Objectives-This study was carried out to investigate possible immunological changes in workers with prolonged low exposure to inorganic mercury in a fluorescent light bulb factory. Methods-29 immunological variables were examined in 34 workers with prolonged low level exposure to inorganic mercury (exposed workers) and 35 unexposed workers as the controls. The selected indicator of mercury exposure was concentration of mercury in the urine (U-Hg), which declined progressively from 36.0 pg/l in 1978 to 6.0 igIl in the study year 1994. Results-None ofthe exposed workers had ever shown signs ofeither acute or chronic inorganic mercury toxicity or had shown any form of hypersensitivity. The only changes found in the exposed workers, compared with the controls, were a reduction of the cells that express cluster differentiation (CD25,(T activation antigen (Tac antigen))) and concentrations of tumour necrosis factor-a (TNF-a) in serum. However, the decrease of cells that express CD25 was unrelated to occupational exposure and was, in all likelihood, a chance finding. Conversely, the decline in serum TNF-a was closely associated with occupational exposure. However, no dose-response relation was found between U-Hg and TNF-a concentrations; nor were TNF-a concentrations affected by cumulative occupational exposure to inorganic mercury in over 20 years. Conclusions-Tentatively, we suggest that reduced serum TNF-a concentrations might be indicative ofan in vivo functional defect of the monocyte macrophage system in this particular group of workers even though they were clinically asymptomatic. (Occup Environ Med 1997;54:437-442)

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