Biological monitoring of workers exposed to 4,4′-methylene-bis-(2-orthochlaroaniline) (MOCA) II.: Comparative interest of '"free" and "total" MOCA in the urine of exposed workers

Objectives: The objectives of the study were to investigate the best urinary marker of exposure to MOCA in urine samples of exposed workers and to study its applicability in exposure evaluation in polyurethane resin production plants. In addition, our intention was to contribute to the question of biological target value in order to reduce exposure levels. Methods: Urinary MOCA markers were measured in urine samples collected at the end of the workshifts. 40 workers from four factories were observed for three consecutive days in the same week. "Free" MOCA in non-acid-stabilized urines, "total" MOCA in urines after alkaline hydrolysis, "acid-labile" MOCA in sulfamic or citric acid-protected urines were measured in all urine samples. MOCA liberated by heating the non-acid-stabilized urines ("heat-labile" MOCA) was also measured in 17 urine samples of exposed workers. Results: The median value of "free" MOCA was lower than that of "sulfamic acid-labile" MOCA: 6.0 and 12.5 mu g/l respectively. The difference between "free" and "total": MOCA was statistically significant (Mann-Whitney test, P = 0.023) for the urine samples collected without acid. Correlations between urinary MOCA concentrations in hydrolysed and non-hydrolysed urine samples were high (r = 0.984, 0.966 and 0.950; P < 0.001) for urine samples with sulfamic acid, with citric acid and without acid respectively. For all the factories, the postshift urinary MOCA concentrations ranged between 1 mu g/l (detection limit) and 570 mu g/l; 25% of "sulfamic acid-labile" MOCA concentrations exceeded 50 mu g/l. Workers handling crystallized MOCA excreted the highest amounts of MOCA in urine. The urinary MOCA concentrations (median) were: 84.0 mu g/l (mixer), 15.5 mu g/l (moulder), 59.0 mu g/l (maintenance) and 3.0 mu g/l (others). Conclusions: MOCA measured in sulfamic acid-protected urine samples without hydrolysis provides a more practical and reliable biomarker than "total" MOCA (after hydrolysis) or "free" MOCA. A biological guiding value of 20 mu g/l expressed as "sulfamic acid-labile" MOCA is proposed.