مطالعه ارگونومیک در نقاشان وقرار گرفتن اسکلت عضلانی درمعرض نقاشی با اسپری دستی در صنعت نجاری

In the present study the ergonomic conditions of manual spray painting in the woodworking industry have been studied. Manual spray painting is, despite rationalisations, quite common in the Swedish woodworking industry. Interviews with spray painters showed that they had higher prevalence of musculoskeletal symptoms in their right shoulder compared with other workers with manual work. A majority of the spray painters painted work-pieces lying on a work-table. A majority of these painters abduct the right upper-arm so much when painting the horizontal surface of the work-piece that they risk supraspinatus tendinitis. The upper-arm abduction when painting horizontal surfaces could be decreased without introducing new ergonomic disadvantages by installing work-tables with powered height control or possibly also by changing the geometry of the spray gun. Gripping the spray gun trigger was for some of the painters identified as causing a high risk for WMSD in the wrist. Lower spray gun trigger force would improve the situation. This could be done by the users in several ways, such as greasing the trigger mechanism and/or decreasing the spring pressure on the fluid needle. Relevance to industry The study highlights some major ergonomic problems for a fairly large group of workers in the woodworking industry.

A number of statistic reports (Andersson, 1995; Swedish National board of Occupational Safety and Health (ASS (1997a), Swedish National board of Occupational Safety and Health (ASS (1997b) and Swedish National board of Occupational Safety and Health (ASS (1997c)) show that the ergonomic conditions are worse in the woodworking and sawmill industry and musculoskeletal symptoms are more common among the workers in this sector, than among workers in most other sectors of Swedish working life. A study of the ergonomic situation in the Danish woodworking and sawmill industry (Christensen et al., 1995) concluded that there was a high prevalence of symptoms from the musculoskeletal system, a high rate of repetitive work with short cycle time, a high rate of manual material handling, and that it is important to identify and quantify critical exposure. Similar data from other countries are missing but the equipment used is mainly the same all over the world, so the working condition may be expected to be the same all over the world. An important task in the woodworking industry is manual spray painting. The Swedish woodworkers union has estimated that approximately 3000 persons have manual spray painting as their main task (approximately 8% of those employed in the sector). Many of them paint flat work-pieces. They either paint both large flat surfaces and edges, or just edges. In the first case the painting is done with the work-piece lying on a work-table and the work-pieces are stored in a drying-rack (in between the painting sessions). In the second case the work-pieces are stacked in piles, and the whole stack is painted on the same occasion. The painters who do not paint flat work-pieces often paint work-pieces which are standing on a work-table or hanging from a conveyor. The spray guns used are either high-pressure, low-pressure, combined, high-volume low-pressure (HVLP) or electrostatic spray guns. High-pressure spray guns create the aerosol through pumping the paint with high pressure through a nozzle, while low-pressure spray guns create the aerosol through spraying compressed air against the paint outside of the nozzle. Combined and HVLP spray guns are combinations of these two methods. Electrostatic spray guns use an electrostatic field to transport the paint to the work-piece. Every method has its advantages in terms of initial costs, productivity, air pollution, etc. The occupational hygiene aspects of manual spray painting have been extensively studied (Bittár et al., 1995), the ergonomic conditions much less so. However, ergonomic studies exist concerning manual spray painting in the engineering industry (Lückner, 1992; Munck-Ulfsfält et al., 1984), and in other occupational groups with similar ergonomic conditions – e.g. welders – musculoskeletal symptoms in the upper extremities constitute a serious problem (Herberts and Kadefors, 1976; Herberts et al., 1981). The aims of the present study were to investigate the prevalence of musculoskeletal symptoms among spray painters, document musculoskeletal exposure when performing manual spray painting, identify hazardous exposure, identify possible relationships between musculoskeletal symptoms and exposure, document the design of workstations and identify possible ways to rearrange the workstations in order to decrease the exposure.

The regions where the spray painters had a higher prevalence of symptoms than the reference material (woodworking and construction-metal workers) were the right shoulder (significant), elbow and hand/wrist (not significant). This over-representation of symptoms may well be related to the spray painting, which in most cases was performed with the right hand only. Upper-arm abduction was identified as highly strenuous for 12 of the 21 assessed painters. The shoulder symptoms may in some cases be supraspinatus tendinitis. A study (Bjelle et al., 1981) has shown that workers with shoulder disorders (to a large extent supraspinatus tendinitis) in median abducted and/or flexed the upper-arm above 60° for about twice as long (about 1 h/working day) as those workers who did not have shoulder disorders. Studies (Järvholm et al (1988a), Järvholm et al (1988b) and Järvholm et al (1990)) have shown that the intramuscular pressure in the supraspinatus muscle (and possibly also to the tendon) is so high that the blood flow is restricted above 30° abduction. Decreased blood flow to the supraspinatus tendon may in combination with mechanical load on the tendon lead to cell death in the tendon (Hagberg et al., 1995), which in its turn may trigger an inflammation. Since the intramuscular pressure and the EMG activity in the supraspinatus muscle are at least as high at 30° abduction as at 60° flexion (Järvholm et al., 1990), the EMG activity in the supraspinatus muscle at 30° abduction resembles about 74–85% of the activity at 60° abduction (Järvholm et al., 1989), and hand activity when the arm is elevated has been shown to increase the EMG activity in supraspinatus (Sporrong, 1997), one may assume that 30° abduction of the upper-arm 1 h/working day with a load in the hand, doing hand activities, without any possibility to unload the arm/hand may cause supraspinatus tendinitis. Personal allowances (e.g. visiting the rest room, basic fatigue) in general sum up to about 9% of working time (Niebel, 1988), and allowances for the work conditions may in the present work sum up to 2–10% in addition to interruptions for discussion of social or work matters, adjustments to equipment, etc. An educated guess is that 15–30% of the time nominally spent on spray painting is taken up with other things. Twelve of 16 spray painters in the work-table group abducted the upper-arm on average more than 30° while painting the upper side of the work-piece. If these spray painters actively spray-painted 70% of the time during which they were nominally occupied with this task, 10 of them would abduct the upper-arm more than 30° for more than 1 h/working day. Thus the upper-arm abduction when painting horizontal surfaces may cause supraspinatus tendinitis among these 10 spray painters. The reason for the abduction of the upper-arm in the work-table group was probably that the work-tables were too high. This forced the painter to abduct the upper-arm so as to establish a proper distance between the surface of the work-piece and the spray gun nozzle to get a proper surface. In addition, they had to abduct the arm more to compensate for the length of the gun and the vertical distance between the crook of the arm and the back of the gun. Nine of these 10 painters who risked supraspinatus tendinitis had less than 25 cm vertical distance between the crook of the free arm and the upper side of the work-piece. Only two out of 13 spray painters who had less than 25 cm vertical distance, did not abduct the upper-arm on average more than 30° when painting the horizontal surfaces, which should be compared to two out of three who had more than 25 cm vertical distance. It is obvious that if the work-tables were lowered, the painters have to decrease the arm abduction, otherwise the distance between the outlet of the spray gun and the work-piece will be too large. The main reason for not lowering the work-table was that if it was low they had to bend forward when spray painting the edges of the work-piece and also when unloading the work-table. A work-table with powered height control would allow the painters to paint the horizontal surface of the work-piece (low position) and also paint the edges and unload the work-table (high position) with comfortable body postures. Normally, they first paint the horizontal surface, then all four edges and then they unload the work-table. This implies that they would only need to adjust the height once during the work cycle. The delay would probably not introduce such a big delay that it would disturb the painters, since between painting the horizontal surface and the edges they even today have a small delay. The delay is caused by the change of posture between the two activities. As a complementary measure the lightest possible spray guns and hoses should be chosen. The angle between the handle and the upper part of the spray gun is generally between 100° and 110°. This angle allows the spray painter to optimise wrist position when painting perpendicular to vertical surfaces. For horizontal surfaces, however, this angle forces the painter to ulnar-deviate the wrist more and/or to abduct the upper-arm more (compared to working at e.g. 90°), to aim the gun at the work-piece. For spray painters who paint a work-piece which lies on a work-table, a spray gun with 90° angle between the handle and the upper part would be advantageous, since it would decrease the upper-arm abduction when spray painting against horizontal surfaces and thus decrease the risks of supraspinatus tendinitis. However, with such an angle, the spray painter may have to hold the wrist slightly radial-deviated when painting vertical surfaces at elbow height or lower. But since they paint horizontal surfaces during longer periods than they paint edges this would probably not be a big disadvantage. However, lowering of the upper-arm under 30° abduction does not eliminate all the musculoskeletal symptoms in the shoulder. A work task such as spray painting demands continuous activation of the stabilising muscles in the shoulder to maintain hand position accuracy. Prolonged activation of these muscles even at low levels may be a risk factor for myalgia in the rotator cuff muscles involved, primarily the upper trapezius muscle (Hägg, 1991; Veiersted and Westgaard, 1993). All of the painters in the work-table group and the conveyor group performed the work-task spray painting for at least 1 h/working day, but they were not asked whether they performed the work-task continuously for at least 1 h/working day. However, at least some of the painters in the work-table group and the conveyor group did perform spray painting continuously for 1 h or more every working day. Thus for at least some of these painters gripping of spray gun trigger caused a high risk for WMSD in the wrist. External force (i.e. trigger force) as well as other parameters which may indicate that the task causes a very high risk for WMSD were not assessed. However, spray gun trigger force has in other studies (Björing and Hägg, 2000; Lee et al., 1997) been shown to be high, the gripping of the trigger has to be rapidly done, carrying the spray gun induces additional load on the wrist and there is a high qualitative demand on output. Therefore, the task may very well cause a very high risk for WMSD in the wrist. The easiest way to improve the situation would be to lower the trigger force of spray guns. There are several ways in which users can decrease the trigger force. The first is to grease the mechanism inside the gun and the second is to decrease the spring pressure on the fluid needle. If there is a leakage of paint because the needle pressure is too low, or if the flow gets too high, one may decrease the pressure from the pump/barrel. Another possibility is to check that the nozzle set is correct for the paint and the desired flow and/or mount an external fluid regulator. The spray painting activity was not identified as causing risks for low-back disorders. However, spray painting against the bottom of the stack was identified as causing the most uncomfortable and strenuous work postures by a large majority of the spray painters in the euro-pallet group. The situation when spray painting against the bottom of the stacks, would be improved if the stacks were raised from the floor. This could easily be achieved through, for instance, stacking the work-pieces on top of three to four European standard pallets instead of one. Loading drying-racks was identified as causing the most uncomfortable and strenuous work postures. However, loading drying-racks is a complementary activity and is therefore not discussed here. Some people in the woodworking industry claimed that manual spray painting is an attractive work task. Therefore, it is possible that spray painters with musculoskeletal symptoms continue to work with the same work task for longer than other workers with the same symptoms. Thus, despite more ergonomic workstations and hand tools future spray painters may still have higher prevalence of musculoskeletal symptoms in their dominant arm compared to other groups of workers with manual work.