Journal of Occupational and Environmental Hygiene, 3: 194–203 ISSN: 1545-9624 print / 1545-9632 online DOI: 10.1080/15459620600584352 Design, Construction, and Characterization of a Novel Robotic Welding Fume Generator and Inhalation Exposure System for Laboratory Animals James M. Antonini, Aliakbar A. Afshari, Sam Stone, Bean Chen, Diane Schwegler-Berry, W. Gary Fletcher, W. Travis Goldsmith, Kurt H. Vandestouwe, Walter McKinney, Vincent Castranova, and David G. Frazer Health Effects Laboratory Division, National Institute for Occupational Safety and Health, Morgantown, West Virginia Respiratory effects observed in welders have included lung function changes, metal fume fever, bronchitis, and a possible increase in the incidence of lung cancer. Many questions remain unanswered regarding the causality and possible underlying mechanisms associated with the potential toxic effects of welding fume inhalation. The objective of the present study was to construct a completely automated, computer-controlled welding fume generation and inhalation exposure system to simulate real workplace exposures. The system comprised a programmable six-axis robotic welding arm, a water-cooled arc welding torch, and a wire feeder that supplied the wire to the torch at a programmed rate. For the initial studies, gas metal arc welding was performed using a stainless steel electrode. A flexible trunk was attached to the robotic arm of the welder and was used to collect and transport fume from the vicinity of the arc to the animal exposure chamber. Undiluted fume concentrations consistently ranged from 90–150 mg/m3 in the animal chamber during welding. Temperature and humidity remained constant in the chamber during the welding operation. The welding particles were composed of (from highest to lowest concentration) iron, chromium, manganese, and nickel as measured by inductively coupled plasma atomic emission spectroscopy. Size distribution analysis indicated the mass median aerodynamic diameter of the generated particles to be approximately 0.24 µm with a geometric standard deviation (σg ) of 1.39. As determined by transmission and scanning electron microscopy, the generated aerosols were mostly arranged as chain-like agglomerates of primary particles. Characterization of the laboratory-generated welding aerosol has indicated that particle morphology, size, and chemical composition are comparable to stainless steel welding fume generated in other studies. With the development of this novel system, it will be possible to establish an animal model using controlled welding exposures from automated gas metal arc and flux-cored arc welding processes to investigate how welding fumes affect health. Keywords inhalation exposure, robotic welder, welding fume Address correspondence to: James M. Antonini, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, 1095 Willowdale Road, M/S 2015, Morgantown, WV 26505; e-mail: jga6@cdc.gov. 194 E lectric arc welding joins metals and alloys that have been made soft or liquid by extreme heat as electricity passes from one electrical conductor to another.(1) Welding processes generate a complex mixture of aerosol and gaseous byproducts. The generated fumes are composed of an array of metals (e.g., iron, manganese, chromium, nickel) volatilized from the welding electrode or the flux material incorporated within the electrode.(2) Gases (e.g., ozone, carbon monoxide) can originate during welding operations due to the use of shielding gases in gas metal arc welding (GMAW), the composition of fluxes in shielded manual metal arc welding (SMAW) or flux-cored arc welding (FCAW), and chemical reactions and ultraviolet irradiation of atmospheric elements and contaminants by the arc.(3) Over the last 30 years, numerous studies have investigated the health effects of welding fume inhalation.(4) Because welders are not a homogenous group, the potential adverse effects of welding fume exposure are oftentimes difficult to evaluate. Differences exist in welder populations, such as industrial settings, the type of welding processes and materials used, ventilation, and other occupational exposures besides welding fumes (e.g., solvents, asbestos, silica). Reviews of epidemiology studies have indicated that many welders have experienced some form of pulmonary illness.(5–8) Respiratory effects have included transient lung function changes, metal fume fever, bronchitis, an increased susceptibility to infection, and a possible increase in the incidence of lung cancer. Even less information is available about the nonrespiratory effects (e.g., neurological, dermatological, reproductive) of welding fume exposure. Recent studies indicate that welding fume exposure may lead to neurological disorders.(9,10) Thus, many questions remain unanswered regarding the causality and possible underlying mechanisms associated with the potential toxic effects of welding fume inhalation. The use of animal models and the ability to control the welding fume exposure in toxicology studies would be helpful Journal of Occupational and Environmental Hygiene April 2006 in developing a better understanding of how welding fumes affect health. Because of the diversity of the welding process and the need to continually generate welding fumes at a reasonably constant concentration over extended periods, very few welding fume inhalation exposure systems have been developed. Early studies have developed welding fume inhalation systems for rats, but experienced welders were required to operate the system(11,12) and the exposures were for only limited periods of time, such as 2–3 hours(11) or 6 hours.(12) In addition, exceedingly high concentrations (400 mg/m3 , 580 mg/m3 , and 1178 mg/m3 ) have been used.(11) An important drawback to most of the previous systems was that as the welding electrodes were consumed and the base metals piece (the surface where the welding occurs) quickly become covered with the newly formed weld, the base metal piece had to be continually changed and replaced every several minutes by laboratory technicians, disrupting the exposure period. Thus, the generation of constant welding fume for extended periods of time at reasonable fume concentrations has proven to be very difficult and has required numerous technicians to operate the fume generation system. It was our objective to construct an automated fume generation and inhalation exposure system that would simulate real workplace exposures and would allow for continuous welding for extended periods of time without interruption. An automated system is needed that would use a computer-controlled, robotic welder that would weld and replace materials as they are consumed during the operation. System operation, reproducibility of the exposure, study of dose-response relationships, and comparison of toxicity of different types of welding fumes would be easier with such a system. This article describes the development of a novel robotic welding fume generator and inhalation exposure system and the physical and chemical characterization of the generated fume. MATERIALS AND METHODS Description of Welding System and Process The welding fume generation system comprised a welding power source (Power Wave 455; Lincoln Electric, Cleveland, Ohio), an automated, programmable six-axis robotic arm (model 100 Bi; Lincoln Electric), a water-cooled arc welding torch (WC 650 amp; Lincoln Electric), a wire feeder that supplied the wire to the torch at a programmed rate up to 300 inches/min, and a automatic welding torch cleaner to keep the welding nozzle free of debris and spatter. For the initial fume characterization studies, gas metal arc welding was performed using a stainless steel electrode (Blue Max E308LSi wire; Lincoln Electric). Welding was performed on A36 carbon steel plates for varying periods of time depending on the analysis at 25 V and 200 amps. During welding, a shielding gas combination of 95% argon and 5% CO2 (Airgas Co., Morgantown, W.Va.) was continually delivered to the welding nozzle at an air flow rate of 20 L/min and over the arc to protect the formed weld from weakening caused by oxidation. Design and Construction of Welding Fume Generation System A diagram of the robotic welding fume generation system is depicted in Figure 1. The system can be divided into four different areas: (1) enclosed control room (Figure 1A); (2) robotic welding fume generator (Figure 1A); (3) animal exposure chamber (Figure 1B); and (4) fume characterization equipment (Figure 1B). The enclosed control room is used for programming and monitoring the operation and performance of the welding system. The room contains the welding power source for the robotic welder and holds the cylinder of shielding gas that is used during the GMAW process. The shielding gas is delivered to the robotic welder with a hose. The room is separated from the robotic welding system by an airtight divider made of aluminum with ultraviolet-protective glass doors that filter UV light from the welding system to protect the investigators in the control room. The welding process is continuously monitored from this room visually through the glass doors. During the welding operation in previously designed welding fume exposure systems, the surface of the base metal piece where the welding occurred quickly became covered with the newly formed weld and needed to be replaced.(11−14) To avoid disruption of exposure to welding fume in a toxicology study, a headstock was designed for the current welding system that holds and rotates a base metal plate holder in different programmed positions (Figure 2). The base metal plate holder has four sides and holds three metal plates per side on which the welding takes place. Because welding arc temperatures can exceed 4000◦ C, it was necessary to devise a system to minimize the excessive amount of heat generated during the process in order to perform long-term exposures. A computer-controlled water circulation unit was designed by National Institute for Occupational Safety and Health (NIOSH) engineers and included within the base metal holder to reduce the temperature at the base metal surfaces where the welding takes place (Figure 2). Excessive heat production at the surface could possibly lead to warping or disfiguration of the base metal pieces that, in turn, could disrupt the arc and result in altered morphology and size of the generated welding particles. Also, to maintain exposures over long periods of time, a programmable torch cleaner was added to the system to automatically and periodically clean the welding gun so that it remained free from debris and spatter to allow the automatic wire feeder to function properly without interruption during the welding process. Exposure Chamber Fume and Gas Determinations A flexible trunk has been positioned approximately 18 inches from the arc to collect the generated fume and transport it the exposure chamber. The generated welding fume was mixed with dry air that was HEPA-filtered and scrubbed with charcoal. Continuous records of chamber fume concentration, temperature, and humidity were maintained during welding fume generation. The generated aerosol was collected onto 37-mm Teflon filters at a rate of 1 L/min, and the particle mass Journal of Occupational and Environmental Hygiene April 2006 195 FIGURE 1. Diagram of the welding fume generation system including: (A) enclosed control room that contains the welding power source and controller [1] and robotic welding fume generator that contains the six-axis robotic arm, wire feeder, torch cleaner, coolers, and base metal holder [2]; (B) animal exposure chamber [3] and fume and gas characterization devices [4] delivered to the exposure chamber was determined gravimetrically. In addition, particle samples were periodically collected gravimetrically onto filters for scanning electron microscopy (SEM) and grids for transmission electron microscopy (TEM) to assess particle size distribution, particle morphology, and elemental composition. Gas samples were withdrawn from the exposure chamber through Teflon tubing with a protective particulate filter in the line during the period of welding, and ozone (ozone analyzer model #450; Advanced Pollution Instrumentation, Inc., San Diego) and carbon monoxide (1312 Photo-acoustic Multi-Gas Monitor; Innova Air Tech Instruments, Ballerup, Denmark) were measured. To maintain welding fume concentrations during animal exposures, fume was collected through an aerosol delivery line above the welding system at a flow rate of 5 L/min drawn 196 from an in-line peristaltic pump. Downstream from the pump, a mass-flow controller was installed as an air dilution system. Dilution airflow rate was adjusted by a solenoid valve regulated using a feedback signal to provide a desired mass concentration in the exposure chamber. The mass concentration in the chamber was monitored in real time by a realtime aerosol monitor (model DR-2000, DataRAM; MIE, Inc. Bedford, Mass.), and the obtained mean electrical signal was compared with a precalibrated signal according to the desired concentration. Based on the difference between the two signals, a signal was fed back to regulate the solenoid valve and adjust a dilution airflow to the desired concentration in the exposure chamber. Depending on the desired concentration, the diluent air in this system was normally controlled between 20 and 80 L/min. Journal of Occupational and Environmental Hygiene April 2006 FIGURE 2. Schematic diagram of the base metal holder and heat exchanger. A programmable headstock was designed that holds and rotates the base metal plate holder in a desired position. The unit holds three steel base metal plates on each of four sides (12 base metal pieces total) on which the welding occurs. Cold water that is circulated through the base metal holder controls the temperature of the base metal plates during extended periods of welding. Welding Particle Morphology and Characterization Inductively Coupled Plasma Atomic Emission Spectroscopy Stainless steel welding particles were collected onto 5.0 µm polyvinyl chloride membrane filters in 37-mm cassettes during 5–30 min of welding. The particle samples were digested and the metals analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) according to NIOSH method 7300 modified for microwave digestion.(15) Briefly, the filters were digested in a microwave in the presence of nitric acid. After digestion, the samples were allowed to cool, transferred to 50-mL flasks, and then diluted with H2 O. The samples were analyzed using a Fisons Accuris inductively coupled plasma emission spectrometer (Fisons Instruments, Beverly, Mass.). Metal content of blank filters were analyzed. A trace amount of iron was measured to be above the limit of quantification (LOQ) in the blank filter and was subtracted from the iron measurements of the collected samples. MSP Corp., Shoreview, Minn.) that is intended for general purpose aerosol sampling, and a Nano-MOUDI (MSP model 115) that is specifically designed for sampling aerosols in the in the size range down to 0.010 µm. By combining the two MOUDI impactors, it was possible to collect particles in the size range from 0.010 µm to 18 µm that were separated into 15 fractions. Besides the special feature of using microorifice nozzles to extend the cut sizes of the lower stages down to 0.010 µm, the MOUDI consists of rotating stages to obtain a nearly uniform particle deposit on the impaction plates, which can avoid particle bounce and is useful for subsequent analysis. RESULTS Characterization of Generated Welding Fume The robotic welder (in combination with the programmable headstock) can be programmed for continuous welding at a rate of 30 cm/min for up to 8 hours/day. Undiluted fume concentrations consistently ranged from 90–150 mg/m3 in the animal exposure chamber during welding. Proposed fume concentrations (1.0–40 mg/m3 ) to be used for animal exposure experiments can be easily attained with air dilution. In a preliminary experiment, welding was performed for 3 hours/day for 3 consecutive days at a desired fume concentration of 40 mg/m3 (Figure 3). Fume concentrations were determined every 30 min during 3 hours of operation for each day and were observed to be near the desired chamber concentration. Mean chamber fume concentration and standard deviation for the 3-day period were 38.4 ± 6.7 mg/m3 . Temperature and humidity remained constant in the chamber during the 3 hours of welding over the 3-day period Scanning Electron Microscopy Welding fume samples were collected onto 47-mm Nuclepore polycarbonate filters (Whatman, Clinton, Pa.). The filters were cut into four equal sections; two sections were mounted onto aluminum stubs with silver paste. The deposited welding particles were viewed using a JEOL 6400 scanning electron microscope (JEOL, Inc., Toyko) and also analyzed using energy dispersive X-ray analysis (SEM-EDS; Princeton Gamma-Tech, Rocky Hill, N.J.) at 20 keV. Transmission Electron Microscopy Welding fume samples were collected at 30-min intervals during 3 hours of welding directly onto bar-coated TEM grids and viewed using a JEOL 1220 transmission electron microscope. Particle Size Distribution Particle size distribution was determined by using a MicroOrifice Uniform Deposit Impactor (MOUDI, MSP model 110; FIGURE 3. Day-to-day variation in exposure chamber fume concentrations after welding for 3 hours/day for 3 days. Desired fume concentration was intended to be maintained at 40 mg/m3 . Mean chamber fume concentration and standard deviation for the 3-day period were 38.4 ± 6.7 mg/m3 . Fume was collected through an aerosol delivery line above the welding system at a flow rate of 5 L/min, and dilution airflow rate was maintained at 80 L/min. Journal of Occupational and Environmental Hygiene April 2006 197 TABLE I. Metal Composition of Generated Stainless Steel Welding Fume Metals Analyzed µg/Sample Weight% of Metals A Iron (Fe) Chromium (Cr) Manganese (Mn) Nickel (Ni) Copper (Cu) 1207.0 ± 323.3 427.5 ± 138.2 295.0 ± 96.8 185.0 ± 47.9 3.3 ± 0.9 57.0 ± 2.6 20.2 ± 3.0 13.8 ± 0.9 8.8 ± 0.4 0.2 ± 0.0 Note: Values are means ± standard deviation; n = 4 welding collection periods of 30 min. A Relative to all metals analyzed. Trace amounts of silicon, aluminum, and vanadium also were present. FIGURE 4. Day-to-day variations in exposure chamber temperature after welding for 3 hours/day for 3 days. Mean temperature and standard deviation for the 3-day period were 20.9 ± 1.4◦ C. (Figures 4 and 5). Mean temperature and percent relative humidity and standard deviations for the 3-day period were 20.9 ± 1.4◦ C and 37.7 ± 2.7%, respectively. Ozone and carbon monoxide concentrations also were measured in the chamber during the welding process (data not shown). Low amounts of ozone (0.041 ± 0.019 ppm) were formed in the chamber during the 3-day period. Ozone levels generated during welding were not significantly higher than background levels (∼ 0.025 ppm) and were lower than the NIOSH recommended exposure limit (REL) and OSHA permissible exposure limit (PEL) of 0.1 ppm.(16) Carbon monoxide levels also were not significantly higher than background levels and were lower than the NIOSH REL (35 ppm) and OSHA PEL (50 ppm).(16) FIGURE 5. Day-to-day variations in exposure chamber relative humidity after welding for 3 hours/day for 3 days. Mean relative humidity and standard deviation for the 3-day period were 37.7 ± 2.7%. 198 The generated welding aerosols from the robotic welder were characterized to ensure that they were similar to the fume examined in other studies. To quantify the amount of each metal present, ICP-AES was performed after collecting bulk particle samples onto filters during 30 min of welding (Table I). Nearly all of the materials in the fume generated during GMAW were derived from the stainless steel electrode that is consumed during the process. The welding particles were composed of (in descending order of amount present) iron, chromium, manganese, nickel, and copper. Trace amounts of silicon, aluminum, and vanadium also were present. Particle collection for different periods of time (5–30 min) and on different days had no effect on the bulk metal profile of the fume (data not shown). Because of the significant number of nanosized particles generated during welding, particle size distribution was determined using a combination of MOUDI and Nano-MOUDI samplers (Figure 6). It was observed in these initial characterization studies using the MOUDI systems that the size distribution of the aerosol generated by the robotic welder may be tri-modal (Figure 6). The most significant mass of particles was in the fine size range with cutoff diameters of 0.10–1.0 µm. Additional ultrafine particles in the range of 0.010–0.10 µm as well as larger, coarse particles with diameters from 1.0– 10 µm in size also were observed. Initial calculations indicate the mass median aerodynamic diameter to be approximately 0.24 µm with a geometric standard deviation (σg ) of 1.39 when considering only the fine size range (0.10–1.0 µm). The mass fraction of the three modes varied slightly from day to day during welding. The primary mode (0.10–1.0 µm) contained 85–95% of the total mass collected, whereas the other two modes contained less than 5–10% each. Depending on operating conditions of welding and fume generation, the large particle mode likely varied with respect to the amount of spatter produced. The small particle mode may have been affected by conditions such as the amount of diluting air and relative humidity in the aerosol delivery line. TEM and SEM images of stainless steel welding fume aggregates are shown in Figures 7 and 8, respectively. Most of the aerosols generated during GMAW were arranged in homogeneous, chain-like agglomerates of primary particles Journal of Occupational and Environmental Hygiene April 2006 FIGURE 6. Particle size distribution of stainless steel gas metal arc welding fume comparing mass concentration versus particle size as measured using the MOUDI and Nano-MOUDI impactor systems. (Figures 7 and 8A). Larger, more spherical particles, referred to as microspatter, were sometimes observed among the agglomerates (Figure 8B). It is possible to use SEM-EDS analysis to determine the elemental distribution of different welding particles. Elemental mapping can be performed on particles collected from different stages of the MOUDI impactor system. For example, metal distribution of different particles that had aerodynamic diameters between 1.8 and 3.2 µm was determined (Figure 9). The metals were observed to be complexed together but distributed heterogeneously when comparing different particles. The metal distribution of particles 1 and 2 were very similar with iron and chromium being the most significant elements (Figure 9B, C), whereas manganese, in addition to iron and chromium, was the most significant element present in particle 3 (Figure 9D). DISCUSSION L ong-term toxicology studies of welding fume are limited in number. Most welding fume toxicology studies have been short term and have used the technique of intratracheal instillation to administer the particles to animals.(17–21) There are several limitations when using the intratracheal instillation method for welding fume toxicology studies. For this procedure, welding fume is collected onto filters, suspended in an aqueous medium, and delivered directly into the lungs of animals. From the time of particle collection and animal treatment, the biological reactivity of the collected fume may be attenuated due to the aging of the particles. Our group has shown that freshly formed welding fume had a concentration-dependent increase in surface free radicals and induced more lung inflammation and injury compared to aged welding fume.(22) Also, exposure to gases (e.g., ozone, carbon monoxide) formed during welding that may affect respiratory health is absent when using intratracheal instillation. Another concern of the instillation method is that much higher concentrations of the particles are usually administered compared with the particle concentrations that may be inhaled by workers over time. This may overload the lungs with particles and overwhelm normal lung clearance mechanisms.(23) The associated lung toxicity may then be a result of particle overload and not the inherent chemical or physical property of the particle. A completely automated system is needed that would perform welding operations and replace materials as they are consumed during the process. Yu et al.(13,14) developed a semiautomated welding fume generation system for exposing laboratory animals to SMAW processes. A circular stainless steel base metal disk powered by an electric motor was slowly rotated, while a second pulley system advanced a welding rod toward the metal disk until an arc was formed. Welding fumes were generated as the disk rotated and the welding rod slowly advanced. The procedure was repeated continually at 5-min duty cycles to process each rod; 3 min to move it forward and be consumed to generate fumes, and 2 min for the holder to be Journal of Occupational and Environmental Hygiene April 2006 199 FIGURE 7. Representative transmission electron micrographs of stainless steel gas metal arc welding fume. (A) Note the many primary particles that are in the nanosize range (<0.10 µm) that form the chain-like agglomerates; bar is equal to 0.20 µm. (B) Lower magnification of an aggregation of chain-like agglomerates. Note the variation in size of the primary particles that form the agglomerates; bar is equal to 0.50 µm. FIGURE 8. Representative scanning electron micrographs of stainless steel gas metal arc welding fume. (A) Note the aggregation of chain-like agglomerates. (B) Larger, more spherical particles (center of the image) were sometimes observed among the agglomerates. Bar is equal to 1 µm. 200 Journal of Occupational and Environmental Hygiene April 2006 FIGURE 9. SEM micrographs of three different stainless steel welding particles (1, 2, 3) collected on one stage of the MOUDI impactor system; SEM-EDS analysis of particles 1 (B), 2 (C), and 3 (D). Elemental mapping of particles indicated that the metals were complexed together but distributed heterogeneously among the different particles. returned and another welding rod installed. Thus, the exposure was continually disrupted every few minutes to replace the used materials (e.g., consumed welding rods). The system developed by Yu et al.(13,14) may be appropriate for SMAW processes where the changing of the electrodes would closely simulate real work place practices. However, this system would not be appropriate in the study of more automated GMAW and FCAW processes. Saito et al.(24) designed an entirely automated robotic welding fume and inhalation exposure system. Animals were exposed for 2.5 hours to welding fume concentrations that varied from 34.2–64.5 mg/m3 depending on the welding current. The welding robot delivered the fume in eight repeated cycles that consisted of welding for 6 min with a pause for 9 min. A transient increase in breathing frequency with a concomitant decrease in tidal volume were observed within several minutes immediately after the start of the welding operation. Unfortunately, no long-term exposures were performed. In addition, the system did not allow for the replacement of the materials that were consumed during the process. It was the goal of the current study to design and construct a welding fume generation and inhalation exposure system that would perform long-term exposures without disrupting the welding process. The welding fume generation system constructed for the study utilizes an automated, programmable six-axis robotic arm and a wire feeder that automatically supplies the wire to the torch at a programmed rate. The system constructed by our group will closely mimic more automated welding processes, such as GMAW and FCAW. Welding exposures can be maintained for up to 8 hours. During the welding operation, the surface on which the welding takes place becomes covered with the newly formed weld and needs to be replaced. To avoid disruption of the welding fume exposure, a headstock was designed that holds and rotates a base metal plate holder in different programmed positions. The base metal plate holder has four sides and holds three metal plates per side where the welding occurs. As one base metal plate becomes covered with the newly formed weld, the robotic welding arm can be programmed to move to the next base metal piece. By having 12 base metal pieces that can be programmed to rotate in any position, a sufficient amount of surface area is provided to maintain the welding process for an extended period of time. The chemical composition of the generated stainless steel welding fume was determined by different analytical methods, ICP-AES and SEM-EDS. Nearly all of the materials in the particulate phase generated during GMAW were derived from the stainless steel electrode that was consumed during the process. Both methods indicated that the particulate was composed of mostly iron and chromium with significant amounts of manganese and nickel present. Small amounts of copper, silicon, and vanadium also were measured in the samples. Other characterization studies have observed similar elemental profiles for stainless steel welding fume generated during GMAW.(19,22,25–27) The specific metals in stainless steel welding fumes that are of toxicological importance include chromium and nickel, two known carcinogens, and manganese, a neurotoxic agent. The toxicological outcome of welding fume inhalation exposure is dependent on the bioavailability of the metals present in the particles. It has been observed that stainless steel welding fumes produced during GMAW are relatively insoluble as opposed to particles which are generated during SMAW and Journal of Occupational and Environmental Hygiene April 2006 201 FCAW processes that use fluxing agents.(26,28) Stainless steel GMAW particles have been observed to persist in the lungs for long periods of time in a mostly unchanged state.(29−31) X-ray diffraction studies have indicated that iron and chromium in stainless steel GMAW fumes are mostly present in trivalent (3+ ) oxidation states, possibly existing as NiFe2 O4 , (Fe, Mn, Ni) Cr2 O4 , and Cr2 O3 .(28,32) Manganese and nickel appear to exist in mixed forms of divalent (2+ ) and trivalent (3+ ) oxidation states, mostly as Mn3 O4 and NiO, respectively. Significant amounts of the biological reactive, carcinogenic species, hexavalent chromium (Cr6+ ), have been measured in stainless steel fumes generated during SMAW and FCAW.(28,32) Furthermore, a study has indicated that measurable amounts of Cr6+ are formed during stainless steel GMAW and the concentration of Cr6+ is dependent on the composition of the shielding gases used.(33) It has been observed that the type of process and the materials used during welding may have an effect on the morphology of the generated welding fume. Aerosols generated during GMAW have been shown to be quite different morphologically compared with particles formed during FCAW and SMAW.(2,27) The aerosols generated during GMAW tend to be smaller and are primarily arranged in homogeneous chain-like agglomerates, whereas the aerosols generated during FCAW and SMAW are larger, more chemically complex and contain a mixture of chain-like and spherical structures. TEM and SEM analysis of the aerosols generated with the robotic welder using a GMAW process in the current study depicted numerous aggregations of chain-like agglomerates that were very similar in appearance to what has been observed by other investigators.(2,27) Aerosol formation during GMAW appears to first include nucleation, a process by which high temperature metal vapors are transformed into primary particles.(34) Nucleation is then followed by coagulation, a dynamic aerosol growth mechanism that occurs when small particles collide to form larger agglomerates. The agglomerates are formed after particleparticle, particle-agglomerate, and agglomerate-agglomerate collisions. After these collisions, the agglomerate are likely held together by van der Waals and other attractive interactions, such as electrostatic and magnetic forces.(34) The strength of these forces are of importance toxicologically because the individual primary particles are in the ultrafine size range (0.01– 0.10 µm). If these attractive forces were to be disrupted by interactions with lungs cells and biological fluids after inhalation, the primary particles could possibly dissociate from the larger agglomerates. Studies have indicated that the adverse effects of particles in the lungs may be due in part to particle size.(35−37) It has been suggested that the pulmonary toxicity observed after exposure to ultrafine or nanosized particles may be a consequence of an increase in surface area. Numerous studies have measured the particle size of formed welding fumes. The selection of the welding process and materials used has an impact on the particle size of the aerosol. In the breathing zone of a welder, the particles are in the form of agglomerates. Therefore, it is the aerodynamic size 202 of the agglomerates, not the size of the single primary particles that form the agglomerates, that determine the sites of pulmonary deposition after welding fume inhalation. Because of the significant number of nanosized particles generated during welding, particle size distribution was determined using a Nano-Moudi impactor. The mass median aerodynamic diameter of the agglomerates generated by the robotic welder was 0.24 µm, giving them a high probability of being deposited in the alveolar regions of the lungs. The particle size results of the current study are comparable to what has been observed by others. Zimmer and Biswas(2) observed GMAW-mild steel welding fume generated in the laboratory to have count median diameters of 0.149 µm. 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