The sensitive and specific assessment of exposure to airborne agents is a precondition for effective prevention measures and health risk assessment. Air monitoring can be a problem because isocyanate aerosols and simultanous exposures to more than one isocyanate, frequently present in the workplace, are not adequately measured by many routine devices . It has been shown that isocyanate exposure can occur despite respiratory protective equipment, and skin absorption or ingestion also having to be considered . Previous studies have shown that the detection of isocyanate-derived (di-) amines in hydrolyzed urine is the most suitable, acceptable and sensitive method for monitoring potential individual isocyanate exposures [23–26]. Earlier studies provided some evidence that the urine excretion time may differ for individual isocyanates [24, 26, 27]. We corroborated the differences in excretion kinetics for different isocyanates and have established the elimination patterns for all major diisocyanates at different exposure concentrations. When looking closer at different isocyanates, it became obvious that the aliphatic isocyanate 1,6-HDI has a shorter excretion time than aromatic isocyanates (4,4'-MDI, 2,4-/2,6-TDI). Notably, aromatic MDA, NDA and cycloaliphatic IPDA were not completely eliminated after 24 h. After pulmonary absorption of 2,4- and 2,6-TDI, the majority had been excreted in urine 6 h after the end of exposure [23, 28].
According to other studies, additional slowly generated TDA fragments were released into urine over days [28, 29]. Other groups could not monitor any longterm release of TDA into the urine . We observed the major excretion peak at 4.1 h and 4.8 h (for 2,6- and 2,4-TDA); the majority of the TDA appeared to be eliminated after 24 h. At high exposure levels, the TDA was eliminated more slowly with a half time of 6 h.
It has to be noted however that the patients were exposed to a mixture of 2,4-/2,6-TDI, which might influence the elimination of a single diamine, a greater exposure group is necessary to prove this hypothesis. Unfortunately, in many studies only pre- and post-working shift data are provided. This may lead to misinterpretation of the actual exposure since only 15-20% of the residual 2,4- and 2,6-TDA is found after 8 h.
In many industrial workplaces, exposure to several isocyanates may take place simultaneously and no information is available about how the different isocyanates are metabolized when the atmosphere contains a mixture of several isocyanates, such as e.g. during thermal degradation of polyurethanes. Other authors have identified MDA in pooled urine samples after exposure to MDI from thermal breakdown [15, 20, 30]. A high variability in TDA and MDA concentrations was described in urine during and between workdays [31–33], but information on the elimination half-times of MDA or NDA was not available as yet. We observed clear excretion peaks between 12-14 h after the end of exposure, revealing urinary elimination of MDA that is significantly slower than for other isocyanate amines. It was also evident that the excretion was not complete after 24 h. We observed similarly slow elimination rates for another aromatic diisocyanate, 1,5-NDI, in another investigation of workplace exposure, with elimination times over 2-5 days in 6 workers (data not shown). We have estimated the excretion half-life for IPDA to be 4-5.5 h (for low and high exposure groups, respectively). In an earlier IPDI exposure study, the urinary elimination half-times of IPDA excretion seemed to be slightly faster, reaching the half-time of excretion values between 1.7-4.3 h for subjects not previously exposed .
Our findings indicate that there is a clear difference in the excretion kinetics for individual isocyanates. Thus the measurements obtained after a working shift may falsely estimate the degree of exposure, especially for the aliphatic HDA with extremely short excretion times or aromatic isocyanates (i.e. MDA, NDA) with their longer excretion times. Interestingly, increasing the isocyanate load during the exposure challenge did not change the overall kinetic patterns, rather inducing a more prolonged horizontal shift (i.e. MDA, IPDA). There were only small differences in the excretion kinetics for the low and high exposure groups of investigated subjects when the individual peak hights were compared. It cannot be excluded that the isocyanate may metabolize differently if air concentrations are higher than those in this study.
Neither prior isocyanate exposure, bronchial hyperresponsiveness nor immunological sensitization to isocyantes were associated with changes in the pattern of the elimination kinetics. It had been proposed that chronically exposed workers might metabolize isocyanates differently than volunteers without prior exposure . We cannot exclude this, but we found similar elimination kinetics for individual diisocyanates despite the different occupational pre-exposure histories of the subjects, their clinical status and different demographic and geographic origins.
It is likely that the same metabolizing enzyme or various (produced) adducts influence the elimination kinetics. The molecular pathomechanism of the isocyanate transport to an affected organ, the development of the disease and its elimination from the body are largely unknown for humans. It is assumed that the isocyanates are hydrolysed to their respective amines and further oxidized by the cyclooxygenase, CYP, to N-hydroxyarylamine and to nitroso compounds with glutathione as an important vehicle [14, 15, 35], with enzyme polymorphisms presumably having an effect. The short lifetime of isocyanate amines means that urinary sampling is often too late, limiting their applicability as a useful biomarker of recent exposure. To monitor longterm exposure, other biomarkers could be considered, with the measurement of DNA- and/or protein adducts offering promise. Novel industrial isocyanates may need modifications of the currently proposed methods for monitoring exposures, especially if they differ substantially from the usual chemical entities.
A major advantage of biomonitoring urinary metabolites is the provision of a measurement that reflects the total dose of isocyanates absorbed by the body by all routes. The simultaneous screening of the urine metabolites of aromatic, aliphatic and cycloaliphatic isocyanates enhances the probability of detecting previously unappreciated exposure. Using this method, we performed the biomonitoring of a group of 55 car industry workers and detected a high exposure to a totally unexpected isocyanate source, which proved to be a novel paint formulation recently introduced into the working process .