XRF-Analysis of Fine and Ultrafine Particles Emitted from Laser

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XRF-Analysis of Fine and Ultrafine Particles Emitted from Laser Printing Devices Mathias Barthel, Vasilisa Pedan, Oliver Hahn, Monika Rothhardt, Harald Bresch, Oliver Jann, and Stefan Seeger* BAM Federal Institute of Materials Research and Testing, Division 4.2 Environmental Material and Product Properties, Unter den Eichen 87, 12205 Berlin, Germany ABSTRACT:

In this work, the elemental composition of fine and ultrafine particles emitted by ten different laser printing devices (LPD) is examined. The particle number concentration time series was measured as well as the particle size distributions. In parallel, emitted particles were size-selectively sampled with a cascade impactor and subsequently analyzed by the means of XRF. In order to identify potential sources for the aerosol’s elemental composition, materials involved in the printing process such as toner, paper, and structural components of the printer were also analyzed. While the majority of particle emissions from laser printers are known to consist of recondensated semi volatile organic compounds, elemental analysis identifies Si, S, Cl, Ca, Ti, Cr, and Fe as well as traces of Ni and Zn in different size fractions of the aerosols. These elements can mainly be assigned to contributions from toner and paper. The detection of elements that are likely to be present in inorganic compounds is in good agreement with the measurement of nonvolatile particles. Quantitative measurements of solid particles at 400 °C resulted in residues of 1.6  109 and 1.5  1010 particles per print job, representing fractions of 0.2% and 1.9% of the total number of emitted particles at room temperature. In combination with the XRF results it is concluded that solid inorganic particles contribute to LPD emissions in measurable quantities. Furthermore, for the first time Br was detected in significant concentrations in the aerosol emitted from two LPD. The analysis of several possible sources identified the plastic housings of the fuser units as main sources due to substantial Br concentrations related to brominated flame retardants.

1. INTRODUCTION The emission of fine and ultrafine particles in the indoor environment has been widely discussed in recent years.1 4 Among the main sources for indoor ultrafine particle aerosols are electronic devices with laser printing function (LPD).5 9 Due to increasing concern about health effects of indoor aerosols numerous recent research activities deal with this topic. Until now, the studies include the determination of the quantity of emitted particles,5,7,10,11 potentially involved formation mechanisms,12 potential health effects of laser printer emissions including volatile (VOC) and semi volatile organic compounds (SVOC),13,14 the impact on real-room5,10,11 as well as on modeled15 office environment, the influence of parameters like fuser roller temperature on the number of emitted particles16 and the use of filter accessories in order to prevent the particles from being released into the indoor air.17,18 r 2011 American Chemical Society

So far, few studies deal with the chemical composition of ultrafine particles emitted from LPD. Morawska et al. stated that the particles mainly consist of recondensated hydrophobic SVOC and a couple of possible candidates such as alkanes, aromatic hydrocarbons, and phthalates have been proposed.12 This hypothesis is supported by a significant relationship between particle number emission rate and fuser roller temperature found by He et al. for some printers.16 Other factors such as fuser heating method, fuser material and structure of the fuser unit were also found to play a role.16 Even though the presence of inorganic solid compounds has been mentioned, up to now no quantification of the number fraction of solid particles is reported.12 Received: May 10, 2011 Accepted: August 2, 2011 Revised: July 18, 2011 Published: August 02, 2011 7819

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Figure 1. Size-resolved particle number concentration time series of LPD 7 and LPD 10 during printing and after.

Understanding particle formation mechanisms in laser printing devices may contribute to a reduction of particle emissions and is therefore of great interest. Possible mechanisms are discussed in recent publications and are mainly based on the fact that the fuser unit of an LPD fixes the toner thermally to the paper.10,12,16,17 The average temperatures during a print job at the outside of the fuser roller typically range from about 130 210 °C.16 Surrounding plastic components of the LPD as well as the paper and the toner material are therefore exposed to heat, which can be high enough and last long enough to evaporate semivolatiles, such as alkanes, alkenes, and siloxanes from these materials. The semivolatiles may then quickly nucleate in the cooler sections of the LPD and form an aerosol. The first aim of the presented work is the chemical characterization of fine and ultrafine particles from laser printing devices by the use of XRF. The evaluation of related possible health effects requires not only information about quantities, but also on chemical composition of aerosols. Second, the identification of potential aerosol sources, as presented in this work, contributes to further knowledge on particle formation mechanisms. In this context, the quantification of volatile and solid aerosol fractions provides useful information. It should be noted that XRF is a convenient technique for the investigation of inorganic compounds; in general it is not suitable for the determination of organic materials. This implies that the presented study basically deals with the minor inorganic contributions to the emissions, but also allows tracing organic compounds containing substituents with Z > 13 such as brominated flame retardants (BFR). BFR in the gas phase or condensated on coarse dust particles are known to be related to emissions from various consumer products.8,19 These compounds have been identified in a variety of dust samples as well as samples from building materials. 19 22 However, until now there has been no evidence of ultrafine aerosols containing bromine emitted from LPD.

2. MATERIALS AND METHODS 2.1. Study Design and Setup. The LPDs were operated in a 1 m3 emission test chamber (ETC) in monochrome print mode according to ECMA 328 (5th edition 2010)23 resulting in a 10 min print job with number of printouts ranging from 75 to 330. Preconditioned standard paper of the same brand and type (80 g/m2 A4 paper with water content between 3.8% and 5.6%) was used. The ETC complies with ISO 16000 9. Environmental conditions such as relative humidity and temperature were continuously recorded as well as the power consumption of the LPD.

Particle number concentrations time series were measured while the particles were simultaneously sampled with a 13-stage cascade impactor (Dekati DLPI, range 30 nm to 10 μm) at 10 L/min flow rate on clean polycarbonate substrates. For the purpose of an improved XRF signal-to-noise ratio, two print jobs were sampled in series on the same substrates, respectively. Furthermore, toner samples, paper and those components of the LPD nearby the fuser units, which are obviously exposed to high temperatures, were also examined by XRF in order to check for elements observed in the aerosols. This was possible using a mobile energy dispersive micro-X-ray spectrometer (ArtTAX, Bruker Nano GmbH, formerly R€ontec-GmbH, Berlin, Germany), which consists of an air-cooled low-power molybdenum tube, polycapillary X-ray optics (measuring spot size 100 μm diameter),24 an electro thermally cooled XFlash detector, and a CCD camera for sample positioning. In general, additional open helium purging in the excitation and detection paths can be used for the determination of light elements. We abandon this feature due to the fragility of the deposited particles. This setup excludes the range of detectable elements with Z e 13. The silicon drift detector with high speed, low-noise electronics permits an energy resolution of 160 eV for Mn KR radiation at a count rate of 10 kcps. It has an active area of 30 mm2 and an 8 μm-thick Dura-beryllium window. The geometry between primary beam, sample, and detector is fixed at 0°/40° relative to the perpendicular of the sample surface.25 All measurements were made using a 30 W low-power Mo tube (50 kV, 600 μA) with an acquisition time of 60 s (live time) to minimize the risk of radiation damage. In order to measure the particle number concentration time series we used different types of butanol-based condensation particle counters (Grimm CPC 5414, TSI CPC 3775). A rotating disk thermodiluter (TSI/Matter Model 379020A-30) between chamber and CPCs was operated at room temperature and kept the CPCs in single count mode. The detectable particle size ranges from 4 nm to 3 μm for both instruments. The calculation of the total number of emitted particles, TP, was carried out according to the procedure described in ECMA 328.23 The emitted particle size distributions between 5.6 and 560 nm, as shown in Figure 1, have been characterized previously with an EEPS (TSI Engine Exhaust Particle Sizer 3090). This size range covers 99.9% of the particle emissions in respect to number concentration values. Background samples were taken in the ETC, loaded with an LPD which was not connected to electric power. During this phase, the background usually was below 20 particles/cm3. Contamination of cascade impactor samples by residual gas phase SVOC and VOC in the ETC are negligible as test measurements 7820

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have shown. Thus, any elements observed in XRF can be assigned to particulate emissions from LPD. Table 1 summarizes the measured TPs. The cascade impactor samples for each printer were analyzed stage by stage providing size-dependent information on the elemental composition of the particles deposited. Each stage of the cascade impactor has a characteristic impact pattern consisting of single jets arranged in a number of concentric circles. The jets typically have a diameter of about 0.3 mm. Taking into account a minimum step width of 0.05 mm of the XRF step motor, a total of four spectra could be recorded across one spot on each sample. The recorded spectra were subsequently averaged. This method ensures that an average surface composition of the particles in each size class is represented by the results. Samples with no visible accumulation of deposited particles could not be analyzed due to a low particle density. In some cases, the visible spots of deposited aerosol did not result in any XRF signal suggesting that the particles mainly consist of organic compounds or elements with Z e 13.

3. RESULTS AND DISCUSSION 3.1. Analysis of Aerosol Samples. It is a general finding that LPD emit particles with aerodynamic diameters near to or below 100 nm.7,5,9,10 Primary toner particles with typical diameters above 1 μm obviously do not become airborne and therefore do

Table 1. Summary of the Total Numbers of Emitted Particles (TP) Obtained by Particle Emission Measurements TP (particles)

printed pages

TPpp (particles/printed page)

LPD 1

2.0  1011

210

9.5  108

LPD 2

2.1  1011

160

1.3  109

LPD 3

12

1.9  10

250

7.6  109

LPD 4 LPD 5

8.2  10 3.8  1011

240 160

3.4  109 2.4  109

LPD 6

2.0  108

75

2.7  106

LPD 7

7.9  10

330

2.4  109

LPD 8

7.3  10

220

3.3  109

LPD 9

3.2  10

230

1.4  109

LPD 10

2.0  10

210

9.5  108

11

11 11 11 11

not contribute to the number concentration of emitted particles. This finding is confirmed by measurements in different laboratories.7,17 Typical size-resolved particle number concentration time series are shown in Figure 1. According to the nomenclature suggested by Schripp, the left print pattern of Figure 1 represents a so-called “initial burst emitter” while the right pattern characterizes a “constant emitter”.9 In our work, most LPD do fit more or less into this categorization while some reveal more complex patterns. The particle size distributions, as roughly characterized by impactor stages with visible particle deposits, are in good agreement with the respective particle size distribution observed with the EEPS. For the interpretation of the XRF data, it must be considered however, that organic particles such as hydrocarbons can be sampled and may form a visible deposit, but cannot be detected and analyzed by XRF. Table 2 gives an overview on elements detected on particle emission samples. Where possible, an assignment to a main source for the element is proposed based on the detailed analysis of possible particle sources as discussed in Section 3.2. In the energy range up to 30 keV, the following elements could be detected: silicon (Si), sulfur (S), chlorine (Cl), calcium (Ca), titanium (Ti), chromium (Cr), iron (Fe), bromine (Br), and traces of nickel (Ni) and zinc (Zn). Previous work by Morawska et al. revealed primarily Ca. Furthermore, Carbon, Fe, Ti, Si, and Mg were observed in varying degrees whereof Ti and Si were assumed to be of mostly environmental origin.12 In contrast, the bulk analysis of large quantities of deposited particles and the comparison with according blank samples performed in this work enables one to clearly attribute the observed elements to the printer aerosol. LPD 6 is an extreme low emitter. In this case, the total number of emitted particles during the print job was only 2.0  108 particles (2.7  106 particles per printed page). The XRF spectra were not evaluable due to an extreme low deposit of particles on the samples. The Ca peaks are observed as a duplet at 3.70 (KR) and 4.01 keV (Kβ), respectively. This element is the most common, found in varying intensity in all LPD aerosol samples except for the low emitter LPD 6. Cl peaks at 2.65 keV are only observed in low intensities in 2 out of 10 printer aerosols (LPD 7 and 8), whereas

Table 2. Qualitative Overview on XRF Results for Aerosol Samplesa sources

paper

toner

structural components

KI stage

1

2

3

4

5

2

3

4

5

2

3

4

d50 (nm)

30

60

100

160

270

60

100

160

270

60

100

160

Si, S, Crb, Feb, Znb

Si, S, Znb

Brb

Br

Br

Si

Si

Si, Tib, Fe

Tib, Feb

Brb

Br

Brb

Cab

LPD 1 LPD 2

Cab

LPD 3

Cab

Cr, Fe, Nib Cab

LPD 4 LPD 5 LPD 7

Ca

Ca Ca

b

Ca Cl, Ca

LPD 8 LPD 9 LPD 10

Clb, Ca Ca

Clb b

Ca

Clb, Ca Ca

Ca

Sib, Sb b

b

Cl , Ca, Ti

Fe

Si, Fe

b

Ca

a

Impactor stages 1 5 are shown and labelled with their according cut points (d50). The elements are assigned to the assumed main sources paper, toner, and structural components, where possible. As discussed in Section 3.2.1 Si and Ti cannot be clearly assigned to a single source. b Traces only; empty cells: data not evaluable. 7821

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Figure 2. Particle size resolved XRF analysis of aerosol samples from LPD 7 and 9. Elemental assignment refers to KR and Kβ lines, respectively. Scattering peaks (Rayleigh/Raman) are marked with SC. The element Ar results from ambient air, the presence of Mo is due to the target material of the X-ray tube.

Si can be detected in three cases (LPD 7, 8 and 9) at an energy of 1.74 keV. For 2 out of 10 LPD (LPD 7 and 9) bromine can clearly be detected in the aerosol samples. The according XRF spectra are shown in Figure 2. The X-ray fluorescence peaks of bromine appear at 11.92 keV (KR) and 13.29 keV (Kβ). Only LPD 7 shows a small peak around 2.3 keV, which clearly indicates sulfur. In other cases, this assignment is challenged by superimposed peaks of the target material. Inorganic compounds such as metals or metal oxides as typical components of toner would be easy to detect by XRF but are in fact not visible on aerosol samples from stages above d50 = 270 nm in any case. Thus, also from size-resolved XRF aerosol analysis it can be concluded that toner particles as such are not present in the aerosol, but as the below discussion reveals, some of their ingredients can be found in smaller particles. In the following, the assignments of the elements to potential sources as listed in Table 2 are discussed in detail. 3.2. Analysis of Particle Sources. As described before, the elements identified in the aerosol emissions are assumed to originate from the materials involved in the printing process that are exposed to high temperatures. In order to evaluate this assumption, toner, paper, and LPD plastic components surrounding the “hot spots” of the fuser units were examined by XRF. 3.2.1. Toner and Paper. A total of four exemplary toner samples was examined and the results are listed in Table 3.The analysis identified Si, Sn, Ca, Ti, V, Cr, Mn, Fe, Ni, and Zn. Fe is the dominant component in toner powder samples taken from the toner cartridges. This is based on the fact that many toners mainly consist of iron(III)oxide Fe2O3, polymer resin, pigment, and additives. Si peaks in some toner samples indicate the presence of fumed silica which is used as a toner additive for enhanced flow and charge stability. It is assumed that further metals which were detected, such as Mn, Cr, V, and Zn, are added to the toner for catalytic reasons in order to improve the oxidation and polymerization of the polymers.26

Table 3. XRF Results of Four Exemplary Toner Samples XRF results of toner samples LPD 3

Si, Sn, Ca, Ti, V, Cr, Mn, Fe

LPD 5

Si, Sn, Ca, Ti, V, Cr, Mn, Fe, Ni, Zn

LPD 7

Ca, Ti, V, Cr, Mn, Fe, Zn

LPD 9

Si, Ca, Ti, Cr, Fe

Toner analysis results by Morawska et al. mainly revealed Fe, some Ti and Sr, along with very small amounts of Al and Si.12 The additional elements found in this study may simply reflect different manufacturers. Figure 3 presents as an example the XRF-spectra from LPD 7 toner and also from office paper samples. The unused polycarbonate substrates are free of contaminations, except for low intensity Cr and Fe peaks which are due to the experimental setup and have been considered when analyzing particle samples. A respective XRF spectrum is also shown in Figure 3. In the paper samples spectra shown in Figure 3 Ca, Ti, Fe, and Sr as well as traces of Cl can be detected. In contrast to more detailed studies on office paper27 no trace elements like Mn, Y, Ba, La and Ce were found due to the limited sensitivity of the method used here. The main component of the elemental composition is Ca. The presence of Ca is most likely due to CaCO3, which is used as mineral filler in order to optimize properties of the paper.28 The black area of the paper printed with LPD 7 contains the elements Cr, Mn, and Zn which are found in the toner. Ti and Fe have also been observed in the processed white paper but show a significantly higher intensity in black printed areas. At least a fraction of Ti is probably due to titanium(IV)oxide often used as white pigment in paper. As the CaCO3 and the TiO2 used in the process of paper manufacturing are no high purity chemicals, it is also possible that further trace elements in the paper derive from that source.26 7822

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Table 4. Overview on Some Flame Retardants (ISO 1043-4) flame retardant

description

FR (16)

aromatic brominated compounds excluding brominated

FR (17)

aromatic brominated compounds excluding brominated

diphenyl ether and biphenyls diphenyl ether and biphenyls in combination with antimony compounds FR (40)

Figure 3. XRF spectra of LPD 7 toner and processed paper. Elemental assignment refers to KR and Kβ lines. Pile-up (PU) and scattering peaks (SC) are marked, respectively.

The elements Si, Cr, Fe, Ni, and Zn are found in toner and in the aerosol emitted from LPD. From this it can be concluded that major parts of these elements in the aerosols can be traced back to the toner. In the case of Si contributions from siloxanes are assumed. Although found in toner as well, Ca is assigned to office paper as it represents its major inorganic component and paper abrasion is a plausible mechanism for the generation of Cacontaining particles. Furthermore, this assignment as well as the assignment of Fe to the toner is in agreement with the findings of Morawska et al.12 Ti occurs both in office paper and toner and thus cannot be clearly assigned to one of the sources, whereas traces of Cl are found in paper only. The assignments are given in Table 2 accordingly. It is worth noting that Br was neither found in the toner powder nor in the paper samples. 3.2.2. Structural LPD Components. The high temperatures close to the fuser unit and the limited space inside the LPD obviously require the protection of inflammable plastic structural components with flame retardants. Manufacturer’s labels found on structural components provide information on plastic materials as well as on flame retardants used. The materials could be identified as glass fiber reinforced polyesters (PBT, PET), acrylonitrile butadiene styrene (ABS) or polycarbonate (PC). Some LPD plastic components contain Si which might be related to the use of fumed silica as filler.29 The composition of the LPD plastic components is not discussed in detail here as the main purpose is to identify possible sources for the elements observed in the LPD aerosols. Table 4 gives an overview on flame retardants identified by manufacturer’s labels according to ISO 1043 4. Six out of ten LPD labels indicate components with brominated flame retardants (BFR), while in the LPD where no label was found on the components, the XRF spectra also suggest the presence of BFR. Figure 4 shows XRF-spectra of fuser unit housings from LPD 1, 7, and 10. An overview on the analysis of printer components is given in Table 5 below. All LPDs feature at least one component where two dominant bromine peaks can be observed in the XRF spectrum at 11.92 and 13.29 keV,

halogen-free organic phosphorus compound

respectively. LPDs 3 and 4 are exceptions where bromine is found in traces only. Additionally, in the XRF spectra from 8 out of 10 printers, the Br peak is accompanied by a Sb peak at 26.36 keV (KR). The peak is observed in the energy range of the Br pile-up peaks, but can clearly be distinguished from the Br (Kβ/Kβ) pile-up (refer to Figure 4, right). Antimony is often used in the form of Sb2O3 (antimony(III)-oxide) in order to improve the flame retarding properties. The fire retarding agent is SbBr3, which is formed when Sb2O3 is heated in the presence of BFR.30 Antimony is present in plastic structural components, but is not observed in LPD aerosol samples. Thus possible contributions from SbBr3 to the Br peaks in the aerosol samples can be excluded. Summarizing these findings strongly suggests that the source of bromine in the aerosols emitted from LPD are the BFR released from plastic components. The pressure roll ensures that the paper is pressed against the fuser roller during the fusing process. Only in one case this mechanical component was accessible to XRF measurements without completely disassembling the LPD. Even though this element is exposed to high temperatures, no indicators for flame retardants such as Br or P could be detected. Basically all component XRF spectra - except for LPD 1 and LPD 8, C2 - contain phosphor. This is a hint to the FR (40) organophosphate flame retardants mentioned above. However, phosphor was not detected in printer aerosols in this study. The detailed analysis of chemical composition and sources of particles has a direct impact on the evaluation of possible health risks such as occupational diseases related to the LPD-particle emissions. Further in vitro and in vivo investigations on LPD emissions now may precisely focus on specific substances and particle sizes rather than on total aerosol concentrations only. The identified particle sources may also form the basis for emission avoidance strategies by means of technical changes of LPD. 3.3. Volatility of the Particles. For the purpose of analyzing the volatility of the particles emitted from LPD and in order to quantify possible solid residues, we used the rotating disk thermodiluter with thermal conditioner air supply (TSI/Matter 379020A-30) at a temperature of 400 °C. Prior to evaporation of volatile components a dilution of the aerosol was applied to avoid recondensation when the aerosol is cooled down again. The total particle number concentrations of the aerosols from LPDs 4 and 7 were measured with this equipment at room temperature and after evaporation at 400 °C. LPD 7 was chosen due to significant XRF peaks indicating a relatively high inorganic fraction in the aerosol, while the XRF results of LPD 4 with only traces of Ca point toward a low inorganic fraction. For both printers, primary dilution was applied by a factor of 125 at room temperature and by a factor of 13 at 400 °C, which is the minimum dilution. The higher dilution at room 7823

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Figure 4. XRF spectra of fuser unit housings (LPDs 1, 7, and 10) (left). Elemental assignment refers to KR and Kβ lines. Pile-up (PU) and scattering (SC) peaks are marked, respectively. Antimony is identified by the presence of the Sb (KR) lines and can clearly be distinguished from the Br pile-up peaks (right).

Table 5. Summary of the XRF Analysis of LPD Fuser Unitsa manufacturer’s label

a b

XRF result

LPD 1

PBT-GF30-FR(17)

LPD 2

PET-(GF+MD)40-FR(17)

Br, Sb P, Br, Sb

LPD 3

C1: no label C2: no label

Si, P P, Brb

LPD 4

C1: PC+ABS-FR(40)

P, Brb

C2: no label

Si, P, Brb

LPD 5

PET-GF40-FR(17)

P, Br, Sb

LPD 6

C1: PBT-GF30-FR(17)

Pb, Br, Sb

C2: PC+ABS-FR(40)

P, Brb

LPD 7

no label

P, Br, Sb

LPD 8

C1: PET-(GF+MD)40-FR(17) C2: PBT-I-GF30-FR(17)

P, Br, Sb Br, Sb

LPD 9

PET-(GF+MD)40-FR(17)

P, Br, Sb

LPD 10

no label

P, Br, Sb

C1 and C2 indicate separate components of the same fuser unit. Traces only.

temperature is required to operate the CPC in single count mode. The calculated TP400 °C values were 1.6  109 particles (0.2% of TPRT) and 1.5  1010 particles (1.9% of TPRT) for LPD 4 and LPD 7, respectively. This is in good agreement with the findings of the XRF analysis. The XRF results show the presence of elements that are likely to be present in an inorganic and solid form such as Ca, Ti, and Fe, especially in the case of LPD 7, where the peak intensities of Ca for example are significantly higher than those of LPD 4. As discussed above, these elements are mainly assigned to contributions from office paper as well as toner. The low inorganic particle number fraction (0.2%) in the aerosol of LPD 4 coincides with the low intensity of the Ca peaks detected by XRF.

Attempts to sample and analyze the remaining solid particles after evaporation of the volatile components have not been successful due to the high dilution of the aerosols. However, the presence of solid particles implies that heterogeneous nucleation processes have to be considered when discussing particle formation mechanisms.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 0049 30 8104 bam.de.

3802, fax: 3877, e-mail: stefan.seeger@

’ ACKNOWLEDGMENT This work was supported by The Federal Environment Agency UBA under grant UFOPLAN 3708 95 301. ’ REFERENCES (1) Wallace, L. A.; Emmerich, S. J.; Howard-Reed, C. Source strengths of ultrafine and fine particles due to cooking with a gas stove. Environ. Sci. Technol. 2004, 38, 2304–2311. (2) He, C.; Morawska, L.; Hitchins, J.; Gilbert, D. Contribution from indoor sources to particle number and mass concentrations in residential houses. Atmos. Environ. 2004, 38, 3405–3415. (3) Matson, U. Indoor and outdoor concentrations of ultrafine particles in some Scandinavian rural and urban areas. Sci. Total Environ. 2005, 343, 169–176. (4) Gehin, E.; Ramalho, O.; Kirchner, S. Size distribution and emission rate measurement of fine and ultrafine particle from indoor human activities. Atmos. Environ. 2008, 42, 8341–8352. (5) Kagi, N.; Fujii, S.; Horiba, Y.; Namiki, N.; Ohtani, Y.; Emi, H.; Tamura, H.; Kim, Y. S. Indoor air quality for chemical and ultrafine particle contaminants from printers. Build. Environ. 2007, 42, 1949–1954. (6) Lee, C. W.; Hsu, D. J. Measurements of fine and ultrafine particles formation in photocopy centers in Taiwan. Atmos. Environ. 2007, 41, 6598–6609. 7824

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