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Mosquito coil is widely known as an efficient
mosquito repellent. The major active ingre-
dients of the mosquito coil are pyrethrins,
accounting for about 0.3–0.4% of coil mass
(Lukwa and Chandiwana 1998). When a mos-
quito coil is burned, the insecticides evaporate
with the smoke, which prevents the mosquito
from entering the room. Pyrethrins are of low
chronic toxicity to humans and low reproduc-
tive toxicity in animals, although headache,
nausea, and dizziness were observed in
male sprayers exposed to 0.01–1.98 µg/m
3
pyrethrins for 0.5–5 hr (Zhang et al. 1991).
No carcinogenic and mutagenic effects have
been found for these insecticides (Ecobichon
1995). The lowest lethal oral dose of pyrethrum
is 750 mg/kg for children and 1,000 mg/kg for
adults [Occupational Health Services (OHS)
1987]. The remaining components of mos-
quito coil are organic fillers, binders, dyes, and
other additives capable of smoldering well. The
combustion of the remaining materials gener-
ates large amounts of submicrometer particles
and gaseous pollutants. These submicrometer
particles can reach the lower respiratory tract
and may be coated with a wide range of
organic compounds, some of which are car-
cinogens or suspected carcinogens, such as
polycyclic aromatic hydrocarbons (PAHs)
generated through incomplete combustion of
biomass (mosquito coil base materials).
Researchers have found that the gas phase of
mosquito coil smoke contains some carbonyl
compounds with properties that can produce
strong irritating effects on the upper respira-
tory tract—for example, formaldehyde and
acetaldehyde (Chang and Lin 1998). Because
coil consumers usually use mosquito coils for
at least several months every year, cumulative
effects from long-term exposure to the coil
smoke may also be a concern.
Epidemiologic studies have shown that
long-term exposure to mosquito coil smoke
can induce asthma and persistent wheeze in
children (Azizi and Henry 1991; Fagbule
and Ekanem 1994; Koo and Ho 1994).
Toxicologic effects of mosquito coil smoke on
rats include focal deciliation of the tracheal
epithelium, metaplasia of epithelial cells,
and morphologic alteration of the alveolar
macrophages (Liu and Sun 1988; Liu and
Wong 1987). For example, when a group of
30 female albino rats were exposed to mos-
quito-coil smoke in a 22.5-m
3
chamber for
8 hr per day, 6 days per week, for 6 months,
these rats lost typical ruffled membranes of
their alveolar macrophages. In addition, the
levels of total protein and lecithin and the
activities of lactate dehydrogenase, acid phos-
phatase, and β-glucuronidase in the lung-
lavage fluid of the rats were significantly
higher than those in a control group that was
exposed to air for the same exposure duration
(Liu et al. 1989).
Despite the fact that mosquito coil smoke
may have many potential adverse health effects,
large populations in developing countries still
use mosquito coils in their daily lives. In previ-
ous studies of various aspects of mosquito coil
smoke, emissions of irritating and carcinogenic
compounds and other pollutants have not been
quantified, which precludes the use of emission
rate data to predict pollutant concentrations in
households and to quantify health risks. Data
are also lacking for comparing emissions from
different types of mosquito coils. To make
informative recommendations to consumers as
to which types of mosquito coil have lower
emissions of health-damaging pollutants, it is
necessary to perform tests of coil emissions in a
systemic manner. In the present study, we
selected for comparison six brands of mosquito
coils popularly used in China and Malaysia.
We measured emission rates of a variety of pol-
lutants of health concern, present in both the
particulate phase and the gas phase of the coil
smoke. We also determined particle size distrib-
ution of the coil smoke and identified a suite of
volatile organic compounds (VOCs), including
human carcinogens and suspected carcinogens.
Finally, using the emission rates determined in
the present study and typical room parameters,
we predicted room concentrations and com-
pared these with reference risk values.
Materials and Methods
Experimental apparatus. The experimental
apparatus is shown in Figure 1, the core of
which is a polyvinyl chloride chamber with a
volume of 0.15 m
3
. The incoming air, after
passing through a series of filters impregnated
with activated carbon (charcoal filters), was
introduced into the chamber through the inlet
valve at a constant flow rate of 5.59 L/min.
Mosquito Coil Emissions and Health Implications
Weili Liu,
1
Junfeng Zhang,
2
Jamal H. Hashim,
3
Juliana Jalaludin,
4
Zailina Hashim,
4
and Bernard D. Goldstein
5
1
Joint Graduate Program in Exposure Measurement and Assessment, and
2
School of Public Health and Environmental and Occupational
Health Sciences Institute, University of Medicine and Dentistry of New Jersey (UMDNJ) and Rutgers University, Piscataway, New
Jersey, USA;
3
Department of Community Health, National University of Malaysia, Kuala Lumpur, Malaysia;
4
Department of Community
Health, Universiti Putra Malaysia, Serdang, Malaysia;
5
Graduate School of Public Health, University of Pittsburgh, Pittsburgh,
Pennsylvania, USA
Address correspondence to J. Zhang, School of
Public Health and Environmental and Occupational
Health Sciences Institute, UMDNJ and Rutgers
University, 170 Frelinghuysen Rd., Piscataway, NJ
08854 USA. Telephone: (732) 445-0158. Fax: (732)
445-0116. E-mail: jjzhang@eohsi.rutgers.edu
We thank L. Zhang and R. Harrington for their
assistance in the laboratory; C. Weisel and X. Xu of
EOHSI for measuring the volatile organic compounds;
and K. Smith of the University of California-Berkeley
for his valuable comments on the manuscript.
J.Z. is supported in part by a National Institute of
Environmental Health Sciences Research Center grant
P30 ES05022.
The authors declare they have no conflict of interest.
Received 18 February 2003; accepted 29 April 2003.
Burning mosquito coils indoors generates smoke that can control mosquitoes effectively. This prac-
tice is currently used in numerous households in Asia, Africa, and South America. However, the
smoke may contain pollutants of health concern. We conducted the present study to characterize
the emissions from four common brands of mosquito coils from China and two common brands
from Malaysia. We used mass balance equations to determine emission rates of fine particles (par-
ticulate matter < 2.5 µm in diameter; PM
2.5
), polycyclic aromatic hydrocarbons (PAHs), aldehydes,
and ketones. Having applied these measured emission rates to predict indoor concentrations under
realistic room conditions, we found that pollutant concentrations resulting from burning mosquito
coils could substantially exceed health-based air quality standards or guidelines. Under the same
combustion conditions, the tested Malaysian mosquito coils generated more measured pollutants
than did the tested Chinese mosquito coils. We also identified a large suite of volatile organic com-
pounds, including carcinogens and suspected carcinogens, in the coil smoke. In a set of experiments
conducted in a room, we examined the size distribution of particulate matter contained in the coil
smoke and found that the particles were ultrafine and fine. The findings from the present study
suggest that exposure to the smoke of mosquito coils similar to the tested ones can pose significant
acute and chronic health risks. For example, burning one mosquito coil would release the same
amount of PM
2.5
mass as burning 75–137 cigarettes. The emission of formaldehyde from burning
one coil can be as high as that released from burning 51 cigarettes. Key words: aldehydes, mosquito
coil, PAHs, particulate matter, smoke. Environ Health Perspect 111:1454–1460 (2003).
doi:10.1289/ehp.6286 available via http://dx.doi.org/ [Online 30 April 2003]
1454
VOLUME 111 | NUMBER 12 | September 2003
•
Environmental Health Perspectives
The chamber was under a slightly positive
pressure to prevent the infiltration of air from
outside by controlling the flow rate of exiting
air via the sampling ports and the outlet
value. During each experimental run, a lit
mosquito coil on the metal stand provided
within the coil packet was placed inside the
chamber. Air samples were drawn out of the
chamber at 10 mL/min through Teflon tub-
ing and diluted with zero-grade air (Air
Products & Chemicals, Inc., Edison, NJ,
USA) 80× in a flask to match the measure-
ment range of an eight-channel optical parti-
cle counter. Additional sampling lines on the
top of the chamber were used to collect sam-
ples for measuring the mass of particulate
matter of 2.5 µm in diameter (PM
2.5
) and for
identifying gas-phase organic compounds.
Tested mosquito coils included two
brands purchased from Malaysia (coils M1
and M2) and four brands from China (coils
C1–C4). Based on the information on the
product labels, all six tested brands had simi-
lar contents of active ingredients (pyrethrins).
However, it is our understanding that the
base material used in making the Chinese
coils was mainly sawdust, whereas materials
used in making the Malaysian coils were
coconut husks/shells.
Measurement of particle number and
mass concentration. We used the eight-chan-
nel optical particle counter (LASAIR model
1002 unit; Particle Measuring Systems, Inc.,
Boulder, CO, USA) to measure number con-
centrations of particulate matter from 0.1 to
10 µm in diameter with eight size ranges:
0.1–0.2 µm, 0.2–0.3µm, 0.3–0.4 µm,
0.4–0.5 µm, 0.5–0.7 µm, 0.7–1.0 µm,
1.0–2.0 µm, and > 2 µm. This particle
counter was operated for 3 hr and 10 min
during each coil test, which included a 10-min
background monitoring before initiating the
coil smoke, a 2-hr coil burning period, and a
1-hr postburning period. A similar set of
experiments were conducted in a room with a
volume of 32 m
3
(4 × 3 × 2.5 m) and an air
exchange rate of about 1/hr (with window
half open) to examine whether there were dif-
ferences in particle size distributions between
the room measurements and the chamber
measurements. An ultrafine particle counter
(model 3007; TSI Inc., St. Paul, MN, USA)
was used in addition to the eight-channel
optical particle counter to monitor particles
with diameters as small as 0.01 µm in the
room experiments.
A personal PM
2.5
sampling head (Personal
Environmental Monitor model 200; MSP
Co., Minneapolis, MN, USA) with a 25-mm
Teflon filter (Pall Co., Ann Arbor, MI, USA)
was located in the middle of the chamber, as
shown in Figure 1, to collect PM
2.5
mass. The
samples were collected using an SKC pump
(model 224-PCXR4; SKC Inc., Eighty Four,
PA, USA) providing a sampling flow rate of
0.8 L/min. Each of these PM
2.5
samples was
collected for 10 min after the coil had been
steadily burned for 1.5 hr. The particle mass
on the filters was determined gravimetrically.
Measurement of PAHs. The PM
2.5
filters,
after mass concentrations had been deter-
mined, were extracted with 150 mL dichloro-
methane individually for 16 hr at 75°C using a
Soxhlet apparatus (Farant and Gariepy 1998;
Sanderson and Farant 2000). The extracts were
evaporated to near dryness and redissolved in
1 mL acetonitrile (ACN). The extracts were
analyzed using a high-performance liquid
chromatographic (HPLC) system (Waters
600E; Waters Corp., Milford, MA, USA)
with a fluorescence detector (Waters 470).
The HPLC column used was a Supelcosil
LC-PAH column (4.6 × 250 mm) (Supelco,
Inc., Bellefonte, PA, USA) under controlled
temperature at 30°C. The mobile phase used
was as follows: solution A = 50% ACN and
50% water; solution B = 100% ACN. The gra-
dient program was 100% A for 20 min, linear
gradient from 100% A to 100% B in 20 min,
100% B for 15 min, then from 100% B back
to 100% A in 10 min, and held at 100% A for
10 min. The mobile phase flow rate was
1 mL/min. The injection volume was 20 µL.
The fluorescence detector program was started
at an excitation wavelength of 270 nm and an
emission wavelength of 350 nm; at 34 min,
the wavelength was changed to excitation at
250 nm and emission at 400 nm and held for
13.5 min; then it was changed to excitation at
280 nm and emission at 425 nm for another
22.5 min. PAH concentrations were deter-
mined through calibration curves prepared
using certified standard solutions of PAHs
purchased commercially (Supelco, Inc.).
Measurement of carbonyl compounds. U.S.
Environmental Protection Agency (U.S. EPA)
method TO-11A was used for collecting and
analyzing carbonyl compounds (U.S. EPA
1999a). Carbonyl compounds (aldehydes and
ketones) in the coil smoke were collected using
2,4-dinitrophenyl hydrazine (DNPH)–coated
C
18
cartridges at a flow rate of 0.07 L/min for
10 min for each test. The samples were slowly
eluted with 4 mL ACN immediately after the
sampling, and the extracts were analyzed using
an HPLC system with a reverse-phase Nova-
Pak C
18
column (3.9 × 150 mm; Waters
Corp.). The mobile phase gradient program
used was as follows: 100% of solvent A
(water/ACN/tetrahydrofuran 60/30/10), hold
for 5 min, then program to 100% solvent B
(ACN/water 60/40) in 28 min, and hold at
100% B for 10 min, and then program back to
100% A in 5 min. The flow rate of the mobile
phase was kept constant at 1 mL/min. The
sample injection volume was 20 µL. The ultra-
violet detector was set at 365 nm. Carbonyl
compound concentrations were determined
through calibration curves prepared using cer-
tified standard solutions of DNPH–carbonyl
derivatives purchased commercially (Supelco,
Inc., and Accustandards Inc., New Haven,
CT, USA).
Identification of VOCs. VOC samples
were collected with stainless steel traps
(0.5 cm inner diameter × 8.8 cm; Perkin
Elmer Inc., Shelton, CT, USA) packed with
0.25 g Tenax TA (Supelco, Inc.) at a flow rate
of 0.1 L/min for 2 min during each test (Xu
and Weisel 2003). A glass fiber filter was
placed in front of the Tenax trap to remove
particles. After sample collection, the samples
were desorbed from the trap at 250°C using
an automated thermal desorption system
Environmental Health Perspectives
•
VOLUME 111 | NUMBER 12 | September 2003
1455
Figure 1. Apparatus and setup for the chamber experiments.
Sampling line for
aldehydes and VOCs
collection
Smoke
(10 mL/min)
Pump
Particle
counter
Mosquito coil
Charcoal filters
Outlet valve
Secondary
dilution system
0.15 m
3
PM
2.5
sampling head
Sampling tubing for
PM
2.5
mass collection
Zero-grade air
(800 mL/min)
Influent air
(5.6 L/min)
Article
|
Mosquito coil emissions and health implications
(ATD-400; Perkin-Elmer Inc., Wellesley,
MA, USA) and transferred to a capillary gas
chromatograph (GC; DB-5 capillary column,
60 m × 0.25 mm inner diameter, 1 µm film
thickness, HP 5890; Hewlett-Packard,
Wilmington, DE, USA) equipped with a
mass spectrometer (MS; HP 5971A; Hewlett-
Packard) system for compound identifica-
tions. The GC temperature program was set
at 35°C for 10 min, gradually increased to
250°C in 20 min, then held at 250°C for
another 10 min.
Measurement of air exchange rate.
Ventilation rate, expressed as air exchange rate,
during each experiment, equals the volumetric
flow rate of the incoming air divided by the
chamber volume. In our experiment, the volu-
metric flow rate of the incoming air was con-
trolled using a needle valve of a flow meter.
The theoretical air exchange rate in this small
chamber should be 2.24/hr. However, this
value may not be exactly reached because of
the variation of the incoming air. The air
exchange rates were thus also determined
experimentally by spiking sulfur hexafluoride
(SF
6
) gas into the chamber after the coil had
been extinguished for 1 hr to avoid the inter-
ference of high-level particles in the smoke. A
real-time SF
6
monitor (type 1302; INNOVA
Air Tech Instruments, Ballerup, Denmark)
was used to measure SF
6
concentrations in the
chamber every 3 min for 30 min. The regres-
sion slope of the plot of logarithm of concen-
tration against time was the air exchange rate.
The experimentally measured air exchange
rates were later used to calculate the emission
rates of gas-phase compounds. The results
show that the experimental determined air
exchange values were about 10% lower than
the theoretical value.
Determination of emission rates. A sin-
gle-compartment mass balance model was
used to describe the whole combustion
process and to determine the emission rate.
Basic assumptions for this model were as fol-
lows: a) background concentration is zero;
b) pollutant concentrations are homogeneous
within the chamber; and c) emission rate and
decay rate of the pollutants remain constant
throughout the entire period of concern. The
relationship between the pollutant concen-
tration C (mg/m
3
) and the emission rate
P (mg/hr) can be expressed as
(when 0 ≤ t ≤ T); [1]
(when t ≥ T), [2]
where k is the total removal rate of pollutant
(hr
–1
), t is the time (hr)—the coil burning
was started at t = 0 and extinguished at t = T,
V is the volume of the chamber (m
3
), and
C
max
is the maximum pollutant concentration
at the time (T) when coil was extinguished.
When the concentration of the pollutant
stabilized in the chamber, that is, had reached
a steady state, Equation 1 can be simplified as
[3]
Therefore, the emission rate P could be easily
obtained using steady-state concentration (C),
pollutant removal rate (k), and the volume of
the chamber (V).
Assuming the removal of the gas-phase
compounds in the chamber is caused only by
ventilation, the removal rate of gas-phase
compounds is equal to the air exchange rate
in the chamber. However, particle removal is
controlled by a number of processes, such as
deposition, coagulation, diffusion, and so
forth. Equation 2 was used to determine par-
ticle total removal rate k. A real-time particle
mass concentration in the chamber during the
postburning period is needed for the calcula-
tion. We converted the real-time particle
number concentration (number per liter) col-
lected by the eight-channel optical particle
counter after the coil was extinguished to
mass concentration (milligrams per cubic
meter) by assuming all particles have a spheri-
cal shape and unit density. For calculation
convenience, the geometric mean of the two
end points of each size range was used as the
diameter for all particles in this size range.
The linear regression slope of the plot of
ln(C) against t would be the total removal rate
of particulate matter in the chamber.
Results and Discussion
Particle behaviors and size distribution.
Number concentration of fine particles at dif-
ferent size ranges as a function of time in one
experiment is illustrated in Figure 2. We did
not include particles larger than 1 µm in
Figure 2 because the concentration of large
particles remained the same as the background
level. This is typical for concentration profiles
of particles in a well-mixed chamber. After the
coil was lit, the concentrations of particles with
C
P
Vk
= .
CC e
–k t T
=
()
−
()
max
C
P
Vk
e
–kt
=−
()
1
1456
VOLUME 111 | NUMBER 12 | September 2003
•
Environmental Health Perspectives
Figure 2. Fine particle number concentration profile
in the chamber as a function of time for C1 during
the whole experimental process. (A) Particles with
size range of 0.1–0.4 µm. (B) Particles with size
range of 0.4–1 µm. Particles larger than 1 µm were
not observed.
050100 150
12 × 10
6
10 × 10
6
8 × 10
6
6 × 10
6
4 × 10
6
2 × 10
6
0
200
050100 150 200
Time (min)
Time (min)
1.6 × 10
6
1.4 × 10
6
1.2 × 10
6
1 × 10
6
0.8 × 10
6
0.6 × 10
6
0.4 × 10
6
0.2 × 10
6
0
A
B
0.1–0.2 µm
0.2–0.3 µm
0.3–0.4 µm
▲
■
◆
0.4–0.5 µm
0.5–0.7 µm
0.7–1.0 µm
▲
■
◆
Number/literNumber/liter
◆ ◆
◆ ◆
0.01
250
200
150
100
50
0
Log D (µm)
A B
0.1 1 10 0.01 0.1 1 10
Log D (µm)
∆N/∆logD (1 × 10
9
/m
3
)
∆N/∆logD (1 × 10
9
/m
3
)
300
250
200
150
100
50
0
◆
■
■
■
■
■
■
■
■
◆ ◆ ◆ ◆ ◆
■
◆
■
■
■
■
■
■
■
◆ ◆ ◆ ◆ ◆
0.01
Log D (µm)
0.1 1 10
6
5
4
3
2
1
0
∆N/∆logD (1 × 10
9
/m
3
)
6
5
4
3
2
1
0
∆N/∆logD (1 × 10
9
/m
3
)
0.01 0.1 1 10
Log D (µm)
◆
◆
◆
◆ ◆ ◆
◆
◆ ◆
■
■
■
■
■
■
■
■
■
■
◆
■
■
■
■
■
■
■
■
◆
◆
◆
◆ ◆
◆
◆
◆
C D
Background
Burning
■
◆
Figure 3. Normalized particle size distributions in the small chamber (A, B) and the room (C, D) for M2 (A,
C) and C2 (B, D). The plots reflect averaged concentrations with standard deviations of the last half-hour
of emissions before the coil was extinguished. Only particles smaller than 0.1 µm were measured in the
room. ∆N represents the concentrations of particles within a specified size interval, and ∆logD is the dif-
ference in the logarithms of the largest and smallest particle sizes of that interval. The logarithm of the
midpoint of a ∆D (logD) is plotted for convenience.
Article
|
Liu et al.
diameters < 0.3 µm increased quickly to a peak
level in only a few minutes, then decreased in
the first half hour and stabilized at a level of
approximately 50% of the peak level until the
coil was extinguished. The concentrations of
particles > 0.3 µm in diameter increased more
gradually and stabilized in the first hour of
burning. After the coil burned out, the particle
number concentration decreased exponen-
tially. Because burning time (2 hr) was long
enough to allow the concentration of pollu-
tants to reach a steady state, the steady-state
burning process can be described using the
single-compartment model.
The complex processes of formation and
removal of particles in the chamber are gov-
erned by a number of mechanisms, including
diffusion, gravitational deposition, convection,
impaction, and coagulation. Diffusion is the
primary mechanism for small particles, whereas
gravitational force is usually the dominant
process controlling the removal of large parti-
cles. Convection and impaction cause deposi-
tion of particles in all directions. Coagulation is
a process whereby small particles collide with
one another through diffusion to form larger
particles. The rate of simple monodisperse
coagulation can be represented as dN/dt =
–KN
2
(Hinds 1982), where N is particle num-
ber concentration and K is the coagulation coef-
ficient. Based on this equation, the coagulation
rate is proportional to the square of the number
concentration of particles. This may explain in
part the concentration changes observed during
the first half hour: The concentration of parti-
cles < 0.3 µm increased dramatically in the first
few minutes of coil burning; then the coagula-
tion process became dominant and the particle
concentration decreased to a stable level.
The particle size distributions of the com-
bustion of M2 and C2 in both the small cham-
ber and the room study are shown in Figure 3.
The plots reflected an averaged concentration
of the last half-hour’s emission before the coil
was extinguished. The plots presented the nor-
malized distribution with ∆N/∆logD versus
logD, where ∆N is the concentration of parti-
cles within a specified size interval and ∆logD
is the difference in the logarithms of the largest
and smallest particle sizes of that interval (Reist
1984). The logarithm of the midpoint of a ∆D
(logD) was plotted for convenience. Particles
> 2.0 µm were not observed.
In the small chamber study, the highest
normalized concentration fell in the size range
of 0.2–0.3 µm. In the room study, we were able
to monitor ultrafine particles in addition to
monitoring fine particles in the eight size
ranges. A bimodal size distribution curve was
observed with peaks at both 0.01–0.1 µm and
0.2–0.3 µm, which means the ultrafine and fine
particles dominate the counts of particles emit-
ted by coil combustion. No particles > 1 µm in
diameter were generated during coil burning.
The room was measured to examine whether
particle size distributions can be “artificially”
affected by the nature of the small chamber
(high surface-to-volume ratio). No measur-
able effects were found on particles > 0.1 µm.
Ultrafine particles were not analyzed in the
chamber experiments.
Emission rates. Table 1 summarizes the
measured coil weight, burn rate, air exchange
rate, PM
2.5
concentration in the chamber,
PM
2.5
removal rate, and PM
2.5
emission rate.
The data were based on an average of five exper-
iments for each brand of coil tested. As shown
in Table 1, all experiments were conducted
under a constant air exchange rate of ~2/hr,
close to the theoretical value, 2.24/hr. The
chamber concentrations of PM
2.5
mass gener-
ated from the combustion of the two brands
of Malaysian coils, 363 ± 12 and 365 ± 19
mg/m
3
, were much higher than from any
Chinese brands tested, the highest concentra-
tion among which was 246 ± 23 mg/m
3
. The
emission rates varied largely across different
brands of coils, from 51.1 ± 7 to 117 ± 14
mg/hr. The particulate matter emission rates of
the Malaysian mosquito coils were markedly
higher than those of the Chinese coils.
However, burn rates of different coils were
similar, ranging from 1.5 to 2.0 g/hr. Different
contents of organic fillers (base materials)
used for smoldering could be the main reason
of the difference in emission rates. The differ-
ence in base materials was reflected by the fact
that the tested Chinese coils had a longer
burning time, ranging from 9 to 11 hr, com-
pared with the Malaysian ones, which can
burn for only 7 hr.
High concentrations of PAHs were
observed in the particulate phase of mosquito
coil smoke (Table 2). The table shows that
the emission rates for low-molecular-weight
PAHs were higher than those for heavier
ones. However, some heavy PAHs are sus-
pected human carcinogens. Benz[a]anthracene,
benzo[a]pyrene, benzo[b]fluoranthene,
benzo[k]fluoranthene, dibenz[a,h]anthracene,
and indeno[1,2,3-cd]pyrene are classified by the
U.S. EPA as probable human carcinogens (U.S.
EPA 1994). Even at trace levels, long-term
exposure to these compounds could increase
cancer risk. Benzo[a]pyrene, benzo[b]fluoran-
thene, and benzo[k]fluoranthene were detected
in the particulate phase of mosquito coil smoke.
Environmental Health Perspectives
•
VOLUME 111 | NUMBER 12 | September 2003
1457
Table 1. Summary of coil weight, burn rate, air exchange rate in the chamber, PM
2.5
removal rates, concentrations, and emission rates for different mosquito coils.
Brand
a
W
b
(g) B
c
(g/hr) A
d
(hr
–1
) K
e
(hr
–1
) C (PM
2.5
)
f
(mg/m
3
) P (PM
2.5
)
g
(mg/hr)
M1 12.7 ± 0.5 1.67 ± 0.04 2.04 ± 0.05 2.14 ± 0.20 363 ± 12 117 ± 14
M2 11.1 ± 0.8 1.70 ± 0.05 1.99 ± 0.09 2.09 ± 0.14 365 ± 19 115 ± 12
C1 20.4 ± 1.2 1.81 ± 0.05 2.02 ± 0.06 2.22 ± 0.18 189 ± 9 63 ± 3
C2 13.3 ± 0.6 1.48 ± 0.03 1.97 ± 0.11 2.33 ± 0.37 246 ± 23 85 ± 9
C3 15.6 ± 1.0 1.56 ± 0.05 1.98 ± 0.10 2.10 ± 0.20 162 ± 15 51 ± 7
C4 18.5 ± 0.9 2.05 ± 0.05 2.09 ± 0.11 2.18 ± 0.16 215 ± 24 71 ± 13
Data expressed as mean ± SD.
a
Number of experiments (n = 5 for all).
b
Coil weight.
c
Burn rate.
d
Air exchange rate.
e
Particle removal rate.
f
PM
2.5
concentration in the chamber.
g
Emission rate of PM
2.5
.
Table 2. Emission rates of particulate phase PAHs in coil smoke (µg/hr).
Compound
a
M1 M2 C1 C2 C3 C4
Acenaphthene 17.4 ± 8.6 ND ND ND ND ND
Fluorene 20.6 ± 0.4 16.7 ± 0.2 0.903 ± 0.102 15.6 ± 0.6 1.46 ± 0.08 2.49 ± 0.04
Phenanthrene 29.0 ± 0.5 20.3 ± 0.7 1.20 ± 0.04 20.6 ± 0.8 3.95 ± 0.35 5.99 ± 0.07
Anthracene 0.988 ± 0.13 1.04 ± 0.04 0.350 ± 0.079 0.146 ± 0.052 0.416 ± 0.078 1.21 ± 0.13
Fluoranthene 94.5 ± 1.1 18.0 ± 0.4 ND 4.26 ± 0.58 1.74 ± 0.15 2.38 ± 0.10
Pyrene 9.37 ± 0.21 14.2 ± 1.4 0.917 ± 0.093 1.90 ± 0.05 0.959 ± 0.031 3.78 ± 0.11
Benzo[b]fluoranthene ND 0.14 ± 0.008 0.043 ± 0.012 0.225 ± 0.046 0.076 ± 0.029 0.158 ± 0.045
Benzo[k]fluoranthene ND 0.028 ± 0.004 0.009 ± 0.003 0.044 ± 0.011 0.011 ± 0.003 0.034 ± 0.006
Benzo[a]pyrene 0.825 ± 0.048 0.237 ± 0.026 0.053 ± 0.016 0.322 ± 0.039 0.109 ± 0.023 0.300 ± 0.061
Benzo[ghi]perylene 3.14 ± 0.24 0.095 ± 0.013 ND ND ND ND
ND, not detected. Data are expressed as mean ± SD.
a
Number of experiments (n = 3 for all).
Article
|
Mosquito coil emissions and health implications
Table 3 lists the emission rates of carbonyl
compounds identified in the mosquito coil
smoke. The results were based on three experi-
ments for each tested brand. The standard
deviation of the means of carbonyl compounds
emission rates was less than 10% for most of
the compounds, suggesting that the experi-
ments had good reproducibility. Among all
the carbonyl detected, formaldehyde and
acetaldehyde had the highest emission rates
and together represented as much as 55% of
the total carbonyl compounds emitted from
the coil combustion. Acrolein, glyoxal, and
methyl-glyoxal, known for their high reactiv-
ity, strong irritation effects, and suspected car-
cinogenic effects, were also detected in the
coil smoke in relatively high concentrations.
Among all the six tested brands, the two
Malaysian coils emitted markedly higher levels
of aldehydes than did the Chinese coils. The
difference is likely due to different smoldering
materials used in making the coils, because the
burning patterns for all the tested coils were
very similar.
Several VOCs in the coil smoke were iden-
tified through the GC/MS analysis, including
benzene, toluene, ethylbenzene, p,m,o-xylene,
and styrene, in relatively high concentrations.
According to the GC/MS library, furan,
1,3-pentadiene, 2-burene, isoprene, cyclopen-
tene, 3-hexyne, 2,4-hexadiene, 1,3-cyclohexadi-
ene, cyclohexene, and 1-heptene were
tentatively identified. The identification of
these compounds indicates that the mosquito
coil smoke contains large amounts of aromatic
compounds, alkenes, and furans, some of which
are known to cause adverse health effects.
Mosquito coil smoke versus environmental
tobacco smoke. Emission factor—presented as
microgram of pollutant emitted per gram of
coil burned—can be obtained from the emis-
sion rate divided by the burn rate. Table 4 lists
a comparison of the emission factors of some
pollutants found in the mosquito coil smoke
with those found in environmental tobacco
smoke (ETS) (Daisey et al. 1998; Klepeis et al.
1999). Incorporating the emission factors, coil
weight, and cigarette weight (0.55 g/cigarette,
excluding the filter), we derived ETS equiva-
lents for all the tested mosquito coils. The
results, shown in Table 4, indicate that PM
2.5
mass released from burning one mosquito coil
would be equivalent to PM
2.5
mass released as
ETS from burning 75 (coil C3) to 137 ciga-
rettes (coil C2). The emission of formaldehyde
from one tested Malaysian coil can be as high
as that from burning 51 cigarettes. However,
total PAHs appeared to be substantially lower
in mosquito coil smoke than in ETS.
Potential exposure and health risk. The
simplified single-compartment mass balance
model, Equation 3, was used to estimate pollu-
tant concentrations in room conditions.
Mosquito coils are usually used overnight on a
daily basis to control mosquitoes in tropical
areas and seasonally in subtropical and temper-
ate areas. Each tested Malaysian coil burned
7–8 hr. Although each tested Chinese coil
burned for up to 11 hr, people may usually
burn coils no more than 8 hr each night.
Hence, we set the average combustion time (t)
for one coil as 8 hr and ignored any exposure
beyond this coil burning period. We assume
the air exchange rate of the bedroom is
between 2 and 5/hr and the volume of the bed-
room (V) is in the range of 50–100 m
3
. The
total pollutant removal rate was considered as
the same as the air exchange rate, given that air
exchange values used here are significantly
higher than possible surface deposition rate.
Using the above assumptions, we calcu-
lated PM
2.5
concentrations resulting from
burning M1 under varying room volume and
air exchange rate values (Figure 4). When vol-
ume is 50 m
3
(5 × 5 × 2.5 m) and the air
exchange rate is 2/hr, the concentration will be
1.17 mg/m
3
. Even this highest estimate is not
unrealistic given that many rooms are smaller
and air exchange rate is not substantially higher
than 2. [The recommended typical air exchange
rate for the United States is 0.45/hr (U.S. EPA
1999b).] Because health-based standards are
given as 24-hr average values, we calculated 24-
hr average concentrations for several measured
pollutants. In our calculation, we assumed that
room concentrations during the other 16 hr
without mosquito coil use are zero. This appar-
ently resulted in conservative estimates as other
sources (e.g., cooking, outdoor penetration,
smoking) may also contribute to the 24-hr
averages. The lower bound and higher bound
of our conservative estimates are shown in
Table 5, where the lower-bound values are asso-
ciated with air exchange rate of 5/hr and room
volume of 100 m
3
, and the higher-bound
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VOLUME 111 | NUMBER 12 | September 2003
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Environmental Health Perspectives
Table 3. Emission rates of carbonyl compounds in the gas phase of coil smoke (mg/hr).
Compound
a
M1 M2 C1 C2 C3 C4
Formaldehyde 7.52 ± 0.61 5.66 ± 0.20 2.21 ± 0.08 1.43 ± 0.07 0.454 ± 0.020 1.31 ± 0.14
Acetaldehyde 2.47 ± 0.19 2.76 ± 0.46 1.00 ± 0.09 2.31 ± 0.21 1.60 ± 0.44 1.66 ± 0.07
Acetone 1.30 ± 0.07 1.48 ± 0.13 0.462 ± 0.009 1.19 ± 0.18 0.880 ± 0.218 0.921 ± 0.072
Acrolein 1.56 ± 0.15 1.22 ± 0.087 0.165 ± 0.022 0.399 ± 0.068 0.0852 ± 0.0059 0.200 ± 0.055
Propanaldehyde 0.852 ± 0.096 0.813 ± 0.083 0.239 ± 0.014 0.507 ± 0.056 0.270 ± 0.052 0.280 ± 0.043
Crotonaldehyde 0.506 ± 0.026 0.459 ± 0.040 0.069 ± 0.119 0.298 ± 0.042 0.321 ± 0.073 0.211 ± 0.009
2-Butanone 2.00 ± 0.34 ND 0.638 ± 0.059 0.723 ± 0.062 0.623 ± 0.173 0.338 ± 0.038
Glyoxal 0.210 ± 0.01 0.352 ± 0.169 0.135 ± 0.092 0.135 ± 0.058 ND 0.051 ± 0.012
o-Tolualdehyde 0.272 ± 0.050 0.209 ± 0.042 0.110 ± 0.095 ND ND ND
4-Methyl-2-pentanone 0.590 ± 0.217 1.09 ± 0.59 0.463 ± 0.058 0.590 ± 0.050 0.354 ± 0.049 0.252 ± 0.040
Methylglyoxal 1.20 ± 0.03 1.34 ± 0.30 0.724 ± 0.186 0.667 ± 0.090 0.402 ± 0.102 0.416 ± 0.036
ND, not detected. Data are expressed as mean ± SD.
a
Number of experiments (n = 3 for all).
Table 4. Emission factors of mosquito coils and emission ETS equivalents.
PM
2.5
Formaldehyde Acetaldehyde Total PAH
a
Brand Ef (mg/g)
b
ETS equivalent
c
Ef (mg/g)
b
ETS equivalent
c
Ef (mg/g)
b
ETS equivalent
c
Ef (µg/g)
b
ETS equivalent
c
M1 70.4 ± 7.3 131 4.54 ± 0.29 51 1.49 ± 0.12 10 64.6 4
M2 68.6 ± 7.7 112 3.38 ± 0.20 33 1.65 ± 0.29 10 19.2 1
C1 34.5 ± 2.9 103 1.21 ± 0.06 22 0.552 ± 0.070 6 0.6 0
C2 57.3 ± 5.1 137 0.967 ± 0.037 14 1.56 ± 0.13 14 4.6 0
C3 32.8 ± 4.9 75 0.291 ± 0.006 4 1.04 ± 0.32 9 1.9 0
C4 34.6 ± 6.0 94 0.639 ± 0.058 11 0.810 ± 0.029 8 3.2 0
ETS 12.4 ± 1.3
d
— 2.04 ± 0.41
d
—3.34 ± 0.53
d
— 340
e
—
Data are expressed as mean ± SD.
a
Total PAH mass was obtained as the sum of all PAHs quantified in the coil smoke.
b
Emission factor: microgram of pollutant emitted per gram of mosquito coil or per gram of cigarette.
c
The number of cigarettes needed to produce the same amount of pollutant emitted from burning one mosquito coil.
d
ETS data were from Daisey et al. (1998).
e
ETS data were from
Klepeis et al. (1999).
Article
|
Liu et al.
values are associated with air exchange rate of
2/hr and room volume of 50 m
3
. Reference
exposure levels for aldehydes derived by the
Office of Environmental Health Hazard
Assessment (OEHHA 2000), and those for
PM
2.5
derived by the U.S. National Ambient
Air Quality Standard (NAAQS) (U.S. EPA
2003a), are used for comparison. Our conserva-
tive estimates indicate that PM
2.5
concentra-
tions can be six times higher than the allowable
NAAQS for 24-hr average PM
2.5
concentra-
tion. In general, for all the tested Malaysian
coils, even the lower-bound estimates are higher
than the reference concentrations, except for
acetaldehyde. Burning mosquito coils such as
the tested Chinese brands may also produce 24-
hr–averaged indoor concentrations exceeding
the standards or reference values if the condi-
tion of the bedroom is not favorable for pollu-
tant removal—for example, if the ventilation is
low and/or room volume is small. For example,
formaldehyde level can be higher than the
reference chronic inhalation concentration,
3µg/m
3
. If the coil is placed close to the bed,
the exposure to the coil smoke may be even
higher. The OEHHA has shown that exposure
to acrolein at 0.19 µg/m
3
for 1 hr can induce
irritation to the eye and respiratory system.
Almost all of the estimated concentrations are
higher than this level (Table 5), suggesting that
exposure to mosquito coil smoke may cause sig-
nificant acute health effects. In addition,
formaldehyde and acetaldehyde are classified by
the U.S. EPA as probable human carcinogens
(U.S. EPA 2003b).
The estimates shown above are derived
based on the assumption that pollutant con-
centration in the room is homogeneous. In
reality, concentrations actually inhaled by
room occupants may be higher than the esti-
mated concentrations because the room air
may not necessarily be well mixed and the
source (coil) may be placed in close proximity
to the breathing zone (the bed level during
sleeping). In houses using mosquito coils, chil-
dren usually sleep in small rooms. To prevent
them from excessive mosquito biting, the win-
dows of their rooms are often closed during
sleeping hours. Thus, the predicted indoor
concentrations above are likely to be very con-
servative and underestimate actual concentra-
tions in children’s rooms. Unfortunately,
children are substantially more susceptible to
air pollution exposure and thus can be more
readily affected than adults (Azizi and Henry
1991; Fagbule and Ekanem 1994).
The pollutants measured in the mosquito
coil smoke, as reported in this article, were gen-
erated from incomplete combustion of base
materials (biomass). Because the coils are made
purposely to have very inefficient combustion
(smoldering effect), large amounts of products
of incomplete combustion are expected from
the burning mosquito coils (Zhang et al. 2000).
The findings from this study can be indirectly
supported by previous studies of other types of
biomass (e.g., wood, crop residue, cow dung),
burning of which produces the same types of
pollutants (Ezzati and Kammen 2001;
McCracken and Smith 1998; Smith et al.
2000; Zhang and Smith 1996). The existing
literature provides strong evidence that indoor
biomass smoke from cooking and heating is a
risk factor for acute respiratory infections
(ARIs) and chronic obstructive pulmonary dis-
ease (COPD). Evidence from 13 studies in
developing countries indicate that young chil-
dren living in homes burning biomass fuels
experience two to three times higher risk of
serious ARIs than do unexposed children, after
adjustment for potential confounders, includ-
ing socioeconomic status (Smith et al. 2000).
An evaluation of eight studies in developing
countries indicates that women cooking over
biomass fires for many years have two to four
times more risk of COPD than those unex-
posed, after adjustment for potential con-
founding factors (Bruce et al. 2000). A recent
World Health Organization report estimated
that indoor smoke from solid fuels (mainly
biomass, also coal) ranked as one of the top 10
risk factors for the global burden of disease in
2000, accounting for 1.6 million premature
deaths each year (WHO 2002). Among all
environmental risk factors, it ranked second
only to poor water/sanitation/hygiene.
Comparing the indoor pollutant concentra-
tions predicted in this article with those result-
ing from biomass fuel combustion, we estimate
that exposure to mosquito coil smoke may
pose comparable or higher respiratory health
risks in people who use mosquito coils for a
large fraction of their lifetimes. However, it is
more complicated to evaluate health risks of
mosquito coils given the obvious benefit associ-
ated with mosquito coil use—prevention of
malaria and other mosquito-borne diseases.
Conclusions
In this study, a comprehensive characterization
of emissions was carried out for six brands
of mosquito coils commonly used in China
and Malaysia. The pollutants characterized
included fine and ultrafine particles, PAHs,
VOCs, and aldehydes, with high irritation or
suspected carcinogenic effects. We found that
all particles emitted from burning mosquito
coils were fine, < 1 µm in diameter. Most par-
ticles were in the size ranges of 0.01–0.1 µm
and 0.2–0.3 µm. In general, the pollutant
emissions from the two tested Malaysian
brands were substantially higher than those
from the four tested Chinese brands. After
comparing health-based standards and guide-
lines, we suggest that exposure to the mosquito
coil smoke poses both acute and chronic health
risks. Before smoke-generating mosquito coils
can be ultimately replaced with nonsmoke
mosquito controlling methods, switching from
a more polluting type (brand) to a “cleaner”
type (brand) may bring substantial reductions
in exposure and respiratory health risks.
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1,000
800
600
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Mosquito coil emissions and health implications
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