the formation of incense smoke

Aerosol Science 38 (2007) 39–51www.elsevier.com/locate/jaerosci

The formation of incense smokeYu-Chen Changa,∗, Hsiu-Wei Leeb, Huan-Hsiung Tsenga

aDepartment of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li, Taiwan 32003, Taiwan, ROCbRespirators and Air Filters Test Center, Health Physics Division, Institute of Nuclear Energy Research, Taiwan, ROC

Received 1 September 2006; accepted 20 September 2006

Abstract

The formation of incense smoke generated from four different types of incense sticks, three manufactured in Taiwan and one inJapan, was investigated in a small controlled chamber. The scanning mobility particle sizer and the quartz crystal microbalance wereused for particle size analyses. The count median diameter (CMD) was found to rise swiftly along the path of the incense smoke.Consequently, a representative sampling location was selected for all measurements performed thereafter. All four types of incensesmoke were shown to exhibit characteristic size distributions, CMDs, and mass median aerodynamic diameters (MMADs). Electronmicroscopy depicted liquid and solid nature of Taiwan and Japan incense smoke, respectively. The different physical states of theparticles were suspected to be a result of different smoke-generating ingredients used by different cultures. Finally, the formationmechanisms of both liquid and solid incense smoke were discussed.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Incense smoke; Formation mechanism; Size distribution; Morphology

1. Introduction

Incense smoke is one of the major indoor air pollution sources in Asian countries such as Taiwan, due to religiousceremonies and for commemorating ancestors. Evidence has accumulated in the past that burning incense is a potentialhazard to human health (Chen & Lee, 1996; Lee & Wang, 2004; Löfroth, Stensman, & Brandhorst-Satzkorn, 1991;Rasmussen, 1987). The toxicity of incense smoke originates from combustion processes rather than unburned incenseconstituents.According to Jetter, Guo, McBrian, and Flynn (2002), who performed a comprehensive review of epidemi-ological studies of toxicity of incense smoke done worldwide, the toxicity differs for different incenses due to the factthat there are a large variety of incenses with different compositions and combinations of ingredients made for differentmarket demands and cultural background. Incense smoke particles are generated from the burning process with sizetypically in the submicrometer size range (Cheng, Bechtold,Yu, & Hung, 1995; Li & Hopke, 1993; Lin, 1992), thus arecapable of depositing in the lower respiratory tract. In additional to its known toxicological effect (Lin & Tang, 1994;Lowengart, Peters, Cicioni, Buckley, Bernstein, Preston-Martin & Rappaport, 1987; MacLennan, DaCosta, Day, Law,Ng & Shanmugaratnam, 1977), information regarding concentration and size distribution of different types of incensesmoke is essential for providing accurate estimate of the amount of particles inhaled and deposited in human lungs.

∗ Corresponding author. Tel.: +886 3 463 8800x2571; fax: +886 3 455 9373.E-mail address: [email protected] (Y.-C. Chang).

0021-8502/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.jaerosci.2006.09.003

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40 Y.-C. Chang et al. / Aerosol Science 38 (2007) 39–51

Lin (1992), Li and Hopke (1993), Cheng et al. (1995), and Mannix, Nguyen, Tan, Ho, and Phalen (1996) investi-gated physical and/or chemical characteristics of incense smoke in test chambers of different sizes. The count mediandiameter (CMD), number density, and chamber size were 0.08–0.1 �m/104 cm−3/200 m3, 0.2 �m/107 cm−3/0.2 m3,0.13–0.14 �m/105 cm−3/34 m3, 0.25–0.42 �m/unknown/4 m3, respectively. Cheng et al. (1995) suspected the dis-crepancies in CMD and number density were due to different chamber size employed, while Mannix et al. (1996)suggested that different humidity, incense type, chamber size, and consequently agglomeration rate may all play a rolein the discrepancy observed. Besides count-based properties, Cheng et al. (1995) reported a mass median aerodynamicdiameter (MMAD) of 0.28 �m for incense smoke, whereas Fang, Chang, Wu, Yang, Chang and Yang (2002) reporteda bimodal distribution with two distinct peaks in 0.18–0.32 and 5.6–10 �m ranges.

Because of the diversity of incenses manufactured worldwide, a basic understanding of how incense sticks areprepared as well as the different ingredients incorporated is essential to the proper interpretation of incense smokeproperties. Incenses have been made and used by western and eastern countries since ancient times. People with differentcultural backgrounds developed their own distinctive recipes using ingredients obtainable locally and/or imported fromspecific areas. For example, incenses made in Asian countries are commonly made of ingredients including woodpowders, herbs, woody plants, spices, plant-based glutinous powders, and water. Besides, the techniques used forprocessing the ingredients, such as mixing and grinding, are also different for different cultures. As the processingtechnique affects the uniformity of the powdery ingredients applied along an incense stick and even among differentsticks, temporal inhomogeneity in both physical and chemical properties of the incense smoke may be resulted forthose prepared using less sophisticated pulverizing and mixing techniques.

For example, in Taiwan, incense sticks are prepared by coating a scented powder paste onto bamboo sticks. Besidesthe bamboo sticks, glutinous powders, fragrance powders, and water are also starting materials used in the process. Thepreparation starts with, first, immersing two-thirds of the bamboo sticks into water. Then, the sticks are spread into afan-like shape before being placed and rolled in a tray filled with glutinous powders. This allows the wetted surfacesof the sticks to form an evenly coated powder layer. The next step is to add the fragrance powders into the glutinouspowder tray. The fragrance powders are added at a fixed fragrance to glutinous powder ratio of, for instance, 3 to 1, andthe two are mixed. Then the first step is repeated in the mixed powder tray till three powder layers are yielded. Afterthat, these freshly made incense sticks are set to dry. Lastly, the uncoated one-third of the bamboo sticks is dyed to adesired color to complete the incense stick manufacturing process.

The fragrances used in Taiwan incense recipes are such as aloewood, sandalwood, Machilus wood, and cypresswood. They are mixed with precious fragrance ingredients selected from a large variety of Chinese medicine herbs.According to a local manufacturer, incense price increases with increasing variety of fragrance ingredients used. Theratio of fragrance to herbs is considered a trade secret and each factory develops own recipes based on experienceswith these ingredients. The binder or glutinous powders used are derived from Machilus Kusanoi (large-leaved Nanmu)trees. The bark of the trees contains glutinous constituents and has long been used to bind fragrance powders to bamboosticks. Among the incenses made in Taiwan, Wuchen, Sinshen and Laoshen are three of the most commonly usedincense types. They are named after the origin or source of the wood powders used. For instance, Wuchen incenses aremade using wood powders of low-grade agar wood (or aloes wood), while Laoshen and Sinshen incenses are preparedusing sandalwood powders originated from India and Australia/Southern Asian countries, respectively.

In contrast, Japan incense sticks are made without the support of bamboo sticks. They are prepared by mixingpowdered incense ingredients, a water-soluble binder named “makko”, and water. All powdery ingredients are carefullypulverized into very fine sizes and sifted to control the particle size. The powder mixture is then blended into dough,which is then extruded into thin sticks. The sticks are then set to dry slowly. The binder, makko, is originated from anatural tree bark of an evergreen tree grown in Southeast Asia named Machillus Thunbergii (red machilus or Makko).Typical ingredients used in Japan incense sticks include sandalwood, aloeswood, clove, spikenard, camphor, star anise,benzoin gum, cinnamon bark, and makko.

In this study, the formation of incense smoke particles was investigated in a small controlled chamber. Four differenttypes of incense sticks were used with three of them manufactured in Taiwan and one in Japan. A representativesampling location was selected based on the CMDs and number densities measured at various sampling locations alongthe incense smoke path. The results were compared with those of Lin (1992), Li and Hopke (1993), and Cheng et al.(1995). The particle size distributions were determined with two particle counters and at different humidity conditions.The morphology and size characteristics of fresh incense smoke particles were examined by electron microscopy. Themechanisms responsible for the formation of incense smoke were discussed based on results obtained herein.

Y.-C. Chang et al. / Aerosol Science 38 (2007) 39–51 41

Fig. 1. Sampling locations [212], [321], [221], [322] versus incense location, [232], in the glass chamber.

2. Experimental

The experimental system is consisted of a glass chamber, a flow control/conditioning console, and an ignition controlunit. The glass chamber is as shown in Fig. 1 and is the principal component of the entire system. The volume of the glasschamber is 0.035 m3. The front panel of the chamber is removable for cleaning and for replacing incense samples aftereach experiment. Before and after each experiment, particle-free humidity-controlled air is delivered through an inlet topurge the chamber, and then exits through an outlet. On the front panel, there is an array of sampling ports. A samplingprobe can be inserted through a selected port to a desirable location inside the chamber. The flow control/conditioningconsole is comprised of, from upstream to downstream, a silica-gel drying column, a HEPA capsule, three de-ionizedwater filled bubblers connected in series, and a static mixer. Wet and dry air can be obtained with and without filtereddry air passing through the bubblers, respectively. With the total flow fixed, calibration of the humidity of the mixedflow against volumetric flow ratio of the dry and wet air yielded a linear relationship. Consequently, the humidity ofthe mixed flow is adjustable by varying the ratio. In addition, as combustion brings forth water vapor and temperaturerise in surrounding air, care was taken to monitor humidity and temperature changes in the chamber. Due to the shorttime span of each experiment, less than 3–4 min, the change in humidity and temperature within the chamber wasdetermined to be negligible.

A typical experiment begins by placing an incense sample vertically on the chamber floor. The chamber is then sealedand purged with filtered dry air till both the humidity and background particle concentration have reached the lowestlevel. After that, a mixed flow of wet and dry air is delivered to the chamber till the target humidity has reached andremained steady for an extended period of time. After the purge air is stopped, the incense sample is ready for ignition.Ignition is conducted remotely by electrical heating of a coiled nichrome bare wire in direct contact with the incensetip. The coiled nichrome wire is connected from inside to a variac placed outside of the chamber. To ignite a sample, thevariac is turned on and then off immediately after the tip turned reddish, which normally takes less than 10 s. In addition,as electrically heated bare wire may generate fine particles, precaution was taken to check such particle generation.With the nichrome wire electrically heated without touching a sample for more than 10 s in a purged chamber, no signof such particle generation was observed.

In this study, the scanning mobility particle sizer (SMPS, TSI, Inc., USA) and the quartz crystal microbalance (QCM,California Measurements, Inc., USA) were used to characterize incense smoke properties. The SMPS has a detectablesize limitation up to 1 �m, whereas the QCM can detect particles up to 25 �m, at least one order of magnitude higherthan the SMPS. Nonetheless, the SMPS provides high-resolution count-based distributions below 1 �m, while the QCMwas used not only to detect particles greater than 1 �m, but also to take advantage of its shorter sampling time, 1 min50 s, rather than 3 min with the SMPS. In other words, the first two consecutive QCM measurements nearly overlap


with the first SMPS measurement, making it possible to examine the dynamic change within the first few minutesof incense smoke formation. In addition, incense smoke particles were collected for scanning electron microscopy(SEM) examination. All particle samples were collected on SEM sample stages stick inversely to the ceiling rightabove the burning incense, [202], under a low relative humidity condition (RH =10%). After removal from the ceiling,they were sputter-coated with a thin layer (about a few nanometers thick) of gold to improve conductivity of sample.A JEOL JSM-5600 SEM and a JEOL JSM-6330F field emission scanning electron microscopy (FESEM) were employedin this study.

Preliminary runs revealed a rising plume and a path the smoke follows within the small chamber suggested the needfor an evaluation of incense properties at various locations. A simple coordinate system was adopted for tagging theselocations. The upper left corner of the front panel was set as the origin, [XYZ] = [000]. The total lengths in X andY directions were divided into three equal segments with the intersecting nodes labeled in ascending order from theorigin. The distance between the neighboring nodes in X and Y directions are 10.5 and 7.25 cm, respectively. In the Zdirection, nodes 1 and 2 are the locations 1 cm (relative to the front panel) and 15 cm (about half way into the chamber)from the origin, respectively. Based on the system, the incense tip is located at [232], or can be conveniently referencedto as the center [X = 2] lower [Y = 3] inner [Z = 2] location. Four sampling locations along the smoke path wereselected for the evaluation. From upstream to downstream, they are [212], [322], [321], and [221]. From the evaluationof sampling locations, a suitable sampling location was selected for all experiments to be carried out afterwards.

Four different types of incense sticks were selected in this study. Sinshen (US$ 8/kg), Wuchen (US$ 20/kg), andLaoshen (US$ 15/kg) incense sticks were purchased in Taiwan. Mainichi-Koh incense was purchased from Japan withits name denoting “incense for everyday use.” It is being sold in Japan and in overseas market such as the United States.The amount of incense used per experiment was about 0.14 g or 3 cm in length. It was scaled down proportionallybased on the amount of incense used per chamber volume given by Cheng et al. (1995). Experiments were carried outto determine if there was any detectable trend between particle diameter and humidity for each type of incense. Theresults are presented and discussed below.

3. Results and discussion

3.1. Incense smoke properties at various locations

Fig. 2 shows the total number densities and the CMDs of Sinshen incense smoke measured at the aforementionedlocations along the path of the rising plume. Note that the sampling interval for each SMPS measurement is 3-min.

10-3

10-2

10-1

100

101

102

105

106

107

221321322212

Sampling Location

N (

cm-3

)

0.0

0.2

0.4

0.6

0.8

CM

D (

µm)

Fig. 2. Comparison of number densities of incense smoke (�) versus background aerosol (�), as well as CMD of incense smoke measured (◦)

along the rising smoke stream (left to right for upstream to downstream). The CMDs are the arithmetic means of the duplicate CMDs measured perlocation and the error bars represent their corresponding standard deviations.


0.01 0.1 1 10 100 1000101

102

103

104

105

106

107

108

Lin (1992)

Cheng et al.(1995)

Li and Hopke(1993)

This Study

NCMD

Chamber Volume (m3)

N (

cm-3

)

0.0

0.1

0.2

0.3

0.4

0.5

CMD

(µm

)

Fig. 3. Comparison of number density (•) and CMD (◦) of fresh Sinshen incense smoke with literature data.

Duplicate measurements were performed per location with each using a new incense sample. At all four locations, thetotal number densities were observed to rise swiftly to 106–107 particles cm−3, about four to five orders of magnitudehigher than the background. The abrupt rise in number density indicates incense particle formation by nucleation froma vapor produced by incense burning. Noticeably, location [322] has the lowest concentration among all. Overall, thediscrepancy in number density among the different types of incense smokes is in one to two orders of magnitude.The CMDs plotted in the same figure were calculated as the arithmetic means of duplicate CMDs measured perlocation. The error bars represent corresponding standard deviations. It is shown that the CMD increases slightly, about20%, towards the downstream. This is perhaps an indication of rapid growth of fresh incense smoke particles whiletraversing from upstream (inner) to downstream (outer) locations. For the purpose of assessing fresh incense smokeproperties, the location [212], which is 15 cm above the glowing tip, is presumably more representative than thosefurther downstream. However, considering the upper limit of number density per channel with the SMPS, [322], aslightly lower concentration and immediate downstream location of [212], was chosen for all measurements performedthereafter. Nonetheless, relative errors involved in the CMDs measured at [322], rather than those at [212], may beestimated from Fig. 2.

While most of the literature cited here did not provide information regarding sampling location and the type ofincense used, plotting all the number densities and CMDs measured along with the literature data as a function ofchamber volume in Fig. 3, disregarding sampling location and incense type, would permit an examination on whetherdata obtained with a smaller chamber are essentially biased. It is shown that the number density of Sinshen incensesmoke is in similar order of magnitude as that of Li and Hopke (1993), one to two orders of magnitude higher thanthat of Cheng et al. (1995), and two to three orders of magnitude higher than that of Lin (1992). Although the amountof incense sample used in this study was scaled down based on that of Cheng et al. (1995), the discrepancy observedin number density may be resulted from different types of incense used in the two studies. Besides, if coagulation wasthe primary particle growth mechanism, the rate of growth would be similar to that of Li and Hopke (1993), and in turnperhaps similar CMD measured. The figure shows that the CMDs measured in this study fall within the 0.07–0.18 �mrange with the highest and the smallest values being very close to that of Li and Hopke (1993) and Lin (1992),respectively. Particularly, even though the particle concentration obtained in this study is as high as that of the Li andHopke (1993), the CMD is closer to theirs only at locations further away from the incense. On the contrary, even withparticle concentration two to three orders of magnitude higher than that of Lin (1992), the CMD obtained at upstream israther close to that of Lin (1992). Collectively, it is shown that incense smoke properties are location-dependent and asmaller chamber does not necessarily mean a higher CMD measured due to coagulation. Through proper planning andexecution, incense smoke properties may be reliably assessed even with a smaller chamber, provided that the amountof incense was properly scaled and an appropriate sampling scheme was carefully devised.


DIAMETER (µµ m)

0.01 0.1 1

NU

MB

ER

CO

NC

EN

TR

AT

ION

(cm

-3)

0

5e+4

1e+5

2e+5

2e+5

Sinshen

RH=10%

A

B

Fig. 4. Number size distribution of Sinshen incense smoke particles (SMPS). The solid (B) and dotted (A) lines represent duplicate runs under thesame condition.

DIAMETER (µµ m)

0.01 0.1 1

NU

MB

ER

CO

NC

EN

TR

AT

ION

(cm

-3)

0

5e+4

1e+5

2e+5

2e+5

Laoshen

RH=10%

A

B

Fig. 5. Number size distribution of Laoshen incense smoke particles (SMPS). The solid (B) and dotted (A) lines represent duplicate runs under thesame condition.

3.2. Incense smoke properties for different types of incense sticks

Figs. 4–7 are the SMPS measured number size distributions of fresh incense smoke of Sinshen (sandalwood,Australiaor Southeast Asia), Laoshen (sandalwood, India), Wuchen (low-grade agarwood), and Mainichi-Koh (Japan) incense,respectively. Comparatively, the number size distribution of Sinshen incense smoke is similar to that of Mainichi–Kohincense smoke, both having total number densities in the 5–8×106 particles cm−3 range, about one order of magnitudehigher than those of Wuchen and Laoshen incense smoke. The number density of Wuchen incense smoke is onlyslightly higher than that of Laoshen incense smoke and both are rather reproducible in magnitude. Besides, Sinshenincense smoke is a single modal distribution with the major peak located in-between 0.1–0.2 �m as depicted in Fig. 4,and is reproducible from sample to sample. In contrast, Laoshen incense smoke in Fig. 5 is a bimodal distribution


DIAMETER (µµ m)

0.01 0.1 1

NU

MB

ER

CO

NC

EN

TR

AT

ION

(cm

-3)

0

5e+4

1e+5

2e+5

2e+5

Wuchen

RH=10%

A

B

Fig. 6. Number size distribution of Wuchen incense smoke particles (SMPS). The solid (B) and dotted (A) lines represent duplicate runs under thesame condition.

DIAMETER (µµ m)

0.01 0.1 1

NU

MB

ER

CO

NC

EN

TR

AT

ION

(cm

-3)

0

5e+4

1e+5

2e+5

2e+5Mainichi-Koh

RH=10%

A

B

Fig. 7. Number size distribution of Mainichi-Koh incense smoke particles (SMPS). The solid (B) and dotted (A) lines represent duplicate runs underthe same condition.

with a major and a minor peak located at 0.07–0.1 and 0.4 �m, respectively. Similar to Laoshen, Wuchen incensesmoke is bimodally to trimodally distributed as displayed in Fig. 6 with the peaks located at 0.05–0.08, 0.1–0.2 and0.4–0.6 �m. The reproducibility of the distribution is apparently not as good as the others. Poor reproducibility of thedistributions may be originated from compositional inhomogeneity of incense powders along a sample stick, inheritedfrom the incense making process, or the stochastic nature of coagulation, or the both. Furthermore, Mainichi-Kohincense smoke is also bimodally distributed with a major peak at 0.1 �m and a minor peak at 0.4 �m, or slightlyhigher. The minor peak was suspected to have resulted from coagulation. When there was only one peak present, thesingle modal distribution resembles that of Sinshen incense smoke. Overall, the discrepancies observed among the sizedistributions of different incense smoke were possibly resulted from the different ingredients used in incense making.


Table 1Count related size statistics for the four types of incense smoke measured at three humidity conditions

Location [322] CMD (�m)a

RH = 10% RH = 70% RH = 95%

A B Avg ± � A B Avg ± � A B Avg ± �

Sinshen 0.134 0.115 0.124 ± 0.013 0.126 0.103 0.114 ± 0.016 0.097 0.097 0.097 ± 0.0Wuchen 0.100 0.131 0.115 ± 0.022 0.156 0.106 0.131 ± 0.035 0.149 0.130 0.139 ± 0.013Laoshen 0.086 0.095 0.090 ± 0.006 0.203 0.151 0.177 ± 0.037 0.125 0.170 0.147 ± 0.03Mainichi-Koh 0.112 0.126 0.119 ± 0.01 0.099 0.135 0.117 ± 0.025 0.125 0.093 0.109 ± 0.02

aBased on 3-min SMPS measurement; A and B represent two separate measurements.

0.01 0.1 1 10

dM/d

Log

Dae

0.0

0.5

1.0

1.5

2.0

SinshenLaoshenWuchenMainichi-Koh

Fig. 8. Mass size distributions of Sinshen, Laoshen, Wuchen, Mainichi-Koh incense smokes (QCM) obtained for the first sampling runs.

The CMDs of the four types of incense smoke, measured under three humidity conditions, were summarized inTable 1. The CMDs of the four types of incense smoke are shown relatively insensitive to humidity except for Sinshen,which appears to decrease with increasing humidity. However, with only two repetitions per humidity, it is inconclusiveas to whether the observed trend with Sinshen is reliable. Furthermore, the fluctuations observed in the CMD, fromlow to high humidity, are in similar magnitude as the discrepancies seen in repetition runs and among different typesof incense, which probably explains why similar trends were not straightforwardly observed with the others. Considerthe growth ratio of about 10% for incense smoke as experimentally determined by Li and Hopke (1993) with a morereliable method, the result shown here proved itself as a less feasible method for assessing small changes in CMDcaused by humidity. Consequently, if neglecting humidity effect, the CMD ranges determined based on a total of sixCMDs per incense type are 0.097–0.134, 0.1–0.156, 0.086–0.203, and 0.093–0.135 �m for Sinshen, Wuchen, Laoshen,and Mainichi-Koh incense smoke, respectively. Notably, since Sinshen and Mainichi-Koh incense smoke have rathersimilar number size distributions, it is not surprising to observe that they share similar CMD range.

The first and the second QCM measurements of the four types of incense smoke were plotted in Figs. 8 and9, respectively. It is shown, in Fig. 8, that Sinshen incense smoke appeared initially as a bimodal mass-weighteddistribution with two distinct peaks located at 0.1 and 0.8 �m rather than a single modal distribution as seen with theSMPS. Particularly, the height of both peaks was comparable. Similarly, Laoshen, Wuchen, and Mainichi-Koh incensesmoke exhibited two to three peaks initially. The peaks were located at 0.1, 0.7, and 10.0 �m for Laoshen, 0.4 and2.0 �m for Wuchen, 0.6 and 6.0 �m for Mainichi-Koh incense smoke. Comparatively, the major peaks for Laoshen,Wuchen, and Mainichi-Koh incense smoke were located at 0.7, 0.4, and 6.0 �m, respectively. From the total mass as


Dae (µµ m)

0.01 0.1 1 10

dM/d

Log

Dae

0

2

4

6

8

SinshenLaoshenWuchenMainichi-Koh

Fig. 9. Mass size distributions of Sinshen, Laoshen, Wuchen, Mainichi-Koh incense smokes (QCM) obtained for the second sampling runs.

Table 2Mass related size statistics for the four types of incense smoke measured at three humidity conditions

Location [322] MMAD50(�m)a

RH = 10% RH = 70% RH = 95%

A B Avg ± � A B Avg ± � A B Avg ± �

Sinshen 0.33 0.19 0.26 ± 0.1 0.32 0.29 0.30 ± 0.07 0.32 0.22 0.27 ± 0.07Wuchen 0.31 0.35 0.33 ± 0.03 0.39 0.52 0.45 ± 0.09 0.26 0.29 0.275 ± 0.02Laoshen 0.33 0.41 0.37 ± 0.05 0.19 0.29 0.24 ± 0.07 0.17 0.16 0.165 ± 0.007Mainichi-Koh 0.19 0.34 0.265 ± 0.1 0.18 0.39 0.285 ± 0.15 0.24 0.15 0.195 ± 0.06

aBased on 110-second QCM measurement; A and B represent two separate measurements.

indicated by the total area under each curve, the result suggests that Sinshen incense sticks produces more particles bymass than Laoshen, Wuchen, and Mainichi-Koh incense sticks.

In less than 2 min, as illustrated by Fig. 9, Sinshen incense smoke evolved into a single modal mass distributionwith its peak located at 0.1 �m, almost coinciding with that measured by the SMPS. In contrast, Laoshen, Wuchen,and Mainichi-Koh incense smoke all grew into rather similar bimodal distributions. It is particularly interesting to notethat they all share the same major and minor peaks at 0.1 and 0.4 �m, respectively, which is very likely an indicationthat they all grow by the same mechanism. Moreover, the emergence of the minor peak at 0.4 �m in the second QCMmeasurement was possibly due to coagulation of the smaller particles in the major peak. Also, the total mass is differentfor different incense with Sinshen being the highest, followed by Wuchen, Laoshen, and Mainichi-Koh. Comparingthe first and the second measurements, it is apparent that Sinshen incense produced more particle mass than the othersin both measurements. In other words, it is more smoke-generating compared with the others. It is also worthwhilenoting that the total particle mass produced by Wuchen surpassed that of Laoshen in the second measurement, and thatMainichi-Koh incense produced the least amount of particle mass among all four of them in both measurements. Inaddition, using the data from the second measurement, the MMAD for 50% collection efficiency, MMAD50, of the fourtypes of incense smoke, measured at three humidity conditions, were calculated and enlisted in Table 2. The MMAD50of the four types of incense smoke were found rather similar to that of Fang et al. (2002).

Likewise, neglecting the 10% error that may be introduced by humidity, the average and standard deviation of allsix CMD and MMAD50 values have been calculated and summarized in Table 3 for each type of incense. The average


Table 3Average and standard deviation of the CMD and MMAD50 for the four types of incense smoke

Incense type CMD (�m) MMAD50 (�m)

Avg ± � Avg ± �

Sinshen 0.112 ± 0.015 0.28 ± 0.06Wuchen 0.128 ± 0.022 0.35 ± 0.09Laoshen 0.138 ± 0.045 0.26 ± 0.1Mainichi-Koh 0.115 ± 0.016 0.25 ± 0.09

Fig. 10. SEM micrograph of Mainichi-Koh incense smokes collected just above ceiling, [202].

CMD appears to be slightly different for different type of incense smoke. Nonetheless, the variations among the averageCMD and MMAD50 values of the four types of incense smoke studied here were within 30%. Among them, Laoshenhas the highest average CMD, followed by Wuchen, while Sinshen and Mainichi-Koh have the lowest and almost thesame CMDs. From the standard deviations, Laoshen has the broadest CMD range, followed by Wuchen, Sinshen, andMainichi-Koh incense smoke. The variation among the standard deviations is attributable to several factors: (1) thedifferent varieties of ingredients used in different types of incense; (2) the processing including grinding, mixing, andcoating techniques used for the ingredients, or (3) the both. Regarding mass-weighted properties,Wuchen incense smokeappears to have the highest MMAD50, followed by Sinshen, Laoshen, and Mainichi-Koh incense smoke. Collectively,the above result indicates that each type of incense generates incense smoke of own characteristic size distribution,which in turn results in a distinctive set of size related properties.

Electron microscopy of the incense smoke particles revealed that Mainichi-Koh and Sinshen incense smoke particleswere mainly comprised of solid and liquid particles, as shown in Figs. 10–12, respectively. Although not shown here,Laoshen and Wuchen incense smoke were also mainly consisted of liquid particles like Sinshen. Performing sizeanalysis on the particles in Fig. 10 revealed that Mainichi-Koh incense smoke particles were mostly 0.1 �m particlesand their aggregates. The irregular shape of the solid aggregates implies that the particles transformed from liquid tosolid phase prior to aggregation. Moreover, as the briefly sputter-coated nanometer-thick gold layer on particle samplesshould not be observable at the magnification, the countless erratically distributed white speckles seen in the figuredemands further inspection. At a higher magnification using FESEM, particles as small as 20–25 nm were revealedand illustrated in Fig. 11. Unfortunately, due to the limited resolution, it was impossible to discriminate whether theseparticles were composed of even smaller identities. Still, it implies that the primary particles are in the lower nanometer


Fig. 11. FESEM micrograph of Mainichi-Koh incense smokes collected just above ceiling, [202].

Fig. 12. Sinshen incense smoke particles collected just above ceiling, [202]. (Scale bar = 1.0 �m).

size range. And if this is true, then the QCM would not be able to detect them at all and that the SMPS has also failedto convey such information probably due to a sampling time too long to disclose particle formation at the early stage.

Performing similar size analysis on the liquid deposits of Sinshen in Fig. 12 yielded roughly two distinct sizegroups—those in 0.1–0.2 �m range and those larger than 0.5 �m. Although electron microscopy cannot convey in-formation as how the larger deposits were formed, in light of the resemblance in the ranges of number density andparticle size among the four types of incense smoke, it is highly possible that the formation and growth mechanismsof the liquid particles of Sinshen, Wuchen, and Laoshen are all very similar to that of Mainichi-Koh incense smokeparticles. Through cleaning of the quartz plates of the QCM and chamber interior after experiment, the oily nature of the


Taiwan incense smoke particles and the ease of preserving deposited particles under low humidity were revealed andimplied high boiling point (Tb) compounds. Being in liquid phase at ambient condition indicates that its melting point(Tm) is below the ambient temperature. Likewise, the solid-state of Mainichi-Koh incense smoke particles implied amelting point beyond the ambient temperature. Also, the difference in physical state of the incense particles is perhapsattributable to the different smoke generating ingredients customarily employed in different cultures.

Based on the above, the formation of incense smoke particles investigated is proposed here. The solid particles ofMainichi-Koh incense smoke were very likely formed as a result of successive nucleation and solidification of incenseingredients or combustion by-products of high-Tb and high-Tm nature such as camphor-like compounds. Further growthof the particles was dominated by coagulation and leads to irregularly shaped aggregates. As mentioned, camphor is oneof the ingredients used in Japanese incense making. It has a melting point and a boiling point of 178.75 and 207.42 ◦C,respectively. Burning camphor-containing incense may cause camphor to sublime at a faster rate or even furtherdecompose, depending on temperature at the incense tip. Once formed, highly concentrated vapor-phase camphormolecules (or decomposed by-products) may nucleate to form nano-sized stable nuclei. Stable nuclei of camphor(or by-products) may solidify rather quickly at a place very close to the burning tip due to a melting point of about30 ◦C below its boiling point. Consequently, given a particle-free background to begin with and in the absence offorced convection such as in this study, the initial growth would less likely be affected by the chamber size, but ratherby particle density in the freshly formed incense smoke. Similarly, liquid incense smoke particles were formed bynucleation of high-Tb/low-Tm ingredients and/or vapor-phase combustion by-products of similar physical nature andgrew rapidly to larger sizes by coalescence.

4. Conclusions

A small chamber study of the formation of incense smoke was demonstrated here. The validity of investigating incensesmoke properties in the small chamber compared with earlier studies using larger chambers was assessed. Incense smokegenerated from four different types of incense sticks were found to exhibit different smoke-generating properties andcharacteristic size distributions. Nucleation and coagulation were suspected to be the principal mechanisms governingthe initial formation and subsequent growth of the particles, respectively. The dissimilar initial mass distributions ofthe particles were probably due to factors including different combinations of ingredients used, processing techniques,the stochastic nature of coagulation, etc. However, further growth by coagulation reshaped the distributions to becomerather similar ones.All three types of Taiwan incense smoke studied here were found to contain mostly oil droplets, whileMainichi-Koh incense smoke contained primarily solid ones. The different physical states observed were probably dueto different thermal properties of the smoke-generating ingredients customarily used by different cultures. Collectively,evidence accumulated here suggests that both liquid and solid incense smoke particles were formed by a vapor phaseprocess. Burning incense caused smoke-generating ingredients to undergo evaporation, thermal decomposition, orthe both. The thermal properties, such as melting point and boiling point, of the vapor-phase precursors determinethe different physical states of incense smoke. Ultimately, through a fundamental understanding of the formation ofincense smoke and further identification of smoke-generating ingredients in incense recipes, the formation of incensesmoke with harmful composition may be prevented or diminished significantly by careful selection of smoke-generatingingredients with physical properties such as non-toxic and with a thermal decomposition temperature exceeding thatof burning incense tip.

Acknowledgements

The authors would like to express their most sincere gratitude to Mr. Hwang Shu Lai of the Atomic Energy Counciland Mr. Ku Chih-Tung for the generous instrumental support by the Center of Health Physics Division of the Instituteof Nuclear Energy Research (INER), Taiwan, Republic of China.

References

Chen, C. C., & Lee, H. (1996). Genotoxicity and DNA adduct formation of incense smoke condensates: comparison with environmental tobaccosmoke condensates. Mutation Research/Genetic Toxicology, 367, 105–114.


Cheng,Y. S., Bechtold, W. E.,Yu, C. C., & Hung, I. F. (1995). Incense smoke: characterization and dynamics in indoor environments. Aerosol Scienceand Technology, 23, 271–281.

Fang, G.-C., Chang, C.-N., Wu, Y.-S., Yang, C.-J., Chang, S.-C., & Yang, I.-L. (2002). Suspended particulate variations and mass size distributionsof incense burning at Tzu Yun Yen temple in Taiwan, Taichung. The Science of the Total Environment, 299, 79–87.

Jetter, J. J., Guo, Z., McBrian, J.A., & Flynn, M. R. (2002). Characterization of emissions from burning incense. The Science of the Total Environment,295, 51–67.

Lee, S.-C., & Wang, B. (2004). Characteristics of emissions of air pollutants from burning of incense in a large environmental chamber. AtmosphericEnvironment, 38(7), 941–952.

Li, W., & Hopke, P. K. (1993). Initial size distributions and hygroscopicity of indoor combustion aerosol particles. Aerosol Science and Technology,19(3), 305–316.

Lin, J. M., & Tang, C. S. (1994). Characterization and aliphatic aldehyde content of particulates in Chinese incense smoke. Bulletin of EnvironmentalContamination and Toxicology, 53(6), 895–901.

Lin, W.-H. (1992). M.S. Thesis, Environmental Engineering Department, National Taiwan University, Taipei, Taiwan.Löfroth, G., Stensman, C., & Brandhorst-Satzkorn, M. (1991). Indoor sources of mutagenic aerosol particulate matter: smoking, cooking and incense

burning. Mutation Research, 261(1), 21–28.Lowengart, R. A., Peters, J. M., Cicioni, C., Buckley, J., Bernstein, L., Preston-Martin, S., & Rappaport, E. (1987). Childhood leukemia and parent’s

occupational and home exposures. Journal of the National Cancer Institute, 79(1), 39–46.MacLennan, R., DaCosta, J., Day, N. E., Law, C. H., Ng, Y. K., & Shanmugaratnam, K. (1977). Risk factors for lung cancer in Singapore Chinese,

a population with high female incidence rates. International Journal of Cancer, 20(6), 854–860.Mannix, R. C., Nguyen, K. P., Tan, E. W., Ho, E. E., & Phalen, R. F. (1996). Physical characterization of incense aerosols. The Science of the Total

Environment, 193, 149–158.Rasmussen, R. E. (1987). Mutagenic activity of incense smoke in salmonella typhimurium. Bulletin of Environmental Contamination and Toxicology,

38, 827–833.

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