The current study measured the 3-D printer emission particles and VOC concentrations at a workplace equipped with two 3-D printers (Sindoh - 3-DWOX 7X and AFINIA 3-D - H800+) and Universal Robots (UR). In addition, VOC sampling was conducted outside the corridor of the workplace. Figure 1A shows a description of the workplace layout, sampling sites, and other contextual information. Four male workers were located in front of Tables 1–4. The workers were computer programmers involved in developing the facility management system and UR robot operation. The UR robots were not operated on the day of the tests. The dimension of the room was 6.75 m × 11.25 m × 2.72 m (206.55 m³). The workplace had only one entrance door and no window and was equipped with three fresh air inlets and three returns located on the ceiling. One ceiling-mounted air conditioner unit was operated during the workday. The manufacturing computer-aided design (CAD) objects consumed 46 and 102 g of ABS filament (containing 95–100% ABS; 0–5% stabilizer; melting point 180–200°C; Plasil, 3Dink Inc, Yangju, Korea) for printers 1 and 2, respectively, to manufacture headphone hangers (Supplementary Figure S1). The sequence of events for a 3-D printing workplace was described in Supplementary Table S1.

Figure 1A shows the workplace setup and layout, air sampling sites for area sampling, and real-time monitoring sites. The area air samples were taken by drawing air through polyvinyl chloride (PVC) filters in sampling cassettes (0.45 µm nominal pore size, 37 mm diameter, 2-inch conductive cowl) obtained from Pall Corp (P/N 64,678; Michigan United States). The filter samples for particles were collected from the location described in Figure 1A, and sampling pumps (MSA, Escort Elf pump) were operated at a flow rate of 1.5–2.0 L/min. The sampling was performed during the normal work period from 08:30 to 17:30.

Two differential mobility analyzing systems (DMAS) combining differential mobility analyzer (DMA-20, 4220, range 6–225 nm, HCT Co., Ltd. Korea; TSI, United States) and condensation particle counters (CPC, 3775, size range 4 nm- 1, TSI INC., Shoreview, MN 0–108 particles/cm³ detection range) were used to monitor the particle size distribution with an electrical mobility diameter ranging from 15 to 710.5 nm. Meanwhile, three types of dust monitors (Model 1.109, range 0.25–32, Grimm, Douglasville, GA; Model PS-1601PM, range 0.25–10, HCTm, Co. Incheon, Korea; PS-1601PMe, range 0.25–10 μm, HCTm, Co. Incheon, Korea) were used to monitor the particle size distribution with a diameter ranging from 0.25 to 32 µm. The workplace air was sampled at a flow rate of 0.3 and 1.2 L/min for the DMAS and dust monitor, respectively. The DMAS scanned the particle sizes at a time resolution of 2.5 min (120 s for up-scan and 30 s for retrace), while the average time for the dust monitor was 1 min. The real-time aerosol monitoring lasted 3 days at the 3-D printing workplace. All the sequences of events that might affect particle monitoring were recorded (Supplementary Table S1).

Benzene, styrene, and formaldehyde were measured at six locations by area sampling using Coconut Charcoal sorbent tubes (SKC Cat. 226-01) (NIOSH (National Institute of Occupational Safety and Health), 2003). Three grab samples were collected from 8:30–11:50 (200 min), 13:30–16:00 (150 min), and 16:20–17:40 (80 min) to monitor changes in concentration over time and to prevent possible analyte breakthrough from the sorbent tube. For measurement and analysis, benzene and styrene were sampled with a low flow (50 ml/min) air sampling pump (Model LFS-113; Gilian Instrument Corp., West Cladwell, NJ, United States) in SKC Coconut Charcoal sorbent tubes (SKC Cat. 226-01) (Stoehr et al., 2015). The samples were analyzed with a gas chromatography-flame ionization detector (GC-FID). Formaldehyde was analyzed by high-performance liquid chromatography - ultraviolet light detector (HPLC-UV) after flow-through with 2,4-dinitrophenylhydrazine coated silica gel tube (SKC Cat. No. 226-119). The actual sampling flow rate of the pump was calculated as an average value after measuring the flow rate before and after the measurement using a DryCal^® primary calibration standard (BIOS, Butler, NJ, United States). The analysis was conducted by the Institute of Occupation and Environment, Korea Workers’ Compensation and Welfare Service, which participates in the American Industrial Hygiene Association (AIHA) Proficiency Analytical Testing (PAT) program. Analysis limit of detection (LOD) was 0.0055 mg/sample for benzene, 0.0088 mg/sample for styrene, and 0.0018 μg/sample for formaldehyde. The real-time TVOC concentration was measured using the EVM-7 Multiparameter Environmental Monitor (Quest Technologies, Inc., United States). The real-time concentration of particle-bound PAH was measured with a real-time photoelectric aerosol sensor PAH 2000 (EcoChem Analytics, League City, TX, United States).

Since most of the VOCs, evaluated by the NIOSH NMAM 1301 (GC FID) (NIOSH (National Institute of Occupational Safety and Health), 1994a), were not detected, further analysis was performed using a high resolution and high sensitivity PTR-TOF-MS (IONICON Analytik, AUT). When the 3-D printer was running, 10 L of air was collected in the Tedlar bag using a grab air sampling pump (SKC Cat. No. 222–2301) from the front of the printer. The air collected in the Tedlar bag was directly injected into the PTR-TOF-MS to analyze the VOCs.

The MMAD of particles generated from the 3-D printer was measured using a foil filter for each stage (diameter, 47 mm; pore size, 5 mm; SKC, Inc., Eighty-Four, PA, United States) with a Nano MOUDI impactor (MOUDI 125 NR, MSP Co., MN, United States) composed of 13 stages (0.01, 0.018, 0.032, 0.056, 0.10, 0.18, 0.32, 0.56, 1.0, 1.8, 3.2, 5.6, and 10 mm). The geometric standard deviation (GSD) for the MMAD was derived from the cumulative mass distribution of the micro-orifice uniform deposition impactor (MOUDI).

An electrostatic precipitator ESPnano (Model 100; ESPnano, Spokane, WA, United States), operating at the standard sampling flow rate of 0.1 L/min, was used to collect aerosol particles on electron microscopy grids. The TEM nickel grid (Formvar/Carbon 200 mesh, TEDpella, CA, United States) or holey TEM grid (Quantifoil 656-200-Cu; Tedpella, Inc., Redding, CA, United States) were further examined under a transmission electron microscope (TEM, H - 7650; Hitachi, Tokyo, Japan) equipped with an EDX (energy dispersive X-ray analyzer, TM200; Oxford Instruments PLC, Oxfordshire, United Kingdom) at an acceleration voltage of 100 kV (NIOSH (National Institute of Occupational Safety and Health), 1994b).

After noticing the 3-D printer emission exposure status, the company took an exposure mitigation measure, as shown in Figure 1B. Two 3-D printers were isolated to the enclosed space (which has ventilation on the ceiling). Real-time aerosol monitoring and formaldehyde sampling were conducted inside and outside of the enclosed space.

The workplace average temperature was 26.9°C and ranged 22.7°C at 8:33 to 28.8°C at 16:47. Relative humidity ranged from 37.3% at 8:41 to 31.1% at 13:44. Carbon dioxide concentrations were 550 ppm at 8:44–9:08 and 1515 ppm at 16:52 (Supplementary Table S1).

TSP concentrations determined from gravimetric analysis during the 9-h of work shift at various locations ranged 0.008–0.011 mg/m³. Sampling inside of the housing of the printer resulted in a non-detectable mass change on the filter concentration (limit of detection µg/m³) (Table 1).

Benzene and styrene measured at various time points were not detected. The TVOC concentrations ranged from 0.2 to 4 ppm with an average of 1.93 ppm. Formaldehyde at various locations was detected at 2.5–21.7 ppb, whereas the inside of the printer was 19.7 ppb, much less than ACGIH TLV-TWA (100 ppb) (2017). Total PAH levels from PAS 2000 were less than 1 ng/m³ throughout the measurement time (Table 2).

A wide variety of harmful gases were detected at low concentrations down to the ppb level (Table 3). However, acetone was at a high ppm concentration because the surface of ABS products was intermittently wiped with acetone. Even though no wiping of ABS surfaces was conducted on the day of this measurement, it is believed that some acetone was residue leftover from previous work. The sum of individual VOCs was 2.085 ppb, which was similar to the average 1.93 ppb result by the TVOC sensor.

UFPs measured by DMAS ranged between 2,569–27,005 particle/cm³ with an average of 16,290 ± 4,468 particle/cm³ during the operation of 3-D printers (Table 1). After switching on the printers, particle number concentration began to increase. When the printers were turned off at 6 PM, the numbers decreased to less than 2000 (Figure 2A). On the other hand, fine particle numbers measured from 16:00 on the day before 3-D printer operation to 8:00 on the day after printer operation showed a different pattern from DMAS measurement. Before starting 3-D printers, fine particle concentration showed 2000–3000 particle numbers/liter and reached approximately 18,000 particle/liter at the start of 3-D printers and continuously increased to 40,000–50,000 particles/liter until the next day morning (Figure 2B). Particle number measurement conducted inside the 3-D printer enclosure resulted in no data. It could be caused by a high-temperature process affecting DMAS measurement or not measurable size by DMAS due to the vapor state of the emission. Our result indicates that the UFPs (∼30 nm) emitted from 3-D printers slowly transformed into fine particles larger than 200 nm (Supplementary Figure S1).

After the initial exposure assessment, the management of the workplace noticed the status of exposure to the 3-D printer emission and initiated exposure mitigation. They isolated the two 3-D printers in the enclosed space, which has ventilation on the ceiling (Figure 1B). The nanoscale particles originating from the 3-D printers were greatly reduced after mitigation measure (Figure 2C), while particles detected by OPC increased little (Figure 2D). After the mitigation measure, formaldehyde at various locations described in Figure 1B ranged 23.1–42.6 ppb at 9:00–13:00 and 9.2–18.6 ppb at 13:00–15:00. The higher concentrations of formaldehyde in the morning time could be caused by floor cleaning and waxing, which may use formaldehyde-containing agents, while the concentrations were low in the afternoon (Table 2).

Count median diameter (CMD) measured by DMAS was 30 nm (Figure 3A), and MMAD and GSD measured by nano-MOUDI were 71 nm and 2.731, respectively (Figure 3B). Size distribution measured by dust monitors showed the formation of slightly smaller particles compared to the background (Supplementary Figure S2).

Well dispersed ABS particles were observed with TEM, where some particles were aggregated (Figures 4A,B). EDX analysis indicated that most particles consisted of carbon when subtracting Cu and Ni as grid and Al as holder compositions (Figure 4D). Unexpectedly, copper metal nanoparticles were also observed, which are believed to be impurities added (pigments, etc.) to ABS materials. The ABS filament was further analyzed by ICP-MS, and the results showed that ABS filament contained some impurity metals (Al 62 ppm; Cr 2.6 ppm; Fe 13 ppm; Cu 1.1 ppm). However, metal nanoparticles were observed only in some areas of the grid, whereas carbon ABS nanoparticles were evenly distributed in a wide area of the grid.