Measurements of fine and ultrafine particles formation in photocopy centers in Taiwan

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Atmospheric Environment 41 (2007) 6598–6609 www.elsevier.com/locate/atmosenv

Measurements of fine and ultrafine particles formation in photocopy centers in Taiwan Chia-Wei Leea, Der-Jen Hsub, a

Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 811, Taiwan b Department of Occupational Safety and Health, Chang Jung Christian University, 396 Chang Jung Road, Sect. 1, Kway-Jen, Tainan, Taiwan Received 9 February 2007; received in revised form 5 April 2007; accepted 11 April 2007

Abstract This study investigates the levels of particulate matter smaller than 2.5 mm (PM2.5) and some selected volatile organic compounds (VOCs) at 12 photocopy centers in Taiwan from November 2004 to June 2005. The results of BTEXS (benzene, toluene, ethylbenzene, xylenes and styrene) measurements indicated that toluene had the highest concentration in all photocopy centers, while the concentration of the other four compounds varied among the 12 photocopy centers. The average background-corrected eight-hour PM2.5 in the 12 photocopy centers ranged from 10 to 83 mg m3 with an average of 40 mg m3. The 24-h indoor PM2.5 at the photocopy centers was estimated and at two photocopy centers exceeded 100 mg m3, the 24-h indoor PM2.5 guideline recommended by the Taiwan EPA. The ozone level and particle size distribution at another photocopy center were monitored and indicated that the ozone level increased when the photocopying started and the average ozone level at some photocopy centers during business hour may exceed the value (50 ppb) recommended by the Taiwan EPA. The particle size distribution monitored during photocopying indicated that the emitted particles were much smaller than the original toner powders. Additionally, the number concentration of particles that were smaller than 0.5 mm was found to increase during the first hour of photocopying and it increased as the particle size decreased. The ultrafine particle (UFP, o100 nm) dominated the number concentration and the peak concentration appeared at sizes of under 50 nm. A high number concentration of UFP was found with a peak value of 1E+8 particles cm3 during photocopying. The decline of UFP concentration was observed after the first hour and the decline is likely attributable to the surface deposition of charged particles, which are charged primarily by the diffusion charging of corona devices in the photocopier. This study concludes that ozone and UFP concentrations in photocopy centers should be concerned in view of indoor air quality and human health. The corona devices in photocopiers and photocopier-emitted VOCs have the potential to initiate indoor air chemistry during photocopying and result in the formation of UFP. r 2007 Elsevier Ltd. All rights reserved. Keywords: Photocopy center; Particulate matter (PM); Ultrafine particle (UFP); Volatile organic compounds (VOCs); Corona charging

Corresponding author. Tel.: +06 278 5123x3115; fax: +06 278 5681.

E-mail address: [email protected] (D.-J. Hsu). 1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.04.016

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1. Introduction Many studies have indicated that ozone, particulate matter (PM) and several volatile organic compounds (VOCs) are emitted during photocopier operation, causing indoor air pollution (Brown, 1999; Lee et al., 2001). Study has shown an association between the sick building syndrome (SBS) and the presence of photocopiers (Skov et al., 1989). VOCs emitted from photocopiers are suspected of causing SBS (Taylor et al., 1984). Consequently, the effect of photocopiers on indoor air quality has attracted substantial interest over recent years. An US EPA report concluded that VOCs from photocopiers may have potential health effects and recommended more research efforts to identify emission profiles from office equipment, and specifically photocopiers (US EPA, 1991). Laboratory and field studies have showed that up to 60 different VOCs may be emitted during photocopier operation (Wolkoff et al., 1993; Leovic et al., 1996; Stefaniak et al., 2000). Due to their ubiquitous sources and potential health effects, benzene, toluene, ethylbenzene, xylenes and styrene (BTEXS) have been the focus of several photocopier-related studies (Wolkoff et al., 1993; Leovic et al., 1996, 1998; Brown, 1999; Stefaniak et al., 2000; Lee et al., 2001, 2006; Mo¨ller et al., 2003; Hsu et al., 2005, 2006). In addition to BTEXS, aerosolized toner powder may be emitted from photocopiers (Brown, 1999; Wolkoff, 1999). Many studies have investigated the health effects of photocopier toner dust and concluded that siderosilicosis and sarcoidosis-like pulmonary diseases are associated with human exposure to photocopier toner dust (Gallardo et al., 1994; Armbruster et al., 1996). However, a toxicity study of rats and rabbits exposed to toner particles indicated no acute oral, inhalation or dermal toxicity (Lin and Mermelstein, 1994). Since the emitted particles during photocopying may differ from the original toners, the composition of toner cannot be used as the only indicator of the hazard of photocopier-emitted particles. However, the characteristics of the emitted particles, which might be more important to health than the original toners, are not fully understood, since most in vitro or in vivo studies have involved original toners only. To date, information on the mass and the number concentration of photocopier-emitted particles remains relatively limited. No information on the size distribution of the photocopier-emitted particles has been reported.

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Photocopying technology is based on electrophotography, in which a corona device is utilized to produce a gaseous ion field. The corona ionization yields an electric field around the wire, which interacts with the electrons in the surrounding air, producing positive or negative ions, depending on the type of voltage applied. Ozone (Yagi and Tanaka, 1979; Chang and Masuda, 1988; Zhou et al., 2003), nitrogen oxides (Rehbein and Cooray, 2001), hydrogen peroxide (H2O2) (Richardson et al.,    2003), free radicals and ions (O, O 2 , O3 , N2 , N ,   H , OH , etc.) (Loiseau et al., 1994) are known to be generated during corona charging by a reaction of charged ions and electrons with atmospheric gases. However, no study has been found to discuss the role of corona devices on the generation and the removal of photocopier-related pollutants. Many studies of photocopier-related emission have been carried out in test chambers (Brown, 1999; Lee et al., 2001, 2006; Hsu et al., 2005, 2006). Field studies on the impact of photocopiers on indoor air quality are relatively limited. Though printer-emitted VOCs and PM has been studied (Kagi et al., 2007), no report has addressed the PM concentration in photocopy centers. Since the size of individual particles influences the degree to which they can be inhaled and the effects that they can cause (Lee et al., 2001), the characteristics of photocopier-emitted particles are needed to evaluate whether exposure control and reduction efforts are necessary. Furthermore, the main difference between the photocopy centers in Taiwan and other developed countries is that such centers in Taiwan are generally small and serve as both businesses and residences. Thus, the pollutants emitted during photocopying would affect the indoor air quality and potentially have adverse health effects on the employees as well as the residents of photocopy centers. The objective of this study is to investigate some selected VOCs concentrations, the number concentration, the mass concentration and the size distribution of the photocopier-emitted particles in some representative photocopy centers in Taiwan. The sources of particles are also discussed and a conceptual model of indoor air chemistry and particles formation and removal during photocopying, involving the effects of corona devices, is proposed. 2. Materials and methods Air sampling was conducted at 12 photocopy centers (A–L) in the Tainan area from November

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2004 to June 2005. Measurements were made on three days, at intervals of approximately two months, at each photocopy center. In Taiwan, most photocopy centers are located in multi-story street houses. The width of each photocopy center is approximately 4.0–4.5 m. In a typical street house, the ground floor is the work area and the upper floors are living areas. Typical interior materials used in photocopy centers include ceramic tile floor, painted concrete ceiling, painted concrete walls and sliding aluminum-framed glass doors. Usually only some metal desks and chairs, and no other furniture are present in the confined space of a photocopy center. Basic information of each photocopy center, including business hours, room dimensions, environmental conditions, types of ventilation and entrance, number of photocopiers, number of copies made and the presence of other VOC-emitting sources were collected. Table 1 lists the physical characteristics of the photocopy centers. Though facsimile machines, color photocopiers and laser printers were seen in some centers, yet they are rarely in use. Therefore, the current study focused on the emission from black-and-white photocopiers. Full-shift area and background samples of BTEXS were collected at each photocopy center. Indoor background sampling works were carried out around 1 h before photocopying operations. Air sampling and analysis was conducted according to the US EPA test methods (US EPA, 1998, 1999) using sorbent tube-automatic thermal desorption (ATD) and gas chromatography/mass spectrometry (GC/MS) or gas chromatography/flame ionization detector (GC/FID), respectively. The samples were collected with Perkin Elmer (PE) stainless steel tubes (1/4 OD, 3.5 in long) packed with 250 mg Tenax-TA 60/80 mesh (Supelco Inc., Bellefonte, PA). Prior to use the stainless steel tubes were conditioned at 250 1C for 4 h by passing through at least 50 mL min1 of pure nitrogen gas. Air was drawn through the tubes using a low-flow SKC Model 222 sampling pump (SKC Inc., Eighty Four, PA) in full shift. Each tube was sampled at a flow rate of 140–150 mL min1 for 2 h, and a total of four tubes were used in a full-shift air sampling (10 am–6 pm). Tests were performed and no breakthrough was found in any of the tubes, verifying the 100% adsorption of the VOCs with PE tubes. The adsorbed VOCs were transferred to GC using an automated thermal desorption system (Model ATD-400, Perkin-Elmer Inc.). The thermal desorption conditions were: primary trap desorbed at

250 1C for 10 min, the secondary trap partially filled (15 mm) with Tenax-TA was held at 30 1C during primary desorption and desorbed at 250 1C for 1 min. The desorbed compounds were transferred to a GC column via a heated (at 220 1C), fused capillary transfer line and then analyzed with Agilent 6890/5973 GC/MSD or GC/FID (Agilent Technologies, Palo Alto, CA). Quality assurance and quality control (QA/QC) of the analytical method such as accuracy and precision were carefully followed by blank test, duplicate analysis, and recovery test. Calibration standards were made by injecting the BTEXS standards in methanol into a heated gas transfer unit connected to an adsorbent tube. The unit contained a 125-mL glass gas sampling bulb (Supelco Inc., Bellefonte, PA), maintained between 200 and 220 1C, through which high purity nitrogen gas flowed at a rate of 150 mL min1. The standards were transferred to the sampling tube over 5 min. The sampling tubes spiked with external standards were analyzed using the same method as the samples. In this study, the spike recovery for VOCs ranged from 92% to 98%. The detection limit of the analytical method for each compound ranged from 0.7 to 1.5 ng. The BTEXS external standard curves were calculated from at least five different concentrations using a linear regression with an acceptable requirement of r2X0.995. TSI Model 8520 DustTrak aerosol monitors (TSI Inc., St. Paul, MN) were used to measure the indoor full-shift and background PM10 and PM2.5 concentration in the photocopy centers. The PM monitoring works were carried out as near to the central positions of photocopy centers as possible. The preliminary results showed that PM2.5 in the photocopy centers accounted for 70–88% of PM10, indicating that most of the PM at photocopy centers was particles that were smaller than 2.5 mm. Hence, only PM2.5 was measured herein. Since DustTrak operates on the principle of light scattering, its response depends strongly on the size distribution and the refractive index of the sampled aerosol (Morawska et al., 2003). A report has demonstrated that the readings of DustTrak can markedly exceed the true PM values if finer particles, such as those commonly encountered in the indoor air, are sampled (Ramachandran et al., 2000). Therefore, Personal Environmental Monitors (PEM) (SKC Inc., Eighty Four, PA) for PM2.5 samplings were used to collect the airborne particles in photocopy centers to calibrate the readings of DustTrak. Based on the 11 comparison

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Table 1 Physical characteristics of the photocopy centers investigated Center

Room volume, m3

Number of photocopiers

Number of copies madea

ACH

Measurement day

Type of entranceb

Ventilationc

A

102

6

7966 (1380)

10

1 2 3

b b b

f a, f a, f

B

289

7

15,733 (10,196)

8.3

1 2 3

b b b

f a, f a

C

101

6

3100 (2051)

5.1

1 2 3

c c c

f a, f a, f

D

74

7

12,219 (13,283)

8.5

1 2 3

c c c

f f f

E

203

8

8233 (3855)

5.8

1 2 3

c c c

f a, f a, f

F

102

6

8833 (577)

7.8

1 2 3

b c c

– f f

G

56

4

8710 (6425)

5.9

1 2 3

b c c

f a, f a, f

H

84

4

6400 (2253)

10.1

1 2 3

a a a

f f f

I

72

9

7200 (2443)

7.2

1 2 3

b c c

f a, f a, f

J

90

7

5300 (360)

6.2

1 2 3

b c c

a a a

K

101

5

7300 (2524)

5.4

1 2 3

c c c

 a a

L

117

5

12,966 (4669)

9.8

1 2 3

b c c

– a, f a, f

30

3

11340

6.5

1

c

–

Md a

The average in three measurement days; ( ) denotes standard deviation. a, wide-opened; b, half-opened; c, automatic or push-and-pull door. c a, air-conditioner; f, fan; ‘‘–’’, neither air-conditioner nor fan. d Only one measurement was made. b

measurements, the following linear regression equation was obtained: PM2:5 ðSKCsampler Þ ¼ 0:4911PM2:5ðDustTrakÞ  0:0003 ðR2 ¼ 0:85Þ.

ð1Þ

All of the PM2.5 readings from DustTrak were calibrated using this equation. The size distributions of the photocopier-related particles were measured at another photocopy center (M) using a Model 3934 scanning mobility

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particle sizer (SMPS) and a Model 3320 aerodynamic particle sizer (APS) from TSI Inc. (St. Paul, MN). The SMPS consisted of a Model 3080 electrostatic classifier and a Model 3025A condensation particle counter (CPC) from TSI Inc. The indoor ozone concentrations were monitored using the ozone analyzer (MLs 9810B) from Ecotech Pty Ltd. (Blackburn, VIC, Australia). A preliminary experiment was conducted in laboratory to investigate the size distribution of original photocopier toners. A home-made aerosol disperser, similar to fluidized-bed aerosol generator, was installed on the top of a stainless steel chamber of 1 m  1 m  2.4 m in dimension. As the toner powders settled from the upper part of the chamber, the size distribution was measured by APS which was located in the lower part of the chamber. 3. Results and discussion 3.1. PM2.5 emission Since PM2.5 values were calibrated by PEM in this study, therefore, it should be noted that PM2.5 values were the approximation of the true values. Table 2 shows the average background and background-corrected 8-h concentrations of PM2.5 measured at the 12 photocopy centers. The background-corrected PM2.5 values ranged from 10 to 83 mg m3 with an average of 40 mg m3. A very large variation in PM concentrations was observed among the photocopy centers and the measurement days. Of the 12 photocopy centers, F and K had the highest PM concentration while I had the lowest. Various factors, such as number of copies made, air change rate and the availability of other emission sources, may affect the PM concentration in indoor environments, and identifying their respective contribution is very difficult. The way the fans were placed in center I might partially contribute to the low PM concentration found. Those fans were placed purposely in the rear and blew towards the entrance, while the fans in other centers were placed without any particular pattern. As shown in Table 1, no ventilation was used in center F on measurement day 1 while only fans were used on days 2 and 3, on which automatic door was set. It is very likely that PM emitted from the photocopiers in center F were held in the room. The reason the PM concentration at center K was high is probably it is the only center of the 12 having a large format printer. The printer ink is speculated to be an extra

Table 2 Average background and background-corrected 8-h PM2.5 concentration of the 12 photocopy centers Copy center

A B C D E F G H I J K L

Background PM2.5 (mg m3)

Background-corrected 8 h-averagea (mg m3)

Mean

SDb

Mean

SD

60 63 70 63 45 73 63 40 30 53 77 47

40 6 10 6 36 12 6 27 17 12 12 35

23 33 50 20 22 83 40 23 10 63 67 47

6 15 17 10 17 23 36 6 0 35 15 12

a Background-corrected background PM2.5). b Standard deviation.

8 h-average ¼ (8 h

PM2.5

average-

source of VOCs for the formation of secondary ultrafine particles (UFP, o100 nm). The source of UFP will be discussed in the following section. In Taiwan, photocopy centers usually open before 9 am and close at 10 pm. If the background PM2.5 value is taken as the particle mass concentration in close hours and the 8-h average PM2.5 value as that during business hours, then the 24-h average PM2.5 concentration can be calculated for each photocopy center by assuming 12 for business and close hours, respectively. Thus, the concentrations of PM2.5 at photocopy centers F and K exceeded 100 mg m3, the 24-h indoor PM2.5 guideline recommended by the Taiwan EPA (Taiwan EPA, 2005). Additionally, most photocopy centers in Taiwan open 6 days a week and 51 weeks per year (they are closed only for Chinese New Year holidays). Therefore, based on the results of this study, the PM2.5 in photocopy centers should be concerned in view of annual human exposure. 3.2. Source of PM The results of the preliminary study indicated that most photocopier toners ranged between 5 and 15 mm, with a number median aerodynamic diameter (NMAD) of 6.5 mm and a mass median aerodynamic diameter (MMAD) of 10.7 mm. Studies have suggested that the aerosolized toner powder may be emitted from photocopiers in

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operation mode (Wolkoff, 1999; Lee et al., 2001) and even in idle mode (Brown, 1999; Hsu et al., 2006). Lee et al. (2001) pointed out that, typically, about 75% of photocopier toner is transferred to the photoconductive drum and that which does not adhere to the drum becomes available for emission to indoor air. Therefore, the particle size distribution at photocopy centers is reasonably expected to be similar to that obtained in laboratory. Thus, APS and SMPS were used to measure the particle size distribution in two periods at photocopy center M, one before photocopying began (to obtain background values) and the other during business hours, when many numbers of copies were being made. The MMAD of the particles obtained by APS before and during photocopying were 10.4 and 7.1 mm, respectively. The MMAD of 10.4 mm obtained in the background air of photocopy center, approximates the value from the laboratory, indicating that the particles are possibly toner particles. The MMAD of 7.1 mm obtained during photocopying suggested large number of smaller particles were emitted to the air, thus size distribution was shifted towards smaller sizes. Fig. 1 shows the particle number concentration measured by SMPS before (i.e. background) and during photocopy operation as well as the outdoor air. As shown, the particle number concentrations in background air and during photocopying were much higher than that in outdoors. This suggested that there are sources of UFP in photocopy centers and

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UFP concentration would remain high when ventilation is inadequate, such as in close hours. Additionally, the number concentration of particles that were smaller than 500 nm was much higher during photocopying than that in the background air and the difference increased as the particle size decreased. The increase in the concentration of submicrometer particles in this study seems to be inconsistent with the results of studies which suggested that PM emitted by photocopiers are aerosolized toner powder (Wolkoff, 1999; Brown, 1999; Lee et al., 2001). Fig. 2 plots the ozone concentration and particle number concentration obtained in photocopy center M. The figure reveals that the concentration of ozone increased when photocopiers began to make copies and continuously increased as photocopying proceeded. The average ozone concentration before operation was about 4 ppb. It rose to about 40 ppb with a maximum of 70 ppb during the monitored period. The few dramatic declines in ozone concentration in the figure coincided with the times when the door was opened, based on our field records. Based on ozone measurement in center M, ozone concentration at some photocopy centers during business hours may exceed the value (50 ppb) recommended by the Taiwan EPA (2005), if ozone emission is not reduced and the ventilation is inadequate. Fig. 2 also indicated that the particle number concentration increased as photocopying began and

Fig. 1. Number concentration of different particle size ranges measured in photocopy center M and outdoor air.

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Fig. 2. Levels of ozone concentration and particle number concentration in photocopy center M.

ozone emission started. The number distribution of photocopier-emitted particles appears to be unimodal and the UFP dominated the number concentrations with the peak concentration appeared at sizes of under 50 nm. A high number concentration of UFP was found with a peak value of 4108 particles cm3 during photocopying. Both ozone and particle number concentrations were found to increase during the first hour of photocopying, however, the former continued to increase while the latter was found to decrease after that period of time. As shown in Fig. 2, the ozone concentration almost doubled (from mid-30 to 70 ppb) in the second half of the monitored period, while the particle number concentration in all sizes (o500 nm) declined by 50% in the same period. The unexpected phenomenon, namely, particle number concentrations declined as operation proceeded for a few hours (usually 1–2 h), is not a special case observed in our study. The same trend was also observed in PM2.5 monitoring conducted in other three copy centers (Fig. 3). Even though the photocopying operations proceeded in the afternoon, the indoor PM2.5 concentrations in those three centers were lower in the afternoon than in the morning.

3.2.1. PM removal in photocopy centers Corona ionizer is widely used in indoor air cleaning. Many studies have confirmed the emission of air ions that charge aerosol particles can reduce the concentration of airborne dust in indoor environments (Krueger and Reed, 1976; Grabarczyk, 2001; Lee et al., 2004). Lee et al. (2004) found that particle mobility at high ion emission rates could enhance the deposition of particles on walls and other indoor surfaces. Those authors also concluded that the particles are charged primarily by the diffusion charging mechanism, and that the particle removal efficiency was independent of the particle size. McMurry and Rader (1985) showed that positively and negatively charged particles of sizes from 0.05 to 1.0 mm were lost at equal rates to Teflon film surfaces at modest levels of bipolar ionization, contributing to particle loss by electrical forces. Shimada et al. (1989) conducted a similar study and concluded that the deposition rate of particles of sizes from 0.02 to 0.2 mm increased by a factor of ten when an electric field was present. In this study, particle removal was also observed to be independent of the particle sizes. The decrease in particle number concentration obtained herein is in agreement with the works cited above. This decrease in the number concentration is likely

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Fig. 3. PM2.5 levels in photocopy centers B, F and G with continuous photocopying operation.

attributable to the surface deposition of charged particles, which are charged primarily by the diffusion charging of corona devices equipped inside the photocopiers. Particle charging is a function of the ion concentration. Based on the monitored results in center M, ozone and particle concentrations increased immediately as the operations proceeded (Fig. 2). During the first hour of operation, ions emitted from corona devices might not be high enough to charge particles indoors; therefore, the increasing trends of ozone and particles were consistent. However, after the first hour of operation, the ion concentrations in indoor environment might reach to a point that can accelerate the speed of diffusion charging and increase the deposition rates of charged particles to nearby surfaces. After this point, the particle removal rates were higher than the particle formation rates and therefore the particle number concentrations decreased, although photocopying process was consistently being conducted and ozone concentrations continued to increase in the same time period under same ventilation conditions. The findings of PM2.5 declining in the afternoon in other three centers (Fig. 3) could also be explained by the same mechanisms of particle charging and removal. 3.3. BTEXS emission Fig. 4 presents the background and average 8-h time-weighted-average (TWA) BTEXS concentra-

tion obtained by area sampling from the 12 photocopy centers. Similar to the reports by Stefaniak et al. (2000) and Lee et al. (2001), toluene was consistently found to have the highest concentration in every photocopy center, whereas the concentration of the other four compounds varied among the 12 photocopy centers and exhibited no particular trend. The background BTEXS concentration, measured before photocopying began, was in most cases found to exceed the 8-h TWA concentration of BTEXS. Once business hours started, and windows and doors were opened or air-conditioners operated, BTEXS concentrations declined markedly. This phenomenon indicates that indoor VOC-emitted sources other than photocopying may be present and VOCs accumulate overnight. More studies should be conducted to identify VOC-emitted sources and improve the air quality in photocopy centers. The current guidelines for BTEXS in Taiwan are 5, 100, 100, 100 and 50 ppm, respectively (CLA, 2003), while American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values (TLV) suggests concentrations of 0.5, 50, 100, 100 and 20 ppm (ACGIH, 2006), respectively. The BTEXS concentrations obtained from the photocopy centers in this study were considerably below the suggested values set by the above organizations. However, some of these compounds are known to react with ozone and form strong

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Fig. 4. Average background and 8-h TWA BTEXS concentration of the 12 photocopy centers.

airway irritants (Wolkoff et al., 2000, 2006a, b). Accordingly, some precautions should be taken to reduce risks to exposed workers, such as developing low-VOC-emission photocopy machines and toners, increasing ventilation to dilute VOC concentrations, periodic cleaning or replacement of HVAC facilities, venting contaminant source emissions to the outside, and using non-ozone emitting air cleaners. Local exhaust ventilation is also recommended to remove pollutants that accumulate in specific areas. 3.4. Indoor air chemistry during photocopying Many studies have focused on the emission rates of photocopier-emitted pollutants (Leovic et al., 1998; Brown, 1999; Lee et al., 2001). However, the indoor air chemistry resulting from photocopieremitted pollutants has not yet been fully addressed. 3.4.1. Formation of VOCs Dry toners are fine powders that consist of synthetic resin, polyethylene wax and carbon black (Furukawa et al., 2002). The fusing stage of the photocopying process provides finishing touches that cause the toner image to remain permanently on a sheet of paper. Depending on the characteristics of the toner and the fuser materials, the toners consistently emit such VOCs as benzene, toluene, styrene, ethylbenzene, xylenes, acetophenone, alkanes and aldehydes (Wolkoff et al., 1993; Brown, 1999; Leovic et al., 1998; Stefaniak et al., 2000; Lee

et al., 2001, 2006; Hsu et al., 2005, 2006). VOCs may be emitted from paper that is heated in the fuser of a dry-process copier. Other materials in photocopy centers, including printed documents, cleaning solvent, office furniture, building materials, flooring materials and other office equipment, can emit chemicals into the indoor environment. 3.4.2. Formation of ultrafine particles The particle size distribution obtained in this study indicated that UFP dominated the photocopier-emitted particles. Different mechanisms might contribute to the formation of UFP during photocopying: (a) Physical process of nucleation and condensation. The first possible formation mechanism of UFP is the nucleation/condensation of low vapor pressure substances, which were vaporized at high temperature and condensed at low temperature to form particles. Some substances from the heated toner or paper were vaporized during the fusing stage, in which the fuser temperature reached around 200 1C, and their concentrations exceeded their saturation vapor concentration. Therefore, particles may form when the saturated vapor condenses at a lower temperature. (b) Oxidation of VOCs. The second possible mechanism of UFP formation during photocopying is the oxidation of indoor VOCs. The byproducts of corona charging during photocopying, such as ozone, NOx and OH-radicals, are both strong

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oxidants for the oxidations of emitted VOCs. Many studies have demonstrated that photo-oxidation products of aromatic hydrocarbons can undergo various reactions to produce secondary organic aerosols (SOA) in the presence of O3, OH radicals, and NOx (Forstner et al., 1997; Yu et al., 1997; Seinfeld and Pandis, 1998; Edney et al., 2000, 2001; Jang and Kamens, 2001). The microenvironment inside the photocopier is very similar to a photochemical smog chamber that contains a light source and higher concentrations of reaction agents. Therefore, SOA formation inside photocopiers might be an important source of indoor UFP during photocopying. Furthermore, many studies have confirmed that ozone may react with unsaturated VOCs (such as terpenes and styrene), causing secondary emission of UFP in an indoor environment (Weschler and Shields, 1997, 1999; Wainman et al., 2000; Wolkoff and Nielsen, 2001; Fan et al., 2003, 2005). Even though UV irradiation is not present in indoor environment (except the spaces inside the photocopiers), SOA may form when ozone reacts with those unsaturated VOCs presented in photocopy center. (c) Ion-induced nucleation. Ions, which are generated by corona devices during photocopying, may play a role in the formation of UFP by ion-induced nucleation of organic vapors. Many works have confirmed the effect of ionizing radiation on aerosol formation (Ramamurthi et al., 1993; He and Hopke, 1993). Ion-induced nucleation is the gasto-particle process causing supersaturated vapors to condense on ions. During ion-induced nucleation processes, the higher particle growth rates are observed because electrostatic forces would enhance

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the stability of electrically charged clusters (Yu and Turco, 2001). Corona air ionizers are extensively used in cleanroom environments to minimize electro-static dissipative problems, but ionizers have been found to generate minute particles (Liu et al., 1987). Ichitsubo et al. (1996) reported an experimental study of UFP generated from organic vapors by corona ionizers. Among the organic compounds tested (aromatics, alcohols, ketones and others), only aromatic compounds undergo gas-to-particle conversion process and yield unstable clusters, which may grow into detectable particles (42 nm) during corona discharge. Based on the results of the above studies, UFP could be formed rapidly during photocopying by the ion-induced nucleation of emitted aromatic hydrocarbons. To date, the information regarding the formations of VOCs and UFP during photocopying is still limited. The mechanism of UFP formation is far from being well understood and a single process is not likely to explain all the sources of particles. Although the formation mechanism remains unclear, Fig. 5 summarizes the possible mechanisms for the formation of UFP during photocopying, including condensation, oxidation and ion-induced nucleation. Corona devices, which can generate ozone, NOx, radicals and ions during photocopying, may be the key element of UFP formation and particle removal in photocopy centers. 4. Conclusions This study concludes that ozone and UFP concentrations in photocopy centers should be concerned in view of indoor air quality and human

Fig. 5. Conceptual model of indoor air chemistry and particle formation and removal during photocopying.

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health. The corona devices in photocopiers and photocopier-emitted VOCs have the potential to initiate indoor air chemistry during photocopying and result in the formation of UFP. Yet, the indoor UFP concentration may decline when particle deposition rate, as a result of the continuous emission of unipolar ions from corona devices, exceeded particle formation rate. Future work should include toxicity tests of the UFP and hazards that arise from co-exposure to ozone and VOCs. Observations of particle size and ion-mobility distributions during photocopying, measurements of gaseous compounds, radicals and ion species involved in nucleation and growth and determination of the chemical composition and other properties of nucleated particles are also needed. Acknowledgements The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract No. NSC 93-2211E-309-001.

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