bridge. Nearly half reported having problems smelling and tasting. Relative to matched controls, the welders exhibited significantly lower scores on the University of Pennsylvania Smell Identification Test (UPSIT); respective means (SEMs) = 29.51 (0.90) and 36.55 (0.88). Eighty-eight percent scored below their individually matched controls. As in idiopathic Parkinson's disease, the olfactory test scores of the welders were unrelated to a broad spectrum of neurological and neuropsychological measures. Although blood levels of Mn were correlated with the time spent working on the bridge, workers with the highest Mn blood levels exhibited better olfactory function than those with the lowest Mn blood levels. The basis of this paradoxical phenomenon, which has been observed previously by others studies, is not clear.