Aerosol Emissions from Fuse-Deposition Modeling ... - ACS Publications

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Aerosol emissions from fuse-deposition modeling 3D printers in a chamber and in real indoor environments. Marina Eller Vance, Valerie Pegues, Schuyler Van Montfrans, Weinan Leng, and Linsey C. Marr Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01546 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Aerosol emissions from fuse-deposition modeling

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3D printers in a chamber and in real indoor

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environments.

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Marina E. Vance*1, Valerie Pegues2, Schuyler Van Montfrans3, Weinan Leng4, Linsey C. Marr4

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*1

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Engineering Drive, Boulder, CO 80309, United States. Email: [email protected],

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phone: (303) 735-8054, fax: (303) 492-3498.

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Department of Mechanical Engineering, University of Colorado Boulder, 427 UCB, 1111

Department of Environmental Health and Safety Virginia Tech, Blacksburg, VA 24061, United

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States 3

William Fleming High School, 3649 Ferncliff Ave. NW, Roanoke, VA 24017, United States 4

Department of Civil and Environmental Engineering, Virginia Tech, 418 Durham Hall, Blacksburg, VA 24061, United States

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RUNNING TITLE: Aerosol emissions from 3D printers

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KEYWORDS: Ultrafine aerosols, incidental nanoparticles, indoor air quality, 3D printing,

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additive manufacturing.

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ABSTRACT

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Three-dimensional (3D) printers are known to emit aerosols, but questions remain about their

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composition and the fundamental processes driving emissions. The objective of this work was to

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characterize aerosol emissions from the operation of a fuse-deposition modeling 3D printer. We

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modeled the time- and size-resolved emissions of submicron aerosols from the printer in a

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chamber study, gained insight into the chemical composition of emitted aerosols using Raman

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spectroscopy, and measured potential for exposure to aerosols generated by 3D printers under

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real-use conditions in a variety of indoor environments. Average aerosol emission rates ranged

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from ~108 to ~1011 particles min-1, and rates varied over the course of a print job. Acrylonytrile-

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butadiene-styrene (ABS) filaments generated the largest number of aerosols and wood-infused

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polylactic acid (PLA) filaments generated the smallest amount. Emission factors ranged from

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6×108 to 6×1011 per gram of printed part, depending on the type of filament used. For ABS, the

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Raman spectra of the filament and printed part were indistinguishable while the aerosol spectra

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lacked important peaks corresponding to styrene and acrylonitrile, which are both present in

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ABS. This observation suggests that aerosols are not a result of volatilization and subsequent

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nucleation of ABS or direct release of ABS aerosols.

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INTRODUCTION

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The field of indoor air quality continually grows to accommodate novel consumer products

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that may release pollutants into the indoor environment. Office equipment such as laser printers

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and photocopiers are known to emit volatile organic compounds (VOCs), ozone, and particulate

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matter.1,2 With the recent development and popularization of three-dimensional (3D) printers,

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studies are needed to understand their potential emissions to indoor environments.

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3D printing (also referred to as additive manufacturing or rapid prototyping) is a bottom-up

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process of creating a three-dimensional object layer by layer. There are several distinct 3D

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printing processes, which can utilize solid, powder, or liquid feedstock materials.3 The 3D

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printing industry has been growing steadily since 2009, when a core fuse-deposition modeling

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(FDM) patent expired.4,5 As the cost of 3D printers has decreased, they have become more

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popular in homes, offices, and schools.

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In FDM, a polymeric filament is extruded through a heated nozzle that moves to create a pre-

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designed object with layers that are typically ~0.25 mm in thickness. Common filament materials

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include acrylonytrile butadiene styrene (ABS), polylactic acid (PLA), polycarbonate, and

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blended polymers.3

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To date, eight research articles have been published on air pollutant emissions of 3D printers.

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Seven of these works evaluated FDM 3D printers6–12 and one evaluated a binder jetting 3D

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printer.13 Studies of FDM printers investigated a variety of polymeric filaments including ABS,6–

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10,12

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nylon,9,11 copolyester,11 and more. Three studies investigated VOCs in addition to aerosols.7,9,10

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Studies have been performed inside stainless steel9,12 and acrylic chambers7,10, in a 60 m3 clean

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room,8 and in indoor environments ranging from 30 to 180 m3 in size.6,10–13 Of these eight

PLA,6,7,9–12 PLA infused with other materials (e.g., copper, wood fiber, bamboo, etc),11

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studies, only Azimi et al. (2016) modeled total ultrafine aerosol emission rates with

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consideration of chamber wall losses and the time-varying nature of emissions, while also

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focusing on situations with low background aerosol concentrations.9 No studies to date have

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reported simultaneous time- and size-resolved aerosol emissions, information that is needed to

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model the fate of the aerosols in detail. Many research questions remain as to the fundamental

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processes driving aerosol and VOC releases from the FDM process and the resulting aerosol

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chemistry and toxicity.

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The objective of this work was to characterize aerosol emissions from FDM 3D printers.

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Specific objectives were (1) to measure the time- and size-resolved emissions of submicron

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aerosols from the operation of a FDM 3D printer using five types of filament material in a

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chamber study, (2) to gain insight into the chemical composition of these aerosols through

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electron microscopy coupled with energy dispersive X-ray spectroscopy and Raman

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spectroscopy, and (3) to determine aerosol concentrations under real-use conditions in a variety

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of indoor environments.

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MATERIALS AND METHODS

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Chamber. To assess the aerosol size distributions and emission rates of different filaments, we

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placed an Afinia desktop 3D printer (H480, Afinia 3D) inside a 520-l polyethylene chamber

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(Atmosbag, Sigma-Aldrich). The following filaments (feedstock materials) were tested: yellow

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acrylonytrile butadiene styrene (ABS) “premium” quality, orange ABS “value” quality, light-

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blue polylactic acid (PLA), wood-infused PLA, copper-infused PLA.

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All filaments were purchased from Afinia 3D. Chemical structures of ABS, PLA, and their

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monomers are shown in Figure S1 in the Supporting Information (SI). We printed a test artifact

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developed by NIST for assessing 3D printers (Fig. 1b),14 scaled to 50% in order to be completed

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within 60 min. Printer settings are described in Tables S1 and S2.

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The chamber was flushed with air that was conditioned through a high efficiency particulate

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air (HEPA, Pall Corp.) capsule filter to a background aerosol concentration of 0 cm-3 measured

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by a Scanning Mobility Particle Sizer (3936NL SMPS, TSI Inc.). The flow rate of air that was

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flushed through the chamber varied by material and is described in the SI file, along with a

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detailed description of the aerosol characterization instruments and their operational settings.

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Aerosol characterization. The concentrations and size distributions of aerosols 14.6–680 nm

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were monitored continuously at 3-min resolution with an SMPS during the entire print job.

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While the focus of this work was the aerosol component of emissions, total VOCs were

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minimally investigated using a GrayWolf (IQ-610, GreyWolf LLC) monitor, which was placed

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inside the chamber during printing with ABS “value” quality and PLA.

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For off-line analyses, aerosol samples were collected on the lowest stage of a three-stage

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impactor (Microanalysis Particle Sampler MPS-3, California Measurements). The cutpoint of

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this stage was 50 nm and the stage upstream had a cutpoint of 300 nm. For scanning electron

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microscopy (SEM), samples were collected on an ethanol-cleaned aluminum stub covered with

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carbon tape and analyzed using an environmental SEM (ESEM, FEI Quanta 600 FEG) equipped

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with electron dispersive X-ray spectroscopy (EDS) capabilities. For Raman spectroscopy,

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samples were collected on a silicon wafer which was attached to an SEM stub. The silicon wafer

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was cleaned with ethanol and sonicated in ultrapure water before use. Raman spectra were

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recorded using a confocal Raman microscope (WITec alpha 500) equipped with a 100× Olympus

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objective. Laser excitation (~10 mW at the sample) was provided by a 785-nm diode laser. Each

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spectrum was obtained as an average of 6 accumulations of 10 s each. The background signal

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from the silicon substrate was removed by subtracting a silicon spectrum.

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A sample of copper-infused filament was also analyzed for metal content by inductively-

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coupled plasma mass spectrometry (ICP-MS, Thermo Electron X-Series, detection limit of 0.5

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ppb). Pieces of filament 40 – 65 mg in mass were dissolved in ~100 mL of dichloromethane

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(DCM, 99.9%, Fisher Scientific). The DCM was allowed to evaporate overnight in a fume hood,

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after which 80 ml of ultrapure water (18 MΩ cm, Barnstead) and 20 ml of trace-metal grade

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nitric acid (HNO3 67 - 70%, PlasmaPure, SCP Science) were added. This solution was heated at

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70 ºC overnight and then 1 ml of this solution was added to 9 ml of ultrapure water and analyzed

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by ICP-MS.

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Aerosol emissions modeling. Aerosol concentrations in the chamber were used to obtain an

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empirical, size-resolved and time-resolved emission rate for each tested material. The chamber

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was modeled as a continuously-stirred tank reactor (CSTR), and size-resolved aerosol wall losses

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were determined experimentally (Table S3). The initial mass balance equation (Eq. 1) was

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solved discretely for the emissions term, which varies over time (Eq. 2). These equations were

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solved separately for each size range of particles measured by the SMPS. The SI file presents a

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detailed description of this model.

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() 

=  + −  − 

Eq 1.

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In Eq. 1, V is the volume of the chamber (cm3), C is the concentration of aerosols (cm-3), Qin is

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the flow rate of air that is introduced into the chamber (cm3 s-1), Cin (cm-3) is the concentration of

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aerosols in the inflow air, zero in these experiments, E is the emission rate (s-1), β is the wall-loss

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coefficient (s-1), Qout is the flow rate of air exiting the chamber (cm3 s-1), and C is the

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concentration of aerosols exiting the chamber (cm-3), which is assumed to be equal to the

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concentration of aerosols inside the chamber.

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( ) =

( )( )  

+  (  ) +  (  )

Eq. 2

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In Eq. 2, tn is the current period of aerosol measurements, and tn-1 is the previous period of

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measurement. For this study, these periods are 3 min apart. Eq. 2 was used to determine the size-

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resolved and time-resolved emission rates in units of number of particles per second.

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Emission rates were summed over each size bin to determine the total aerosol emission rate

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over time. While the emissions model accounts for size-specific wall-losses, it does not account

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for aerosol agglomeration, which is likely to occur at high aerosol concentrations.15 To adjust for

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this effect, negative emission values were eliminated before total aerosol emissions were

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calculated, as done in a previous study.9 Finally, the aerosol emission factor was determined by

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summing the total number of aerosols emitted during the printing activity and dividing it by the

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mass of printed part.

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Field measurements. Field measurements took place in five sampling locations at Virginia

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Tech. Each location had an unenclosed FDM 3D printer equipped with ABS filament, and users

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volunteered to print a typical part as they would during normal operations. The locations

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included two laboratories, two offices, and one classroom. Tables S5 and S6 describe all printers,

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filament materials, parts printed, and indoor environments. Figure S2 presents photos of typical

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parts printed for this study.

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Field measurements occurred in two phases: (1) using a handheld condensation particle

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counter (CPC model 3800, Kanomax USA, Inc.), and (2) using both the CPC and a portable

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SMPS (Nanoscan model 3910, TSI, Inc.). The classroom was only tested during phase 1 because

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the printer was not available during the second phase, and one of the offices was only tested

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during phase 2 for the same reason. Instruments were placed at seated breathing height (1 – 1.2

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m) and ~1 m away from the 3D printer nozzle, to mimic a realistic exposure scenario of a user

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working near the printer. The maximal contribution of the 3D printer to indoor aerosols was

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estimated by subtracting the background size distribution, measured before printing began, from

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the size distribution obtained at the time of maximum aerosol concentrations.

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RESULTS AND DISCUSSION

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Aerosol size distributions in chamber experiments. As shown in Figure 1, the aerosol size

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distributions at the time of maximum total concentration differed substantially by type of

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filament.

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Figure 1. Aerosol size distributions at the time of peak concentration inside the chamber for each

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filament material. For improved legibility, the results for copper-infused and wood-infused PLA

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are plotted on a separate panel (b), whose y-axis range is two orders of magnitude smaller than in

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the top panel (a). Dark lines represent average among replicates, and shaded areas represent

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standard errors.

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The ABS filaments introduced the largest number of aerosols in the chamber, with the less

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expensive, “value” ABS introducing significantly more aerosols than the “premium” ABS

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material. Despite this difference in concentration, both “value” and “premium” ABS filaments

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yielded similarly-shaped size distributions, with modes at 51 ± 9 nm and 51 ± 4 nm, respectively.

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These results are in agreement with Yi et al., who reported SMPS aerosol geometric mean

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diameters (GM) ranging from 45 to 79 nm for different color ABS filaments in a chamber of

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similar volume (0.5 m3).12 The maximum total aerosol concentrations in the chamber were quite

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high, 2×106 cm-3 and 5×105 cm-3 for ABS “value” and “premium”, respectively. Agglomeration

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may have shifted the size distribution and the mode to larger diameters than originally emitted.

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Very near the printer nozzle, it is likely that particle concentrations were even higher and that

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some agglomeration occurred, even if concentrations in the bulk chamber air did not exceed 106

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cm-3. Analogous to studies of particles in vehicle exhaust, these measurements represent

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emissions after some near-source aerosol processing has occurred.

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The PLA filament yielded the smallest aerosols, with a mode of 22 ± 2 nm. For PLA, Yi et al.

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reported a GM of 28 – 32 nm. Stabile et al. performed measurements in a 40 m3 room and

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reported a mode of 13 – 15 nm for PLA.11,12 The copper-infused and wood-infused PLA

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filaments yielded peak concentrations about 2 orders of magnitude smaller than did the other

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materials. Peaking at approximately 28 ± 3 nm, the size distribution generated by wood-infused

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PLA was similar in shape to that produced by PLA. The copper-infused PLA emitted much

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larger aerosols than did other filaments, with a peak at 470 ± 13 nm. Stabile et al. reported a

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mode 12 – 24 nm for wood-infused and copper-infused PLA.11

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Size-resolved aerosol emissions over time. Using Equation 2, we calculated the aerosol

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emission rate as a function of size and time. The results obtained for ABS filaments appear in

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Figure 2 and those for PLA-based filaments are shown in Figure 3.

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Figure 2. Size- and time-resolved aerosol emission rates (min-1) obtained in chamber studies

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using (a) ABS “value” and (b) ABS “premium” filaments. This figure is better visualized in

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color as the scale on the right represents a color code for emission rate magnitude. The color

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scale is the same for Figures 2 and 3. The results represent one replicate for each material.

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Duplicates in S3 show good visual agreement.

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The ABS “premium” filament led to lower aerosol emissions than did the ABS “value”

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filament, although aerosol size distributions were similar in shape. Figure 2 shows that aerosol

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emissions continued throughout the print job for ABS “value,” whereas they seemed to stop after

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the first ~ 10 min for ABS “premium,” at which time ultrafine aerosol emissions decreased by an

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order of magnitude, even when printing the same object. Figure 2b shows a characteristic

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“banana” shape that describes aerosol formation and subsequent coagulation and growth.16

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Figure 3. Size- and time-resolved aerosol emission rates (min-1) obtained in chamber study using

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(a) PLA, (b) copper-infused PLA, and (c) wood-infused PLA filaments. This figure is better

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visualized in color as the scale on the right represents a color code for emission rate magnitude.

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This color scale is the same for Figures 2 and 3. The results represent one replicate for each

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material. Duplicates in Figure S4 show good visual agreement.

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The aerosol mode for ABS “premium,” PLA, and wood-infused PLA continually increased

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throughout the printing activity (Figure S5), indicating that the aerosols present in the chamber

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continued to grow over time due to agglomeration, even after total concentrations dropped below

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106 cm-3. The aerosol mode for ABS “value” behaved differently, decreasing twice throughout

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the printing activity, indicating new aerosol emissions.

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While the emissions model accounts for size-specific wall-losses, it does not account for

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aerosol agglomeration, which occur at concentrations > 106 cm-1. Rapid agglomeration is likely

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to occur in close proximity to the nozzle, as discussed previously. Agglomeration leads to

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negative values (Table S4) of emission factors for smaller aerosols and may lead to positive

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emission factors for larger aerosols even when actual emissions are zero. Thus, the emission

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rates shown in Figures 2 and 3 are “effective” emission rates representing a scenario in which the

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air in a small volume around the printer is relatively stagnant, where particles can accumulate

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and agglomerate, or the printer is in an enclosure. This phenomenon was also observed by Azimi

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et al.9 and in one of the indoor environments, as described below.

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Total aerosol emissions over time. Figure 4 shows the total aerosol emission rate, in units of

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min-1 in chamber experiments, averaged among replicates for the five tested filaments.

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Figure 4. Total aerosol emission rates measured in chamber experiments for each filament

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material. For improved legibility, the results for copper-infused and wood-infused PLA are

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plotted in a separate panel (b), whose y-axis range is three orders of magnitude smaller than in

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the top panel (a). The dark line represents the average among replicates, and the shaded areas

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represent standard errors.

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As reported in other studies,9,11,12 aerosol emission rates were not constant over time.

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Emissions may be affected by the different activities performed by the printer. All filaments

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generated a peak of aerosol emissions at the start of printing activities that lasted ~5 – 10 min,

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the period when the support raft was printed. At ~20 min, the printer finished printing the bottom

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shell of the object and began printing the filling. At ~38 min, the top shell began to be printed

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and at ~47 min, the pins at the top of the object started being printed.

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For copper-infused PLA, a large, broad peak also occurred at 24 – 44 min. Average aerosol

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emission rates, presented in Table 1, ranged from ~108 to ~1011 min-1, in agreement with

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measurements performed by Azimi et al.9 for ABS and PLA filaments. Emission factors in terms

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of particle number per gram of printed part, which can be used to estimate total emissions from

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printed jobs of different sizes, ranged over three orders of magnitude, from 6×108 g-1 for copper-

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infused PLA to 6×1011 g-1 for ABS “value.”

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Table 1. Average aerosol emission rates and emission factors (per mass of 3D printed part) for

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each filament material (± standard errors). Average aerosol emission rate (min-1)

Aerosol emission factor (g-1)

ABS “value”

(1.08 ± 0.01) ×1011

(6.2 ± 0.3) ×1011

ABS “premium”

(1.25 ± 0.01) ×1010

(7.8 ± 0.2) ×1010

PLA

(1.48 ± 0.01) ×1010

(7.6 ± 0.2) ×1010

Wood-infused PLA

(1.10 ± 0.01) ×108

(5.7 ± 0.5) ×108

Copper-infused PLA

(1.58 ± 0.01) ×108

(6.4 ± 0.8) ×108

Filament material

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Aerosol composition, ABS. Optical images of impacted aerosols resulting from ABS “value”

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3D printing are shown in Figure 5 alongside Raman spectra of aerosols, the raw ABS filament,

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and a printed part.

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Figure 5. Optical images of ABS “value” aerosols impacted onto a silicon wafer (a) and insets

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with larger (≥10µm) aerosols (b) and smaller (≤1µm) aerosols (c) and resulting Raman spectra

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(d). Broader peaks are associated with smaller aerosols (c). Peaks associated with the benzene

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ring present in styrene are located at 620, 1001, 1031, 1157, and 1183 cm-1. The peaks associated

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with the carbon-nitrogen triple bond in acrylonitrile (ABS) is located at 2236 cm-1.

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The Raman spectra of the filament and printed part were indistinguishable while the aerosol

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spectra lacked important peaks that correspond to the benzene ring present in styrene and the

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carbon-nitrogen triple bond present in acrylonitrile, both present in the ABS structure. The

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extrusion temperatures (210 – 270 ºC) were higher than the boiling points of both styrene (145

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ºC) and acrylonitrile (77 ºC). This disproves the hypothesis that the aerosol is a result of

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volatilization and subsequent condensation/nucleation of ABS or direct release of ABS aerosols.

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A second hypothesis is that aerosols may be formed as a product of the thermal degradation of

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the ABS polymer, which would involve the release of semi-volatile compounds—likely

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oligomers—that are volatile at extrusion temperatures but not at room temperature, thus causing

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condensation, nucleation, and subsequent growth of aerosols. Suzuki and Wilkie performed a

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thermogravimetric analysis coupled with infrared spectroscopy (TGA / FTIR) on ABS among

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other polymers and observed that its degradation begins at 340 ºC with the release of butadiene

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monomer, which is quickly followed at 350 ºC with the release of aromatics from the original

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styrene monomer. The release of acrylonitrile occurs last, at ~400 ºC.17 These thermal

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degradation processes occur at a significantly higher temperature than the extrusion temperature

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observed in a 3D printer (up to 270 ºC). In contrast, a study by Tiganis et al. demonstrated that

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after 168 h at 90 – 120 ºC the polybutadiene component of ABS may undergo thermo-oxidative

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degradation that may lead to the formation of carbonyl and hydroxyl products.18 Unwin et al.

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reported the release of styrene, 1,3-butadiene, acrylonitrile, and 4-vinyl-1-cyclohexene from the

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vacuum forming of ABS sheets at 160 – 180 ºC.19

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A third hypothesis is that this type of thermal degradation may be occurring, but in chemical

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additives rather than in the ABS polymer itself, or perhaps in both. Bai et al. used gas

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chromatography/mass spectrometry (GC/MS) to investigate the release of additives from virgin

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ABS polymer and waste ABS plastics from reprocessing at 230 – 270 ºC. They found that

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commonly-used antioxidants and lubricants oxidize, degrade, and, in some cases, volatilize at

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that temperature range.20

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Pigments and dyes are another class of commonly used additive in ABS filaments, whose

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original color ranges from translucent to white. To our knowledge, there have not been studies

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on the thermal degradation of pigments and dyes present in ABS plastics.

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Aerosol composition, PLA. The Raman spectra of filament, printed part, and aerosols are

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shown in SI Figures S6 – S9 online. In contrast to ABS, the spectrum of the PLA part differed

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from that of its raw filament material (Figure S7), and the printed part itself was not spatially

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homogeneous with regards to its Raman spectra (Figure S8). Aerosols presented broad Raman

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peaks at ~1300 and ~1600 cm-1. Unlike ABS, the chemical structure of PLA does not contain

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any bonds with characteristic Raman peaks (Figure S9). Aerosols emitted from copper-infused

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PLA were investigated using SEM/EDS and no discernible copper peak was identified in the

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aerosols (Figure S10), although the copper filament contained 21.1 ± 0.3 % copper as determined

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by ICP-MS. The filament also contained 500 ± 76 ppm zinc, 214 ± 53 ppm silicon, and 175 ± 29

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ppm iron. We hypothesize that the presence of copper affected aerosol emissions but the copper

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itself was not aerosolized. However, further investigation into the aerosol composition is needed

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to prove the absence of copper or other metals.

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VOC and CO emissions. The total VOC concentrations in the chamber reached 0.83 ± 0.01

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ppm and CO concentrations reached 0.69 ± 0.01 ppm above background during printing with

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ABS “value”. During printing with PLA, neither pollutant reached detectable levels inside the

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chamber.

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Field measurements. In five of the eight indoor measurements, aerosol concentrations

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increased after the start of printing activities (Figure 6), which demonstrates that the operation of

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3D printers can impact the air quality of indoor environments. The average aerosol background

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level for indoor environments ranged from 854 ± 12 cm-3 in the classroom to (1.7 ± 4)×104 cm-3

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in Laboratory 2. Maximum aerosol concentrations ranged from 3.9×103 cm-3 in Office 1 to

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6.6×104 cm-3 in Laboratory 1. In one set of measurements in Office 1, there were large peaks in

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aerosol concentration during background measurements ~15 min before printing and at ~40 – 45

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min, which could not be explained by 3D printing activities.

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Dilution appeared to impact concentrations, even though all measurements were conducted ~1

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m from the printer. The smallest increase in aerosol concentrations during printing was observed

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in Office 1, which was also the largest room, with a volume of ~780 m3 (Table S6). One of the

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largest increases in concentrations was observed in Office 2, which was the smallest environment

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investigated (36 m3). For comparison, Steinle was able to detect a 1.2 × 103 cm-3 increase in

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aerosol concentrations at a distance of ~0.3 m but not at 2.5 m from the printer in a 180-m3 room

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with a 2 h-1 air exchange rate.10

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Figure 6. Total aerosol concentrations observed in real indoor environments during typical 3D

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printing activities, using ABS filaments only. These filaments were purchased from different

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manufacturers and were not subcategorized as “premium” or “value”. At time 0 min, printing

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activities began and continued through the end of measurements. The Classroom and Office 2

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were only tested once because the printer was not available during the second phase. Sampling

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inlets were placed at seated breathing height (1 – 1.2 m) and ~1 m away from the 3D printer

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nozzle.

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The y-axis range in the bottom two panels (Office 1 and Classroom) is one order of magnitude smaller than in the upper three panels.

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Figure S11 shows the estimated size distribution of aerosols contributing to indoor

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environments by 3D printing activities. These were obtained by subtracting background size

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distributions from those obtained at the time of maximum concentration. The size distributions

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differed by location, likely reflecting the different types of printers used in each location. In

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Office 2, the mode occurred at 15.4 nm, which was significantly smaller than the ~51 nm mode

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observed in chamber experiments with the same printer and filament material (Afinia H480 and

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ABS “premium”, respectively). Figure S12 shows aerosol emission rates calculated for Office 2

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in two replicate measurements, one of which shows aerosol growth from ~15 nm to ~54 nm

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throughout the first ~45 min of printing. This finding confirms that the aerosol growth observed

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in some of the chamber experiments may also occur in indoor environments and might especially

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occur in enclosed, unvented 3D printers.

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Office 2 had a much larger volume than the chamber (36.2 m3 versus 0.52 m3) and a different

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air exchange rate. It appears that dilution in the room was sufficient to avoid the high

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concentrations that led to agglomeration in the chamber experiments. The air exchange rates of

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indoor environments were not measured for this study; they are typically 0.5 h-1 but may reach as

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high as 5 – 7 h-1.21,22 The American Society of Heating, Refrigerating and Air-Conditioning

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Engineers (ASHRAE) recommends a minimum ventilation rate of > 0.35 h-1 for living areas.23

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The air exchange rate in the chamber was ~2.5 h-1 for ABS measurements.

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INHALATION EXPOSURE

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The size-resolved emission rates developed in this study can be used to estimate the inhalation

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exposure to aerosols.24 Assuming that an average adult engaged in light work25 operates a 3D

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printer in a 30 m3 room with an air exchange rate of 0.5 h-1 and remains in the room during a 1-h

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print time (the print duration of this study), the person would inhale 107 – 1010 particles or up to

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5.6 µg of aerosols, assuming spherical aerosols of 1 g cm-3 density15 (Tables S7 and S8). Of

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course, actual exposure may differ because emissions will not mix instantaneously and

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homogeneously throughout the room. In terms of particle number, inhalation exposure associated

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with 3D printers appears to be similar to that for laser printers, as the variability and range of

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concentrations is comparable to that observed in a study of laser printers in offices in Australia26

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and Germany.27 The mean particle number emission rates of 3D printers in this study, and thus

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the resulting exposure, are lower than for combustion sources in the home and comparable to

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those generated by appliances such as an electric space heater, laser printer, and vacuum

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cleaner.28,29 Of course, differences in particle composition are also important for assessing effects

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of these sources. The results presented in this work can be used by exposure science and life-

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cycle assessment modelers as input data for further modeling and, more importantly, by toxicity

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researchers as realistic dosing metrics for FDM 3D printers.30,31

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ASSOCIATED CONTENT

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SUPPORTING INFORMATION

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Includes detailed descriptions of the 3D printers, filaments, printed parts, as well as

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descriptions of the chamber setup, emissions model and inhalation dose calculations, 12 figures,

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and 8 tables. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

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Corresponding Author

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* Phone: (303) 735-4567; Email: [email protected].

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval

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to the final version of the manuscript.

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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Funding for this work was provided by the Institute for Critical Technology and Applied

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Science (ICTAS) at Virginia Tech, the Center for Sustainable Nanotechnology (VTSuN), the

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Center for Innovation-Based Manufacturing (CIBM), and the National Science Foundation

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(NSF) and the Environmental Protection Agency (EPA) under NSF Cooperative Agreement EF-

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0830093, Center for the Environmental Implications of Nanotechnology (CEINT). This work has

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not been formally reviewed by EPA, and no official endorsement should be inferred. This work

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used shared facilities at the Virginia Tech National Center for Earth and Environmental

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Nanotechnology Infrastructure (NanoEarth), a member of the National Nanotechnology

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Coordinated Infrastructure (NNCI), supported by NSF (ECCS 1542100). We acknowledge A.

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Tiwari for assisting in the chamber set up, E. Vejerano for repairing the SMPS in a time of need,

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C. Williams for Coordinating the CIBM research experience for teacher’s program that allowed

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for the participation of S. V. M. in this project. We also thank A.J. Prussin, J. Parks, and Y. Wu

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for their assistance in performing ICP-MS sample preparation and analysis. Finally, we thank the

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many 3D printer users who allowed for sample collection in their workspaces.

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