In this study, a previously published tape-strip sampling method (Fent et al., 2008
) was used to quantify dermal concentrations of individual polyisocyanates in automotive spray painters. Because quantitative dermal concentration and covariate data were obtained, LMM could be used to evaluate the fixed effects of covariates on dermal concentration while estimating within- and between-worker variance components via random effects. However, there were several limitations in this study that were taken into consideration when we evaluated the results.
First, the potential for selection bias exists because we used several different sampling schemes throughout the study (). However, arms and hands were sampled several times whether protective clothing was worn or not. As a result, the significant effect of protective clothing (gloves and coveralls) in the isocyanurate model () should be valid. Nevertheless, our exposure estimates may not be entirely accurate or comparable between and/or within painters. Second, >60% of the dermal concentrations were below the limits of detection for HDI, uretidone, and biuret. Consequently, statistical power was reduced for the analysis of this data. We restricted our analysis of the HDI, uretidone, and biuret data to unprotected workers in order to increase our statistical power, but much of this data (≥39%) still fell below the detection limits. As a result, multiple imputation was used for dermal concentrations below the detection limits in an effort to obtain better estimates of the parameters of interest (Lubin et al., 2004
). Other limitations are described in Part I of this series (Fent et al., 2009
Despite these limitations, the mixed models developed in this study described a considerable amount of variability (R2 ≥ 0.36) in dermal concentrations of isocyanurate in all 47 painters as well as dermal concentrations of HDI, uretidone, biuret, and isocyanurate in 15 painters who did not wear coveralls or gloves during spraying.
The product of analyte-specific BZC and paint time was the most significant variable in all the mixed models. The effect of this variable on dermal concentrations of polyisocyanates in painters who did not wear protective clothing can be seen in . Using the same product of analyte-specific BZC and paint time (e.g. 5.0 μg min m−3), the models in predict ~2, 10, and 17 times higher dermal concentrations of uretidone, biuret, and isocyanurate than HDI. Because HDI (0.05 mmHg at 25°C) exists partially as vapor in overspray, HDI may supply less exposure to the skin or evaporate off the skin. The oligomers, on the other hand, have relatively low vapor pressures (e.g. biuret 4.7 × 10−7 mmHg at 20°C). Therefore, any differences between predicted dermal concentrations of individual HDI oligomers are likely due to the different rates of skin absorption or chemical reactivity. Further investigation into dermal absorption and reactivity differences among polyisocyanates is warranted.
Although the products of BZC and paint time were able to describe much of the variability (32–60%) in dermal concentrations in painters who did not wear protective clothing, other variables (i.e. gun type and airflow) when included in the mixed models were able to increase the explained variability (36–68%). While these models were developed to describe the variability in unprotected painters, the mixed model described in was developed primarily to identify additional determinants of dermal concentration, as well as to evaluate the effectiveness of protective clothing used by painters in this study.
As expected, gloves and coveralls were significant predictors in the mixed model (). However, wearing a hood was not significant, possibly due to low statistical power, inadequacy of loose-fitting hoods for protection, and/or less intense overspray surrounding the face and neck compared to the arms and hands during painting. The variables related to material type, age, and thickness were not significant in the model, which may suggest that similar protection was achieved for the different types of protective clothing used by painters. However, the effects of material type, age, and thickness are likely to be subtle compared to the major protective effects of wearing coveralls and gloves, and as such, would be difficult to identify with LMM. Therefore, more controlled experiments are needed to fully evaluate the effectiveness of protective materials.
Sampler type (one stage versus two stage) was significant in the isocyanurate model (). The effect of two-stage sampling was discussed in Part I of this series (Fent et al., 2009
). Briefly, two-stage samplers are more likely than one-stage samplers to underestimate the BZC of isocyanurate due to the potential for polymerization on the untreated pre-filter. This finding is important as both one- and two-stage samplers are commonly used to sample atmospheres containing polyisocyanates.
In Part I of this series, we observed significantly higher (P
≤ 0.05) BZCs of HDI, biuret, and isocyanurate in WA than in NC (Fent et al., 2009
). However, in this study, we observed that painters in NC had significantly higher dermal concentrations of HDI, uretidone, and isocyanurate than painters in WA (). The mixed model for predicting dermal concentrations of isocyanurate in all painters (), which included the protective effect of coveralls and gloves, could not explain this difference (i.e. the effect of location was significant when added to the model). It is possible that climatic differences could be the cause of these differences. However, temperature and humidity were not significant variables in the mixed model (). In addition, the dermal sampling scheme we used differed depending on whether or not coveralls and gloves were worn. This could have biased the results by location since protective clothing was worn more frequently in WA than in NC. Further investigation is needed to determine the cause of the observed differences in dermal exposures to isocyanurate between painters in NC and WA.
To our knowledge, statistical modeling has not been used to investigate dermal exposure to polyisocyanates in the automotive refinishing industry. However, Brouwer et al. (2001)
developed a deterministic model for predicting dermal exposure to overspray in airless spray painters. The primary factors of this model were overspray generation rate, transmission of overspray, and aerosol deposition efficiency. These factors could not be measured directly in our study, but may be estimated by the variables in this study. For example, BZC may be representative of the overspray generation rate, airflow and booth type may be important factors in the transmission of overspray, and gun type, which influences the size of overspray particles, may affect the aerosol deposition efficiency. All these variables were significant in one or more of the models.
Use of high-volume low-pressure (HVLP) guns was associated with lower dermal concentrations, most likely due to the improved transfer efficiency of HVLP guns compared to the conventional guns. Increasing airflow was associated with decreasing dermal concentration, most likely due to the increased capture and removal of overspray from the painters’ personal space at higher airflows. It is important to note that transmission of overspray may be influenced by factors other than the airflow and booth type. These factors may not have been characterized in this study. However, it is possible that BZCs were measured in such close proximity to the painters’ skin that, in effect, transmission of overspray had occurred. Under this scenario, instantaneous BZC would be related to instantaneous dermal concentration by a factor related to aerosol deposition. Consequently, the product of BZC and paint time would be related to cumulative dermal concentration for the paint task, which is essentially what was estimated in this study.
The random effect of visit day was significant in the final isocyanurate model. This suggests that painters’ dermal exposures varied between visits due to factors not evaluated in this study, such as the size and orientation of the objects being painted, the busyness of the shop, and the condition of the work equipment during the sampling day. Further research is needed to identify the primary variables associated with the inter-visit variability. Nevertheless, the inter-visit variability observed in this study emphasizes the importance of sampling personal exposures at various times throughout the year in order to obtain the most representative exposure estimates.
Isocyanurate was the most abundant polyisocyanate collected from the skin whether or not coveralls and gloves were worn (). The reason for the higher levels and detection rate of isocyanurate in skin may simply be due to the greater abundance of isocyanurate in the atmosphere (GM = 1410 μg m−3) compared to the other analytes (GM ≤ 7.85 μg m−3).
For all the measured polyisocyanates, the highest dermal concentrations were in painters who sprayed in crossdraft booths. The isocyanurate model generated in this study predicted higher dermal concentrations for workers painting in semi-downdraft and crossdraft booths than for workers painting in downdraft booths. According to the findings in Part I of this series (Fent et al., 2009
), painters who sprayed in downdraft booths had lower BZCs than painters who sprayed in the other booths for all the measured polyisocyanates. Flynn et al. (1999)
observed that, depending on worker orientation, crossdraft booths may actually draw overspray across the painter's body. It is conceivable that this effect may also occur in semi-downdraft booths. Thus, the higher concentrations of polyisocyanates in the air coupled with the inability of the ventilation system to draw air away from the painters personal space may have led to higher dermal concentrations in painters who used crossdraft and semi-downdraft booths.
The results reported here are consistent with the tape-strip validation measurements previously reported (Fent et al., 2008
). Few investigators have quantified and reported exposure to polyisocyanates in human skin. Bello et al. (2008)
used wipe sampling to quantify dermal exposure (ng cm−2
) to TRIG in painters who did not wear protective clothing (GM = 1.9, GSD = 10.9, n
= 49 measurements) and in painters who wore coveralls (GM = 1.0, GSD = 3.2, n
= 3) and gloves (GM = 1.0, GSD = 5.2, n
= 17). After converting regional dermal exposure estimates (ng cm−2
) of individual polyisocyanates into estimates of TRIG for this study, it became clear that we measured higher levels of polyisocyanates in the skin of painters who did not wear protective clothing (GM = 50, GSD = 8.2, n
= 306) than in the skin of painters who wore coveralls (GM = 2.3, GSD = 7.4, n
= 487) and gloves (GM = 1.5, GSD = 8.0, n
= 314). Given the specificity of the analytical method, the polyisocyanates measured and reported here do not necessarily represent all the possible polyisocyanate species in automotive paint. For example, monomeric and polymeric isophorone diisocyanate, which is sometimes present, and polymers of HDI larger than isocyanurate were not quantified. Therefore, the actual TRIG concentrations are most likely to be higher than what we were able to measure with our analyte-specific liquid chromatography-mass spectrometry method. Nevertheless, compared to the wipe sampling method used in Bello et al. (2008)
, it appears the tape-strip method we describe has superior collection and quantification efficiency. Furthermore, the specificity of the analytical method provides a means to investigate individual monomeric and polymeric HDI concentrations in the skin. Tape-stripping is also the only method available to quantitatively measure polyisocyanate species in the non-viable skin layer, thus, providing an estimate of the absorbed dose.
This study provides a significant contribution to the characterization of the processes governing dermal exposures to individual polyisocyanates (HDI monomer and its oligomers) in automotive spray painters. Through LMM, we were able to identify the primary determinants of dermal exposure to monomeric and polymeric HDI. The mixed models developed related dermal concentration to the product of BZC and paint time. As a result, these models may be particularly useful for exposure reconstruction studies where information on BZC and paint time is readily available or can be estimated. However, further validation is necessary to determine the accuracy of these models. Although this study was able to demonstrate the effectiveness of the use of coveralls and gloves to reduce exposure, isocyanurate was detected in the skin of painters wearing coveralls and gloves for 93% of the paint tasks. This underscores the importance of reducing BZCs in the painting atmosphere. By reducing BZCs, the amount of overspray available for deposition will be reduced, thus providing less loading onto protective clothing and exposed skin. Moreover, this study describes exposure assessment tools to estimate the doses of individual polyisocyanates to the skin and lungs. This information may be used to investigate the roles of monomeric and polymeric HDI, as well as dermal and inhalation routes of exposure, in the development of respiratory sensitization and occupational asthma.