Automotive spray painters in the Puget Sound region of Washington State were recruited to participate in an exposure assessment study. Painters were recruited as described previously.⁽²⁴⁾ Recruitment, sampling, and data handling procedures were approved by the Washington State Institutional Review Board (WSIRB) at the Washington State Department of Social and Health Services, and by the Institutional Review Board in the Office of Human Research Ethics at the University of North Carolina at Chapel Hill. The exposure assessments were conducted between August 2006 and February 2007. A total of 33 painters were recruited from 25 autobody shops. One large shop had three painters, six shops had two painters each, and 18 shops had one painter each enrolled in the study. Although initially we aimed to sample all 33 painters on three separate occasions (i.e., a repeated measures study design), attrition was such that eight painters were monitored twice and one painter monitored only once. Painters were lost to the study due to shop closure, job change, or personal leave of absence. Information concerning the use of respiratory protection and other industrial hygiene data was recorded during each sampling visit. A summary of the dermal protection worn by painters in this study is reported by Ceballos et al.⁽²⁵⁾

Personal breathing-zone concentrations were measured whenever a painter applied an isocyanate-containing coating using a spray gun. To determine whether isocyanates were present in a product, the painter was consulted, container labels were reviewed, and material safety data sheets (MSDSs) were obtained prior to spray application. Isocyanate air sampling cassettes were made and analyzed by the Washington State Labor and Industries’ Laboratory. The dual-stage closed-face cassettes made in-house were identical to commercially available ISO-CHEK samplers (SKC Omega Specialty Division, Houston, Texas). The first (aerosol) filter (37-mm, 5.0 um polytetrafluoroethylene; PTFE, SKC Inc., Eighty Four, Pa.) was derivatized in the field with 1-(2-methoxyphenyl) piper-azine (MOPIP, 1.0 mg/mL; Sigma-Aldrich, St. Louis, Mo.) in reagent grade toluene and analyzed for polyisocyanates. The second (vapor) filter (37-mm glass fiber, SKC) was preimpregnated with 9-N-methylaminomethyl anthracene (MAMA; Sigma-Aldrich) and diluted in the analytical laboratory with reagent grade methylene chloride before being analyzed for isocyanate monomers. Air sampling was conducted at 2/L min airflow and was initiated prior to the painter entering the spray booth and stopped when the painter exited the spray booth. Research staff recorded the actual time spent spraying (i.e., paint time t), which was used to calculate isocyanate concentration during the spray painting task.

The analytical methods used were capable of chromatographically resolving derivatives of HDI and IPDI monomers and polyisocyanates and are additionally described by Ceballos et al.^(26–28) High-pressure liquid chromatography (1100 HPLC series; Agilent Technologies, Santa Clara, Calif.),with diode array detection and fluorescence detection, was used. Both the monomer and polyisocyanate analyses used a reverse-phase column at room temperature (hypersil ODS 5 μm, 200 mm, 2.1 mm I.D with guard column, Agilent). The chromatographic elution was different for the monomer and polyisocyanate analysis. The monomer derivatives were eluted with a flow of 0.3 mL/minute (mL/min) in two steps. In Step 1, 60% acetonitrile with 40% 215 mM triethyl amine (TEAP, pH 3) was held for 24 min. In Step 2, this was then ramped over 1 min to 70% acetonitrile with 30% TEAP and held for 23 min. The polyisocyanate derivatives were eluted in a gradient: initially 40% acetonitrile with 60% TEAP held for 14 min; then ramped slowly over 6 min to 50% acetonitrile/50% TEAP; then ramped over 8 min to 80% acetonitrile/20% TEAP; and then held at 80% acetonitrile/20% TEAP for 16 min. Finally, the program ended with a 2-min ramp to 40% acetonitrile with 60% TEAP.

The calibration standards are summarized in Table I. HDI and IPDI monomer commercial standards, as well as diluted HDI and IPDI polyisocyanate bulk products from Bayer Material Science (Leverkusen, Germany), were used in calibration. The analysis parameters such as limit of detection (LOD), limit of quantitation (LOQ), recovery, and precision for the analytes are summarized in Table I. The LODs were based on a detectable standard or spiked media sample that did not meet the LOQ requirements. The LOQs were based on spiked media with an average acceptable recovery of ≥75%. Overall recovery was expressed as an average of the percent recoveries established at five concentration levels having six replicates each. There was a bias for HDI uretdione recovery, which could result in overestimation of this species. While the source of the bias is unclear, it may be caused by the presence of impurities in the standards (bulk products) or by solvents used during sample preparation that affected the uretdione's response. The overall assay precision was calculated as 1.96 multiplied by the total coefficient of variation (CV).

Quality control was performed to ensure that the standard concentrations for HDI and IPDI monomer were within ±15% of the theoretical value. Calibration checks (three to five concentrations) were run before, during, and after all sample analyses. The number of field and laboratory blanks run per sample set was at least 10% of the total number of samples for each type of blank.

The analytical laboratory provided results in units of mass (μg) per filter for each isocyanate species. Following the procedures described by Hornung and Reed,⁽²⁹⁾ an analyte mass below the LOD or the LOQ was assigned surrogate values by dividing the respective limits by √2. Because the analytical method identified each individual isocyanate species, it was possible to calculate the percentage fraction of each species measured within a given sample. The total HDI polyisocyanate air concentration (Table II), expressed as μg/m³ time-weighted averaged (TWA) over 15 min, was calculated using Eq. 1: (1)MHDIIsocyanurate+MHDIUretdione+MHDIBiuretV×tT where M = mass, V = air-sample volume, t paint time, and T = min.

The Total HDI Polyisocyanate NCO and Total All Isocyanates NCO expressed as μg/m³ NCO STEL over 15 min (Table III) was calculated using Eq. 2: (2)∑i=15MiFiNCOV×tT where M = mass and F = NCO correction factor as follows:

M₁ (F_{1 NCO}) = HDI isocyanurate (F = 4.64)

Samples for which the paint time was greater than 15 min were not time weighted. If t >15 min, then the operation “t/T” was deleted from Equations 1 and 2. The paint time was an average of 40 ± 17 percent (range 8 to 87) of the total pump run time and lasted an average of 8.1 ± 6.3 min (range 1 to 53 min). The maximum use concentrations (MUC, Table II, and Table III) for the respirator worn were calculated by multiplying the applicable STEL by the respirator's OSHA-assigned protection factor (APF).⁽³⁰⁾

Booth ventilation was evaluated at the exhaust grills using a rotating-vane anemometer (Model RVA501; ALNOR Instruments, Huntington Beach, Calif.) with an accuracy of ±20 linear feet per min (LFM, or six linear meters per min, LMM). The exhaust grills were located on the floor of downdraft booths and on the walls of semi-downdraft and cross-draft booths. A minimum of nine measurements were taken in a transect pattern across the grill area, with the anemometer held approximately one inch from the exhaust grills. The total cubic feet per minute (CFM or Q) was calculated using the equation Q = VA for standard exhaust banks, where V = velocity and A = area of exhaust bank.⁽³³⁾ Two downdraft booths had slots in the floor with an aspect ratio (width/length) less than 0.2. The equation Q = 2.6 LVX, where L = slot length, V = velocity, and X = distance between the anemometer and the exhaust slot was used to calculate Q for these two booths.⁽³²⁾ The air changes per min (ACM) were calculated by dividing CFM by the room volume.

A total of 29 different isocyanate-containing products from 11 manufacturers were sampled. Clearcoat products were the most frequently sampled, representing 72% of the collected air samples. Single-stage products, in which a color basecoat is combined with the clearcoat, represented 20% of the samples. The remaining air samples were collected during the application of sealers (5% of samples), pigments (<1%), and basecoats (<1%). The most frequently used products were manufactured by PPG Industries (29% of the total) followed by DuPont (28%) and BASF (11%).

A total of 228 personal breathing zone samples were collected. Quantile-quantile (Q-Q) plots, Shapiro-Wilk tests, and skewness coefficients revealed that the distributions of all analytes were non-normal (p ≤ 0.05) and positively skewed. In 26% of samples, monomeric HDI was below the LOD (Table IV). In 92% of the samples, monomeric IPDI was below the LOD; consequently, air concentrations of monomeric IPDI were not calculated. HDI isocyanurate and IPDI polyisocyanate were present at the highest concentrations, with only 4% and 26% of the samples below their respective LODs. HDI uretdione and HDI biuret concentrations were below their LODs in 55% and 65% of the samples, respectively.

Ventilation was evaluated in 36 booths in 25 shops (Table VII). The majority of the booths were downdraft, with an average exhaust velocity of approximately 130 LFM (40 LMM) and volumetric airflow of approximately 8800 CFM (250 m³/min). Air velocity in the painter's breathing zone was not documented in this study. The International Fire Code and the ACGIH require that booths be maintained with an average air velocity over the open face of the booth or booth cross section at or above 100 LFM (40 LMM).^(32,33) Although all seven of the semi-downdraft booths met this requirement, 52% of downdraft booths and none of the cross draft booths met this requirement. Some booth manufactures specify booth performance in terms of air changes per minute, and the average ACM was 3.0 for the downdraft booths. Within-booth performance variability was evaluated over the course of this 7-month study; 22 of the 36 booths were evaluated on three separate visits. Four of these booths exhibited a coefficient of variation (CV) of less than 10% while the majority (N = 14) had a CV between 10 and 30% (Table VIII).

Because the analytical method used identified individual isocyanate species, it was possible to calculate the percentage fraction of individual isocyanates within a sample. The minimum percentage of IPDI polyisocyanate, HDI biuret, HDI uretdione, and HDI isocyanurate in a given sample ranged between ≈0 to 4%, while the maximum percent ranged from 58% to 96%. We conclude, therefore, that there is considerable variability in the possible isocyanate composition among the 29 paint formulations produced by the 11 manufactures investigated here.

While several of these individual isocyanates were predominantly measured at levels below their respective LODs or LOQs, they were measured occasionally at concentrations that exceeded the STELs. Further, as individual species, they were measured at concentrations that may have exceeded the respiratory protection used by the painter. Because HDI isocyanurate was the predominant species, it may be tempting to base exposure assessment on this species alone. However, the paint formulation analysis presented here identified two products very low in HDI isocyanurate but very high in HDI uretdione. Similarly, there was an inverse relationship between the presence of HDI isocyanurate and IPDI polyisocyanate. It is reasonable to expect that paint formulations will change over time and that not all the isocyanate species are recorded on the product's MSDS. Consequently, it is important to quantify all possible isocyanate species in the spray paint when conducting exposure assessment for these mixtures.

Very few samples contained HDI isocyanurate below the LOD. The within- and between-worker variability was calculated for this species and observed to be low, indicating that the spray painters were exposed to HDI isocyanurate in a homogenous manner. This homogeneity may reflect the fact that the samples were collected only during spray painting operations. The inclusion of other tasks such as paint mixing or vehicle taping would likely introduce more variability in the data.

The Wald-type statistic, based on measurements of HDI isocyanurate, was very high and indicates overexposure (i.e., the probability that a painter's mean exposure is greater than either the OR-OSHA or UK-HSE STELs is larger than the predetermined acceptable probability). Although identifying overexposure to HDI isocyanurate is valuable, all painters wore some type of respiratory protection, which must be incorporated into their personal exposure assessment. Simple counts and percentage calculations of the number of samples exceeding the STEL_15min or the respirator's calculated MUC were therefore described for all of the individual isocyanate species.

Exposures were compared with the two most applicable STELs: the OR-OSHA HDI polyisocyanate STEL_15min (1000 μg/m³) and the UK-HSE all isocyanate STEL_15min (70 μg NCO/m³). Applying the OR-OSHA STEL_15min alone to spray painter's exposure may not fully assess exposure risk. Exposure to HDI uretdione should be included in exposure assessment, even though this compound was not part of the toxicity testing that led to the establishment of the OR-OSHA STEL_15min for HDI polyisocyanates. Exposure to IPDI should be accounted for, as 15% of samples had concentrations of IPDI that exceeded the OR-OSHA STEL_15min.

The UK-HSE STEL_15min standard assumes equivalent toxicity between monomeric and polymeric isocyanates.⁽³⁴⁾ We noted that 98% of samples exceeded the UK-HSE STEL_15min for all isocyanates. Similarly, Woskie et al.⁽³⁵⁾ reported that 79% of HDI polyisocyanate exposures for spray operations in collision repair shops exceeded the UK-HSE STEL_15min. Applying the UK-HSE STEL_15min, which is based on the toxicity of the monomer, may not be appropriate for the data presented here because polyisocyanates are the predominant species, rather than monomers. Differences between polyisocyanates and monomeric isocyanates are discussed by Bello et al.⁽²³⁾ Unfortunately, the differences in toxicity are not well understood, in part because polyisocyanates have not been investigated as extensively as the monomers.

High breathing-zone concentrations of several individual polyisocyanates were documented, illustrating the need for polyisocyanates to be sampled comprehensively, analyzed, and compared with comprehensive OELs. A hypothetical comprehensive polyisocyanate OEL, expressed as μg total polyisocyanate NCO/m³, would address all of the possible polyisocyanates found in automotive paint formulations. The advantages of a comprehensive polyisocyanate OEL, expressed as total NCO, are (1) no prior knowledge of the individual isocyanates present in a formulation would be required (the isocyanate composition of a product may be proprietary and not available from MSDSs); (2) changes in paint formulations would be accommodated, and (3) investigators could still compare monomer concentrations with their respective monomer OELs. The greatest limitation to a comprehensive polyisocyanate NCO/m³ OEL is that the differences in toxicity between the individual polyisocyanates are not accounted for.⁽²⁴⁾ While this is a significant limitation, the alternative of developing OELs for all possible individual polyisocyanates would be challenging. The need for a comprehensive polyisocyanate OEL has been suggested previously by Bello et al.⁽²³⁾ and the results reported here demonstrate this necessity.

The majority of painters enrolled in this study (67%) wore either negative pressure half-face respirator (APRs) with replaceable filters or disposable half-face APRs. This observation is consistent with a Washington State industry-wide collision repair survey, in which 69% of shops reported that painters wore half-face APRs when spraying two-part clearcoats.^(10,11) Sparer et al.⁽²⁾ reported that 86% of Connecticut shops provided half-face APRs to spray painters. Using the OR-OSHA and UK-HSE STEL metrics, we conclude that 21% and 63% (respectively) of the painters in this study were not adequately protected against polyisocyanates when wearing half-face APRs having an OSHA APF of 10. The concentrations documented in this study compared with the OR-OSHA STEL_15min suggest that painters should use respirators with an APF of 25 or greater, such as full-face APRs, powered air-purifying respirators (PAPRs), or any supplied air respirators operated in continuous flow mode. Full-face respirators have the additional advantage of providing eye protection.

Liu et al.⁽³⁶⁾ used a PortaCount Plus respirator fit tester to calculate workplace protection factors and concluded that half-face APRs provided effective protection against isocyanates in properly trained and fitted workers. However, respirator training and fit testing is a challenge for an industry where only 25% report having contracted with a safety and health consultant.^(10,11) The use of SARs when applying auto paints has been recommended by NIOSH, the U. S. Environmental Protection Agency, and some paint manufacturers. Air-purifying half-face respirators are unlikely to protect against intense overspray conditions (we observed a two-person spraying operation in which painters were inadvertently spraying each other) or when painting large objects such as buses, boats, or trailers.⁽²¹⁾

Because respirators are not always worn and maintained properly, adequate exhaust ventilation is equally important as it can reduce the burden of exposure. While all seven of the semi-downdraft booths in this study met an average of 100 LFM exhaust velocity, only 52% of the downdraft booths and none of the four cross-draft booths met this target. The ventilation measurements presented here are discussed by co-authors in Fent et al.⁽²⁴⁾ and shown to be correlated with polyisocyanate breathing zone concentrations. Increased airflow and downdraft booths (vs. cross- or semi-draft booths) were associated with reduced polyisocyanate breathing zone concentrations. Gaines et al.⁽³⁷⁾ and Flack et al.⁽³⁸⁾ documented that the type of spray booth was a significant predictor of HDA level (1, 6-hexamethylene diamine, an HDI biomarker) in the urine and blood plasma of autobody spray painters, regardless of the painters’ choice of respiratory protection. Education and outreach are needed to improve respiratory protection and spray booth ventilation in the collision repair industry.

The strengths of this study include comprehensive sampling and analysis of all isocyanate-containing coatings used throughout a spray painter's workday, sampled up to three times over several months. While not all analytical methods identify specific isocyanate species, the analytical method described here identified all species currently known to be present in automotive paints, including IPDI. This allowed us to describe the percent fraction of individual species within the air samples.

Limitations of this study include the loss of subject follow-up due to the length of the study. In addition, because shops participated in this study on a voluntary basis, they may not have been representative of the industry as a whole. Participating shops may have used different paint lines or had different safety and health programs compared with non-participating shops. As mentioned previously, the prevalence of half-face respirators in this study population was nearly identical to that reported by Washington shops in a 2006 industry-wide survey.^(10,11)

In addition, the median (1) and average (1.4) number of painters per shop was very similar to that reported in the 2006 survey (median of 1 and an average of 1.8 painters).^(10,11) The paint products documented in this study were also similar to those reported in the 2006 industry survey. In these respects, we conclude that the study population was representative of all collision repair shops in Washington. The air sampling conducted here was specific to the task of spray painting and based on the spray time, which was 40% of the total sampling time. Concentrations based on total sampling time would be lower than those presented here. Exposures to isocyanates during other tasks, such as paint mixing, were not evaluated.