Background concentrations obtained from the shop surveys were in line with both observations and knowledge of the fabrication shop operations with the overall background levels attributable to the reliance on dilution ventilation in a high particle-generating environment. The significant differences between shops is in agreement with the asymmetric distribution of workload, machinery, and manpower. Shop B/A produces the largest structures that require higher weld deposition levels; has the most experienced staff, and houses the welding robot, which may explain its higher geometric mean in relation to production shops C and D. The detail shop houses virtually all of the CNC/PC machinery, which has been shown to generate a substantial amount of particulate. Shop E levels reflect the denser employee spacing, lower ceilings, and older ventilation system. The manual detail section of shop D, which is referred to in Figure 6
, is where many particle-generating tasks occur, including tacking, grinding, and oxyacetylene cutting. Operationally, it has many employees working in close quarters, which is afforded by the comparatively small size of the workpieces; the consequence is that exposure to particulate generated by adjacent activities is more likely than in other areas. For these reasons, the manual detail section has been selected for ambient air sampling. In general, our findings on high background levels are in agreement with previous studies done in welding training facilities [11
] and various workplaces [6
Manual task findings showed that grinding generated more respirable particulate than welding, and similar particulate levels to oxyacetylene cutting, which is in agreement with other findings showing thermal cutting exceedances over welding [11
]. Particle concentrations obtained in our study for SMAW, FCAW, and MCAW are in the same range of what was observed in previous studies [6
The depressed numbers seen with the welding robot, burner table, and milling machine are reasonable; the welding robot positions the material about five feet from the ground, and uses the MCAW process, requiring all welds to be performed in the flat (1G/1F) position. Therefore, the material being welded creates a floor, and the thermal vectors direct the particles upward, which is away from the operator in this case. The milling machine drills holes with a very slow rotational speed; the slower the drilling speed, the lower (and larger) the particle generation [21
]. In addition, the use of copious amounts of cutting fluid acts to contain generated particles. The burner table is a water table-type cutting machine, which is designed to mitigate particle release.
Analogously, the CNC plasma generated similar particulate levels as the beamline. That the beamlines, which perform the same task, were significantly different is likely due to the unequal number of samples per process (i.e., sawing versus drilling) that was taken for each machine. Other possible explanations include machine model (size/specifications) differences and machine setup (position of the operating station relative to the processes) differences. We found no difference between welding processes, which deviates from the literature [22
]. This may be attributable to the similarity in profiles below one micron—which is the size limit of the P-Trak—between SMAW, FCAW, and GMAW [22
]. It may also be due to the conservative nature of non-parametric analyses, and/or the difficulty in sampling SMAW welding. Grinding, gouging, and oxycutting produced higher particle levels than any welding tasks, therefore putting fitters and helpers at risk for exposure to metal particles.
In welding fume, vaporized metal condenses, forming oxide particles that are primarily of respirable size, and often in the ultrafine range [19
]. Welding fume is composed largely of consumable rather than base metal [23
], although fume constituents are affected by welding parameters, including shielding gas, polarity, potential difference, and current [2
]. For example, concentrations of particles during metal active gas welding (MAG–aka GMAW) were shown to increase with increasing amperage and/or increasing CO2
in the gas mixture [24
]. Exposure is also dependent on the relative position to the weld, the volume of space, and ventilation [23
]. For example, monitors distanced vertically from the weld had orders of magnitude greater deposition than monitors distanced horizontally [25
], as thermal vectors channel particles upwards; also, welders in confined spaces have poorer lung function than those welding in open air [2
]. The welding parameters used at the fabrication shop are quite uniform, with direct current electrode positive (DCEP) polarity and a 3:1 argon:CO2
gas mixture held constant. Also, consumable Safety Data Sheets (SDS) show that FCAW and MCAW electrodes are largely indistinguishable, citing manganese, iron oxide, fluorspar (CaF2
), and amorphous silica as hazardous constituents [17
]. In this fabrication shop, welders using the MCAW process may have a higher exposure potential due to the flat (1G/1F) position required with this consumable; beyond this, welders are likely to experience similar exposures.
The SDS for SMAW electrodes indicate that they contain quartz silica in minute quantities [26
], potentially posing a risk to fitters; however, worksite observations showed that welding is both brief and sparse. Therefore, between the low amount found in the consumable and the limited volatilization, it would seem highly unlikely that quartz silica exposure would exceed the Time-Weighted Average (TWA) of 0.025 mg/m3
]; however, without occupational sampling, this remains speculation. The SDS for the grinding discs were uninformative about whether phenolic resin particulate is of greater concern than nuisance dust [14
]. There is some evidence that phenolic resin dusts can impair lung function [28
]; therefore, it merits investigating how much abrasive dust exposure exists for welders, trade helpers, general helpers, and fitters.
When generating SEGs, it is important to remember that exposure is dependent on a number of factors, including process, material used, ventilation, and intermittency of work [20
]. The welding processes, mechanical and thermal cutting methods, and abrasive polishing techniques all generate particles of concern. Therefore, the most hazardous task is not necessarily the greatest source of exposure. An example of this is with fitters, who do weld, but do so infrequently and discontinuously. Therefore, the majority of exposures likely come from mechanical and thermal cutting, which is an exposure profile that they share with trade helpers and, occasionally, general helpers. All of the thermal cutting methods generate metal oxides of the base metal [29
]. Plasma cutting, and the grinding and drilling of mild steel all create a significant amount of respirable particulate, with 42% of particulate generated below 2 µm, and grinding and drilling producing a full third and 20% below 1 µm, respectively [30
]. This presents a challenge, as plasma table operators are likely to have a similar amount of exposures as other beamline operators, and yet are exposed to the oxides of metal, rather than elemental metal, and oxidation states do affect toxicodynamics [32
]. In practice, the Alberta Occupational Health and Safety Code does not distinguish between elemental and oxides of iron or manganese [27
] which makes mechanically and thermally generated metal exposure comparable, and allows plasma drill and beamline operators to belong to the same SEG.
In contrast to welding fumes, particulate from grinding tends to remain close to the ground, as there are no thermal vectors to displace them upwards [34
]. Thus, the beamline operations may only be a regional hazard and contribute less to the ambient particulate found in the shop. It is our opinion that CNC/PC stations do not require near field air sampling, because operators’ personal samples would act as a surrogate, based on work practices. The same logic could be applied to the workers in the manual detail section of shop D, and shop E workers. However, as they perform a great number of manual tasks, it would be wise to distinguish between ambient and task exposures. Thus, both shop D and shop E should undergo ambient sampling to address the survey data observed in shop D, and the overall particulate level in shop E. Finally, to address concerns voiced by the shippers and the occupational nurse about the sandblasting abrasive spreading beyond the blast bay, near field air sampling should be undertaken in the shipping section of shop C, directly east of the wall separating the blast bay from shop C, as well as the paint shop. Finally, in our view, there is no need to perform near field air sampling in the main building shops, because repurposing personal air samples from positions classified as indirect ambient would be more representative of the overall exposure, as determined by worksite observation.
There are limitations to this study, since the P-Trak is uninformative of the true nature of the particles being counted. The instrument itself functions well in comparison to larger condensation particle counters when used indoors; while counts differ, the correlation between the P-Trak, the Scanning Mobility Particle Sizer (SMPS), and mini-disc particle counters is considered good [35
]. The P-Trak can underestimate freshly emitted particles from combustion sources, and underestimates counts at the low end (<40 nm) of the detection range [35
]; primary welding particles are generally lower than 50 nm in diameter, although agglomerates form with all welding processes [36
]. Therefore, the measurements taken, including welding, oxyacetylene, and plasma cutting, may be underestimates of actual particle exposure. With regard to shop comparisons, although thousands of data points were logged, they result in minutes of total sampling time per day. Consequently, it would be premature to consider the ranking of shop particle concentrations found here as conclusive, and gravimetric comparison is recommended. However, the use of the instrument was useful in determining final exposure groups, and presented a fast and cheap way to estimate exposure levels. The likely chemical composition of particles can easily be determined from work descriptions and field observations.
4.2. Similar Exposure Groups and Monitoring Program
Similar exposure groups are simply groups of workers sharing similar exposures and frequencies; the classification serves to create a larger pool from which to sample. Preferred candidates are the positions with the most ideal fit into their respective class, which require the least amount of verification to ensure they are appropriately classified on a given sampling session, and have been listed here for illustrative purposes. The complete employment roster is assigned in the program supplied to the fabrication shop (not shown). Final SEGs are presented in Table 4
, and include a range of particle concentrations, as well as the type of exposure for each group.
Exposure classification was determined as (1) Very Low to Low Exposure, with negligible exposure to mixed particles (group 1); (2) Medium Exposure, with exposure between 10,000 and 70,000 part/mL to either mixed (group 2) or metal particles (group 3); (3) High Exposure, with exposure above 70,000 part/mL to metal particles (group 4); and (4) Specific Exposure, which includes spray painters (exposure to solvents, group 5) and sandblasters/descaling operators (exposure to crystalline silica, group 6). Preferred candidates were determined as follows:
group1: Production Manager, Corporate Maintenance Manager, General Foreman, Building Maintenance Operator, Mechanics, Shipping Clerk, Yard Man, Truck Driver, Mod Yard Crane Operator
group 2: Lead Hand/Foreman and Ironworker
group 3: Electrician and Shipper
group 4: Welder, Fitter, Trade Helper, CNC/PC Operator
group 5: Spray Painter
group 6: Sandblaster
It is recommended to collect personal exposure samples from preferred candidates from groups 2 to 6. In addition, area samples should be collected from the shop C shipping area and paint shop to monitor crystalline silica from sandblasting operations, as well as the manual detail section of shop D and shop E, due to their high particulate levels.
It is preferable to perform sampling six to 10 times yearly for a minimum of eight hours, since shifts are 10 hours long. Due to their high exposure or exposure to very toxic chemicals, groups 4 to 6 should be sampled 10 times a year to ensure monitoring of the worst case scenario. Groups 2 and 3 can be monitored less frequently (i.e., six times a year). Note that the exposure groups can be refined once regular sampling is performed, since this work gave us only broad estimations of exposure. Refining SEGs from both a between-group and within-group perspective allows for more tailored exposure interventions and, eventually, reduced sampling requirements. Samples collected in the shipping area of shop C and from the sandblaster/descaling operator should be analyzed for crystalline silica. Samples collected in the paint shop and from the painter should be analyzed for crystalline silica, metals, and solvents to account for exposure from blasting bay, paint, and background metal particles. All other samples should be analyzed for metals and particulate.
The facility does not have any local exhaust ventilation, and uses only general dilution ventilation. Currently, the company relies upon PPE to reduce exposure to hazardous particles. The sandblaster is required to wear a helmet-style supplied air respirator (SAR); the spray painters are required to wear half-face air-purifying respirators (APRs) with gas/vapour cartridge filters; and the welders are required to wear a half-face air-purifying respirators (APRs) with P100 (oil proof HEPA) filters. However, the PPE policy does not mandate that other personnel wear any respiratory protection. Our recommendations are to have all members of SEG group 4 wear a half-face air-purifying respirators (APRs) with P100 filters. It was observed that the sandblaster would remove his helmet immediately after abrasive blasting; it ought to be policy that he or she would not be allowed to remove the helmet while in the blast bay. Although installing a local exhaust ventilation system (LEV) would be very costly, it might be interesting for the company to investigate using portable extractors for welding fumes, which should help reduce the background levels in the shops.