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  A CONTROL MATRIX FOR SPRAY PAINTING
AT AUTOBODY REPAIR SHOPS
BY

William A. Heitbrink


US Department of Health and Human Services
Public Health Service
Centers for Disease Control and Prevention
National Institute for Occupational Safety and Health
Division of Physical Sciences and Engineering.
4676 Columbia Parkway R5
Cincinnati Ohio 45226-1998


Report Date
May 1998

For submission as a Case Study to Applied Occupational and Environmental Hygiene.

Abstract. (Author's note - abstracts are not published in Dawn Tharr's Column in AEOH)

A control strategy for protecting autobody repair workers from polyisocyanate exposure is presented. A combination of a downdraft spray painting booth and an air purifying respirator can be an effective means of controlling worker polyisocyanate exposures. When autobody painting is done in crossdraft or semi-downdraft spray painting booths, the workers should use respirators with an assigned protection factor of 25 or greater. These recommendations are based upon empirical field studies and criteria to judge the applicability of these recommendations are stated. These recommendations assume that the companies using this exposure matrix have a respiratory protection program, a hazard communication program, and a program to ensure adequate flow rates in the spray painting booths. In addition, there is a need for the painting companies and the autobody repair industry to continuously collaborate in an effort to keep these recommendations current.

Introduction

Workers who spray paint cars apply a clear coat which contain polyols and hardeners that are prepolymerized forms of diisocyanates. Generally, these polyisocyanates are trimers of hexamethylene diisocyanate (HDI). Exposure to these polyisocyanates can cause skin and eye irritation, respiratory sensitization, asthma, and reduced lung function.1 Exposure limits for these polyisocyanates have not been addressed by Occupational Safety and Health Administration permissible exposure limits (PELs), American Conference of Governmental Industrial Hygienist's Threshold Limit Values (TLVs), or recommended exposure limits (RELs) developed by the National Institute for Occupational Safety and Health. Because of the reported health effects and the number of painters seeking medical attention, the State of Oregon promulgated permissible exposure limits for HDI- BASED polyisocyanates. This Oregon PEL includes an 8-hour time-weighted average of 0.5 mg/m3 and a short-term exposure limit of 1 mg/m3.2 This exposure limit is consistent with the exposure recommendations developed by a manufacturer of the HDI polyisocyantes.3 Between 1980 and 1990 two-thirds of the personal air samples for HDI polyisocyanates exceeded the Oregon STEL with a geometric mean of 1.6 mg/m3.4

During spray painting in autobody repair shops, workers are exposed to all of the paint components which are atomized. In addition to polyisocyanates, these constituents include organic solvents, metals such as lead and chromium, and trace quantities of HDI monomer. In a series of field evaluations of spray painting booths in the autobody repair industry, these exposures were generally found to be below exposure limits recommended by NIOSH, ACGIH, and OSHA.5, 6, 7, 8, 9, 10 However, in some shops, worker exposure to chromium did exceed the NIOSH REL for hexavalent chromium. The NIOSH REL for hexavalent chromium is 1 µg/m3. In this shop, the available material safety data sheets did indicate that the chromium compound had a valance of +6.

A study of control measures for paint overspray in autobody repair shops found that the type of spray painting booth and the choice of spray painting gun can be used to minimize worker exposure to paint overspray.11 Three types of spray painting booths are used in the autobody repair industry. These are downdraft booths, semi-downdraft booths and cross draft booths. The concept and operation of these booths are discussed in Figure 1. Previously reported measurements found that exposures to paint overspray in downdraft booths are lower than in the other two types of booths.5 In a downdraft booth the paint overspray was dispersed without flowing back into the worker's breathing zone. In a crossdraft booth and in semi-down draft booths, the paint overspray is dispersed into the incoming fresh air, causing the workers exposure to be elevated. As a result, geometric mean particulate overspray concentrations were 1.9 and 2.7 mg/m3 at two different downdraft booths versus 23 mg/m3 at a cross draft booth.

HVLP spray painting guns are more efficient than conventional spray painting guns. This can cause particulate overspray concentrations to be reduced by a factor of two and spray painting gun efficiency appeared to increase by 30%.12 As spray painting gun efficiency increases, the painter uses less paint and there is a more than proportionate decrease in worker exposure to particulate overspray.

During spray painting, painters are frequently exposed to excessive polyisocyanate concentrations.4 In addition, respirator usage in this industry is reported to be largely inappropriate.5 Due to concern mainly for worker health risks, the Environmental Protection Agency's New Chemicals Program has been regulating polyisocyanate components of automotive refinishing paints and coatings for a number of years. The end results have always been that regulation under the Toxic Substances Control Act essentially banned new polyisocyanates through strict regulation. Since automotive refinishing shops are generally small businesses with inadequate worker protection, an effort was made to address their exposure issues through a Product Stewardship Partnership. The partnership is a voluntary program which includes members from the following federal government agencies and industry: Environmental Protection Agency, Occupational Safety and Health Administration, National Institute for Occupational Safety and Health, National Paint and Coatings Association, National Automobile Dealers Association, Automotive Service Association, Automotive Service Industry Association, Collision Industry Conference, National Institute for Automotive Service Excellence, Inter-Industry Conference on Automotive Collision Repair, and individual paint manufacturers. An important element of this program is a control strategy matrix describing the type of respiratory protection to be used with specific spray painting booths.

This matrix was used to develop and specify a training program utilized by paint companies as part of their product stewardship program. As a practical matter, this matrix will also be used as an exposure assessment tool. The shop owner simply chooses the respiratory protection based upon the type of spray painting booth. Good industrial hygiene practice dictates that exposures are measured before respirators are used. However, there are about 50,000 autobody repair shops in the USA using commercially available spray-painting booths. These booths have one of three different designs. The OSHA respirator standard requires that the employer shall identify and evaluate the respiratory hazard and "this evaluation shall include a reasonable estimate of employee exposure to respiratory hazards."13 Thus, exposure assessments can be made at a limited number of booths and applied to the entire industry. Monitoring the polyisocyanate exposures in all of the spray painting booths would appear to be a practical impossibility.

The Matrix

The matrix presented in Table I is intended as a guide to compliance with manufactures recommendations for protecting workers adverse health effects attributed to polyisocyanate exposures during spray painting in autobody repair shops. The manufacturer has recommended that exposures to polysisocyanates be kept below 1 mg/m3 as a ceiling limit and 0.5 mg/m3 as an eight-hour time weighted average. Because most spray painting tasks require 10-20 minutes and the total painting time is generally less than four hours per day, the compliance this ceiling limit is the relevant hygienic issue. The matrix in Table I permits the use of air purifying respirators in properly operated down draft spray painting booths. In such booths, total particulate and polyisocyanate concentrations remained below 10 mg/m3. In a properly operated downdraft booth, the air flow around a car being painted averages 80 fpm with no point below 60 fpm. To do this, the total air flow in these booths needs to exceed 10,000 cfm.

When individual car parts are painted in such a booth, the part's orientation is critical. In downdraft spray painting booths, eddies can be formed near the walls of the booth as illustrated in Figure 2. When paint overspray enters such eddies, the paint overspray is transported to the top of the booth where it is mixed with the incoming fresh air. This can cause the worker's exposures to be elevated. In one study, this elevated paint overspray concentrations from a geometric mean of 5.6 to 14.9 mg/m3. To avoid this problem, parts need to be placed over the central exhaust and oriented so that the paint overspray is directed away from these cornices and toward the front and back of the spray painting booth (See Figure 3). The filters in the ceiling of the booth extend to the front and back wall, thus minimizing the formation of eddies which can transport contaminated air into the incoming fresh air.


The air purifying respirators used in downdraft spray painting booths need to be outfitted with prefilters mounted on cartridges for organic solvents. The prefilter is needed to protect the worker from particulate matter including polyisocyanates and other solids which may be present in the paint. The organic vapor cartridges are needed to protect the worker from solvent vapors and the traces of HDI monomer which is present in the paint. Experimentally, HDI monomer has been shown to experimentally not break through a respirator prefilter and organic respirator cartridges for a period of 25 hours of exposure to concentrations between 25 and 150 mg/m3 of paint mist.14

Table I specifies the use of respirators with an assigned protection factor of at least 25 when painting is done in a cross draft or semi-down draft booths with flow rates of at least 10,000 cfm. When painting the sides of cars in these booths, half the paint overspray is dispersed into the incoming fresh air. This causes paint overspray concentrations that were as a high as 25 mg/m3 for short term samples collected during painting tasks. In the crossdraft and semi-downdraft booths studied by Heitbrink and by McCammon5, 15, 16 polyisocyanate concentrations generally remained below 10 mg/m3. However, the amount of polyisocyanate may have been reduced to minimize the cost of the painting job as clear coats are primarily used on the more expensive cars. In this situation, the use of air purifying respirators is justified if monitoring shows that employee exposures are consistent with the appropriate use of air purifying respirators. In reviewing exposure measurements, one must realize that painting conditions vary from job to job, and one must make sure that respiratory protection decisions are based upon sampling during representative conditions.

Underlying Assumptions

The matrix in Table I is based empirical studies. As a result, one cannot extend the matrix beyond the conditions for which data was taken. The conditions for using the matrix are listed in Table II. These conditions address the paint application rate, the polyisocyanate content of the sprayed paint, and the spray painting booth. When the painting conditions are outside of the conditions defined in this table, exposure measurements are needed to select an appropriate respirator.

Painting Conditions

The first four specifications in Table II address the application of paint to the vehicles. During Heitbrink's study, paint application rates were measured at some shops and these paint application rates were under 175 g/min. This matrix assumes that one worker is painting a vehicle in a properly operated and maintained booth. Because the booths in this study were less than 8 feet high, the study findings do not extend to larger vehicles. The data for Heitbrink's study was taken primarily with passenger cars where roofs were rarely painted. When painting the sides of cars, the distance from the painted surface and the chin was at least 2 feet below one's chin. On vans, a worker may need to stand on a ladder to ensure a reasonable gap between the roof and the breathing zone. Only one worker is painting in the booth at a time. If more than one worker is painting, the worker's exposure could be elevated because paint overspray may be directed at a coworker.

Booth Conditions

During Heitbrink's study, the spray painting booths operated at flow rates between 6,000 and 12,000 cfm.5 Typically, these booths were 24' × 13' × 8' high. The maintenance of some of these painting booths was inadequate resulting in increased exposures. In using these booths, the matrix assumes that the booth is not grossly different from the booths studied. Furthermore, use of the matrix assumes that the booth flow rates and velocities comply with ACGIH standards for crossdraft and semi-downdraft booths (item 9 in Table II).17 Down draft booths are assumed to comply with a standard developed by the Institute National de Recherche et du Securite ( INRS, a French equivalent to NIOSH).18 This will require air flows of 10,000 cfm. As stated in item 8 in Table II is a translation of this standard from the French text in the standard. Furthermore, this matrix requires a maintenance program to ensure that these conditions are met.

The consequences of truck or bus painting are not addressed by this matrix. The spray painting gun needs to be kept below shoulder level and the paint should never be dispersed into the incoming fresh air. If large vehicles are painted from ladders or platforms, the painter should be able to avoid dispersing the paint into the incoming fresh air. In the absence of exposure monitoring data for painting oversized vehicles, one should assume that polyisocyanate concentrations are greater 10 mg/m3 and a respirator with an assigned protection factor of 25 or greater should be used.

HDI Monomer Considerations

The matrix presented in Table I assumes that the exposure to HDI monomer is less than 0.14 mg/m3 which is the short term exposure limit recommended by NIOSH.19 In the data collected by McCammon, 2 of 28 samples were positive for HDI monomer.7,8, 20 In McCammon's data set, 750 µg/m3 of monomer was measured in one sample. Because the trimer of HDI were not detected in this sample, this result is considered an anomaly. In Heitbrink's data, 2 of 35 samples were positive for the monomer. Based upon a 20 minute sample and a detection of limit of 0.4 µg per sample, the detection limit for HDI monomer would have been 13 µg/m3. In a summary of Oregon OSHA data reported by Janko et al, the geometric mean HDI monomer concentration in all autobody refinishing operations was 0.02 mg/m3 and the maximum HDI monomer exposure was 0.3 mg/m3. In a summary of isocyanate exposures in the transportation after market, Meyer et al reported a geometric mean HDI concentration of 0.03 mg/m3 and 95 percentile value of 0.14 mg/m3 for 35 samples.3 Thus, the available data, suggests that exposure to HDI monomer is below the NIOSH ceiling of 140 micrograms/m3 (0.02 ppm).

The reason for the low HDI monomer exposure is the low concentration of HDI in the hardener. The monomer is present in the pure hardener at concentrations less than 0.2% (Biuret can be as high 1.6% due to storage at high temperature). In the paint mixture sprayed on the car, the hardener and polyol are mixed together in a ratio of one to at least two. Thus, the HDI monomer concentration is only 0.067% of the total mass of nonvolatile components. Assuming a maximum particulate concentration of 10 mg/m3 in a properly operated downdraft booth, the maximum HDI monomer concentration in the liquid aerosol is 6.67 µg/m3 and for the biuret, this concentration could be as high as 50 µg/m3.

The low monomer concentration also depresses the saturation vapor pressure of HDI monomer. The mixture which is sprayed is about 50% nonvolatile paint components and the remaining components are volatile organic solvents. In ideal solutions, the saturation vapor pressure of a dilute component is the product of the component's saturation vapor pressure and the mole fraction of the component in the liquid phase. 21 For the polyisocyanate which is widely used in the autobody repair industry, the estimated saturation vapor phase concentration of the HDI monomer is about 0.2 mg/m3. Based upon manufacturer's material safety data sheets, this trimer resin has a maximum HDI monomer content of 0.2%. If formulation involving higher HDI monomer contents such as the HDI biuret, monitoring is needed to evaluate whether hygienically significant exposures to the monomer could occur.

Thus, the low HDI monomer content of the polyisocyanate resin explains the fact that the HDI monomer exposures measured on workers are generally less than 0.14 mg/m3, the short term exposure limit recommend by NIOSH. A low monomer content reduces the saturation vapor pressure of the HDI monomer in the paint solution and the liquid phase concentration in the spray droplets. Thus, item 10 of Table II specify a maximum content of HDI monomer.

Managerial Programs

The matrix presented in Table I assumes that the autobody repair shop has a respirator program, a ventilation maintenance program, and a hazard communication program. The respirator and hazard communication programs are required by OSHA standards.22, 23 An effective respirator program is needed to ensure that the respirators are properly maintained and used. An effective hazard communication program is needed so that shop owner can review the hazards posed by the paints and evaluate whether this matrix is appropriate for their autobody repair shop. The ventilation maintenance program is needed to ensure that the booth operates at the assumed air velocities and flow rates specified in Table II. Booth flow rates need to checked every six months. A hazard communication program is needed to ensure that the paint solids do not involve components with exposure limits below 1 mg/m3 as 15 minute short term exposure limit and a 0.5 mg/m3 8-hour time weighted average.


Conclusions

Furthermore, the paint companies and the autobody repair industry need to form a partnership to maintain and update the information basis for this matrix. In the future, spray painting conditions change and technology will advance. Undoubtedly, these changes will affect the painter's exposure. To ensure the appropriateness of the recommendations contained in Table I, there is a continuing need to routinely evaluate exposures in actual body shops.

Because it is impractical and expensive to collect air sampling data under all possible conditions, exposure modeling for workers performing spray painting is needed. Assuming spray painting gun efficiencies are known and paint application rates are either known or are measurable, computational fluid dynamics can be used to model and predict the worker's exposure to paint overspray.24, 25 Such a model would need to be experimentally verified. Once verified, such a model could be used to examine various painting scenarios for exposure impact to identify factors which greatly increase or decrease exposure. This could lead to further experimental data and a refinement of the recommendations contained in Table I.

Figure 1

Figure 1. The three different styles of spray painting booths in the autobody repair industry.

Figure 2

Figure 2. A downdraft spray painting booth with cornices. Eddies from under the cornice in this booth.

Figure 3

Figure 3. Orient parts so that paint overspray is directed at the front side of the booth. If the paint overspray is directed at the space under the cornice, an eddy with transport noticeable quantities of overspray into an eddy which will transport the overspray into the incoming fresh air.

Table I. Control Matrix for Painting in Autobody Repair Shops
 
Condition type of booth gun respirator
painting car in booth downdraft with an average air flow around car of 80 fpm and no point with an air flow less than 60 fpm. hvlpa APFb> 10 (e.g. half face piece air purifying or better)
painting car parts of painting car in booth semi downdraft; crossdraft hvlp, conventional APF> 25 (e.g. supplied air continuous flow, powered air purifying, etc.)
painting car parts that are not attached to car downdraft, paint overspray directed at front or back of booth. Hvlp APFb> 10 (e.g. half face piece air purifying or better)
a hvlp high volume low pressure spray painting gun. A spray painting gun with an atomization

Table II. Physical Dimensions to ensure adequacy of matrix.
 
  dimension specification
1 paint application rate under 150 g/min of paint with a solids content of 50%
2 Polyisocyante content of applied paint solids Under 33%
3 number of painters one painter
4 minimum distance between painted surface and workers chin 2 feet
5 booth flow rate 10,000 - 14,000 cfm
6 booth size approximately 12' × 25' × 8' ft high
7 cornice for illuminating parts no more than 1 foot on each side of the booth
8 criteria for air velocity around a car in a downdraft booth.
(INRS):		 The air velocity around the perimeter of a car is to be measured at 10 points. Three points are on the side of each car and two are next to the front and rear of the car. These measurements are taken 0.5 meters (m) from the side of the car and 0.9 m above the booths floor. The mean value of these points is to be greater than 0.4 m/sec and no point is to have a velocity less than 0.3 m/sec. These measurements are based upon integrated 60 second samples.
9 Criteria for crossdraft and semi-downdraft spray painting booth flow rate 100 cfm/ft2 of cross sectional area. When width times height is greater than 150 ft2, the criteria is 50 ft/ft2.
10 HDI monomer content of paint HDI monomer content of the sprayed liquid shall be less than 0.2% of the polyisocyanate. If the monomer content is greater than 0.2%, the shop must either show the workers exposure to HDI monomer remains below the NIOSH REL or have the workers use supplied air respirators in the spray painting booth.

References
  1. Tornling, G., Alexandersson, R, Hedenstierna, G, and Plato N. (1990). Decreased Lung Function and Exposure to Diisocyanates (HDI and HDI-BT) in Car Repair Painters: Observation On Re-Examination 6 Years After Initial Study. American Journal of Industrial Medicine 17:299-310.

  2. Oregon Occupational Safety and Health Division: Interim this is the Oregon PEL check it out.

  3. Myer HE, ST O'Block, V. Dharmarajan: A Survey of Airborne HDI, HDI-Based Polyisocayanate and Solvent Concentrations in the Manufacture and Application of Polyurethane Coatings. Amer. Ind. Hyg. Assoc. J. 54(11):663-670.(1993)

  4. Janko M., McCarthy K., Fajer, M, Raalte, J. (1992). Occupational Exposure to 1,6 Hexamethylene Diisocyanate-Based Polyisocyanates in the the State of Oregon, 1980-1990. Am. Indl Hyg. Assoc. J. 53(5):331:338

  5. Heitbrink WA, Cooper TC, Edmonds MA, Bryant CJ [1993]. In-Depth Survey Report: Control Technology for Autobody Repair and Painting Shops at Blue Ash Autobody Shop, Blue Ash, Ohio. U.S. DHHS, PHS, CDC, NIOSH, NTIS Pub. No. PB-93-215838.

  6. Heitbrink WA, Cooper TC, Edmonds MA, Bryant CJ, Ruch W [1993]. In-Depth Survey Report: Control Technology for Autobody Repair and Painting Shops at Valley Paint and Body Shop, Amelia, Ohio, May 6, 11-15, June 8, and October 15, 1992. U.S. DHHS, PHS, CDC, NIOSH, NTIS Pub. No. PB-93-216190.

  7. Cooper TC, Heitbrink WA, Edmonds MA, Bryant J, Ruch WE [1993]. In-Depth Survey Report: Control Technology for Autobody Repair and Painting Shops at Jeff Wyler Autobody Shop, Batavia, Ohio, June 16-19, and July 21, 1992. U.S. DHHS, PHS, CDC, NIOSH, NTIS Pub. No. PB-93-216182.

  8. Heitbrink WA, Cooper TC, Edmonds MA [1993]. In-Depth Survey Report: Control Technology for Autobody Repair and Painting Shops at Cincinnati Collision Autobody Shop, Blue Ash, Ohio, July 27-30, 1992. U.S. DHHS, PHS, CDC, NIOSH, NTIS Pub. PB-94-118361.

  9. Heitbrink WA [1993]. In-Depth Survey Report: Control Technology for Autobody Repair and Painting Shops at Team Chevrolet, Colorado, Springs, Colorado, December 8-11, 1992. U.S. DHHS, PHS, CDC, NIOSH, NTIS Pub. No. PB-94-151677.

  10. Heitbrink WA, Fischback TJ, Edmonds MA [1994]. In-Depth Survey Report: Evaluation of Spray Gun Technology for Occupational Exposure to Auto Paint Shop Hazards at DeVilbiss Automotive Refinishing Products, Maumee, Ohio, September 14-19, 1992, and July 6-8, 1993. U.S. DHHS, PHS, CDC, NIOSH, NTIS Pub. No. PB-94-194669

  11. Heitbrink, W., Wallace M,, Bryant C., and Ruch W: Control of Paint Overspray in the Autobody Repair Shops. Am. Ind. Hyg. Assoc. J. 56(10)1023-1032. (1995)

  12. Heitbrink WA, Verb RH, Fischbach TJ, Wallace ME: A Comparison of Conventional and High Volume -Low Pressure Spray-Painting Guns. Am. Ind. Hyg. Assoc. J. 57(3):304-310 (1996)

  13. Title 29 Code of Federal Regulations Part 1910.134(subpart d 1 ii) Respiratory Protection. January 8, 1998.

  14. Vasta JF: Respirator Cartridge Evaluation for Isocyanate Containing Imron and Centrari Enamels. Am. Ind. Hyg. Assoc. J. 46(1):39-44 (1985).

  15. National Institute for Occupational Safety and Health: Health Hazard Evaluation Report 95-0406-2609, Matrix Autobody, Englewood Co. Authors C. McCammon and B Sorenson. NIOSH, Cincinnati Ohio.

  16. National Institute for Occupational Safety and Health: Health Hazard Evaluation Report 95-0312-2593, Martin's Car Star Inc, Lakewood. Authors: C. McCammon and B Sorenson. NIOSH, Cincinnati Ohio

  17. American Conference of Governmental Industrial Hygienists (1995). Industrial Ventilation - A Manual of Recommended Practice. 22nd edition. Cincinnati Ohio.

  18. Institut National de Recherche et du Securite: Guide Pratique De Ventilation-9. Ventilation Des Cabines et Postes de Peinture (Practical Ventilation Guide - 9: Ventilation of Paint Stations and Cabins). 1992. Paris France: INRS, 1992. P16 (French)

  19. National Institute for Occupational Safety and Health: NIOSH Pocket Guide to Chemical Hazards. DHHS (NIOSH) Publication No. 97-140. NIOSH, Cincinnati Ohio, 1997.

  20. National Institute for Occupational Safety and Health: Health Hazard Evaluation Report 95-0405-2600, Spence's Car Star Denver Colorado. NIOSH Cincinnati Ohio

  21. Denbigh K: The Principles of Chemical Equilibrium. 2nd edition. Cambridge University Press: Cambride UK. (1966)

  22. Title 29 Code of Federal Regulations Part 1910.134. Respiratory Protection.

  23. Title 29 Code of Federal Regulations Part 1910.1200. Hazard Communication.

  24. Flynn MR and GN Carlton: Exposure Models for Contaminant Control Decisions Involving Ventilation. In: Ventilation '97: Global Developments in Industrial Ventilation. Proceedings of the 5th International Symposium on Ventilation for Contaminant Control Ottawa Ontario Canada, September 14-17, 1997. pp 15-19

  25. Rota R, G Naon Scarra, P Torricelli, MA Citteriao, C Sala: Analysis of Painting Booths: Experiments and CFD Simulations. In: Ventilation '97: Global Developments in Industrial Ventilation. Proceedings of the 5th International Symposium on Ventilation for Contaminant Control Ottawa Ontario Canada, September 14-17, 1997. pp 85-96.
 
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