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Inhalation Toxicology
International Forum for Respiratory Research
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/iiht20
Characterization of E-cigarette coil temperature
and toxic metal analysis by infrared temperature
sensing and scanning electron microscopy –
energy-dispersive X-ray
Haley A. Mulder , James B. Stewart , Ivy P. Blue , Rose I. Krakowiak , Jesse L.
Patterson , Kimberly N. Karin , Jasmynne M. Royals , Alexandra C. DuPont ,
Kaitlin E. Forsythe , Justin L. Poklis , Alphonse Poklis , Shelle N. Butler ,
Joseph B. McGee Turner & Michelle R. Peace
To cite this article: Haley A. Mulder , James B. Stewart , Ivy P. Blue , Rose I. Krakowiak , Jesse L.
Patterson , Kimberly N. Karin , Jasmynne M. Royals , Alexandra C. DuPont , Kaitlin E. Forsythe ,
Justin L. Poklis , Alphonse Poklis , Shelle N. Butler , Joseph B. McGee Turner & Michelle R.
Peace (2020): Characterization of E-cigarette coil temperature and toxic metal analysis by infrared
temperature sensing and scanning electron microscopy – energy-dispersive X-ray, Inhalation
Toxicology, DOI: 10.1080/08958378.2020.1840678
To link to this article: https://doi.org/10.1080/08958378.2020.1840678
Published online: 03 Nov 2020. Submit your article to this journal
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RESEARCH ARTICLE
Characterization of E-cigarette coil temperature and toxic metal analysis by
infrared temperature sensing and scanning electron microscopy –energy-
dispersive X-ray
Haley A. Mulder
a
, James B. Stewart
a
, Ivy P. Blue
a
, Rose I. Krakowiak
a
, Jesse L. Patterson
a
, Kimberly N. Karin
a
,
Jasmynne M. Royals
a
, Alexandra C. DuPont
a
, Kaitlin E. Forsythe
a
, Justin L. Poklis
c
, Alphonse Poklis
a,c,d
,
Shelle N. Butler
a
, Joseph B. McGee Turner
b
and Michelle R. Peace
a
a
Department of Forensic Science, Virginia Commonwealth University, Richmond, VA, USA;
b
Department of Chemistry, Virginia
Commonwealth University, Richmond, VA, USA;
c
Department of Pharmacology and Toxicology, Virginia Commonwealth University,
Richmond, VA, USA;
d
Department of Pathology, Virginia Commonwealth University, Richmond, VA, USA
ABSTRACT
Introduction: Electronic cigarettes (e-cigarettes) have rapidly evolved since their introduction to the
U.S. market. The rebuildable atomizer (RBA) offers user-driven modification to the heating element
(coil) and wicking systems. Different coil materials can be chosen based on user needs and preferen-
ces. However, the heating element of an e-cigarette is believed to be one-source for toxic
metal exposure.
Methods: E-cigarette coils from Kanthal and nichrome wires were constructed in a contact and non-
contact configuration and heated at four voltages. The maximum temperatures of the coils were
measured by infrared temperature sensing when dry and when saturated with 100% vegetable gly-
cerin or 100% propylene glycol. The metal composition of each coil was analyzed with Scanning
Electron Microscopy-Energy-Dispersive X-Ray (SEM-EDX) when new, and subsequently after 1, 50, and
150 heat cycles when dry.
Results: The coils reached temperatures above 1000 C when dry, but were below 300 Cinboth
liquid-saturated mediums. Metal analysis showed a decrease of 9–19% chromium and 39–58% iron in
Kanthal wire and a decrease of 12–14% iron and 39–43% nickel in nichrome wire after 150 heat cycles.
Significant metal loss was observed after one heat cycle for both coil alloys and configurations.
Conclusions: The loss of metals from these heat cycles further suggests that the metals from the coils
are potentially entering the aerosol of the e-cigarette, which can be inhaled by the user.
ARTICLE HISTORY
Received 16 March 2020
Accepted 17 October 2020
KEYWORDS
E-cigarettes; coils; metal
analysis; metal aerosol;
SEM-EDX
Introduction
Electronic cigarettes (e-cigarettes) have rapidly evolved since
their introduction to the U.S. market in 2007. All e-cigarettes
operate under the same, basic principle. The device consists
of two main components: a battery power supply and an
atomizer that contains a coil and wicking system (Breland
et al. 2017). The coil is typically a metal filament wrapped
around a wicking material of silica or cotton, saturated in a
refill formulation liquid (e-liquid). The e-liquid consists of a
ratio of propylene glycol and vegetable glycerin, nicotine, and
usually, a flavoring agent (Peace et al. 2016). When the bat-
tery is activated, it delivers an electric charge to the coil,
which heats and aerosolizes the e-liquid. The generated aero-
sol is then inhaled by the user (Peace et al. 2016; Breland
et al. 2017).
The first e-cigarette, designed in China in 2004, resembled
a traditional cigarette and was known as the ‘ciga-like.’Future
generations of e-cigarettes consisted of the cartomizer (a cart-
ridge atomizer) and clearomizer (a transparent tank cartom-
izer), which contained replaceable tank-and-atomizer systems
(Breland et al. 2017). A popular generation of e-cigarette is
known as the rebuildable atomizer (RBA), which allows for
complete user-driven customizability. The intent of the RBA
is to enable the user to modify the device to create a nicotine
delivery system that an e-cigarette consumer would find satis-
fying (Grana et al. 2013). Users take advantage of adjustable
power supplies and coil configurations to impact heat produc-
tion with the desire to maximize drug delivery.
The use of rebuildable atomizers caused polarizing dis-
cussions about what components will create the best ‘build’
and therefore, the best vaping experience. Early e-cigarette
models used nichrome 80:20 wire for coil builds in commer-
cial atomizers (Williams et al. 2013). The metal is comprised
of nickel and chromium and has a maximum operating
temperature of 1200 C (BNC 2020). Kanthal A-1 is a resist-
ance wire that grew popular with RBA builders in the vap-
ing community. The metal is comprised of iron, chromium,
and aluminum alloys and has a maximum operating tem-
perature of 1400 C (Kanthal 2018a). With wire composition
preferences came choices in wire configurations. Generally,
the coils in the e-cigarette can be wrapped into two
CONTACT Michelle R. Peace mrpeace@vcu.edu Department of Forensic Science, Virginia Commonwealth University, Box 843079, Richmond, VA 23284, USA
ß2020 Informa UK Limited, trading as Taylor & Francis Group
INHALATION TOXICOLOGY
https://doi.org/10.1080/08958378.2020.1840678
configurations: contact or non-contact. The contact build
has the coil wraps touching together and the non-contact
build has the coil wraps separated in space. Despite the
numerous ways to create an e-cigarette coil, there is no
definitive answer on which type of coil build provides the
best user experience and most efficient drug delivery.
Potential toxic metal exposure from inhaling e-cigarette
aerosols is of concern with the use of metallic filaments as
the heating element. Chromium and nickel, two major metal
alloys present in nichrome wire are listed as carcinogens
and are linked with chronic lung infections and other
respiratory illnesses such as reduced lung infections and
bronchitis (ASTDR 2005,2012). Chromium is also present
in Kanthal wire. Some studies for iron oxides also suggest that
inhalation over time can lead to lung cancer (Kornberg et al.
2017). Early studies assessed e-cigarette aerosol for metals and
identified tin, silver, iron, nickel, aluminum, and cadmium
present in the aerosol (Williams et al. 2013; Hess et al. 2017).
Several studies highlighted a (Hess et al. 2017; Halstead et al.
2019; Williams et al. 2019; Zervas et al. 2020) variability in
metal concentrations in aerosols between e-cigarette brands,
potentially resulting from the different heating units within
the devices, device age, and access of the liquid to metal. One
study assessed the metal content present in e-liquids directly
from the manufacturer bottle verses the tank and the resulting
aerosol. The study determined that there was a higher metal
content present in the e-liquid from the tank and the aerosol,
suggesting that the coils had an effect on metal concentration
(Olmedo et al. 2018). A later study found that factors such as
e-liquid composition and boiling temperature of the e-liquids
had an effect on the transfer of metal from two commercial
coils to the e-liquid aerosolized in the device (Zervas
et al. 2020).
The purpose of this study was to characterize two types of
common metal wires used to construct coils for electronic cig-
arettes with typical user modifications. The temperature out-
put of the two coils when in a contact and non-contact
configuration under dry conditions and wet conditions when
saturated with 100% vegetable glycerin, 100% propylene glycol
e-liquids, and a 50:50 mixture was determined using an infra-
red (IR) temperature sensor. The metal profile of the two coils
before and after heating was determined using scanning elec-
tron microscopy energy-dispersive X-ray (SEM-EDX).
Experimental
Reagents and supplies
For all studies, a Kayfun Lite Styled Rebuildable Atomizer
(RBA) was purchased from FastTech (Hong Kong, China).
The Pure-Atomist Kanthal A-1 wire of 30, 32, and 34
American Wire Gauge (AWG) and ceramic tweezers were
purchased from Lightning Vapes (Brandenton, FL). The
consolidated chromium wire of 30, 32, and 34 AWG were
purchased from McMaster Carr (Elmhurst, IL). The 2 mm
high quality silica wick and Master 5-in-1 coiling kit were
purchased from Amazon (Seattle, WA). The 100% pure
vegetable glycerin (VG) and 100% pure propylene glycol
(PG) were purchased from Wizard Labs (Almonte Springs,
FL). A thermoMETER M3 and M1/M2 temperature sensors
ranging from 100–600 C and 650–1800 C were purchased
from Micro-Epsilon (Raleigh, NC). The Sorensen power
supply was from Amtek (San Diego, CA) and a multimeter
was purchased from Fluke (Everett, WA).
Coil building
Kanthal A-1 and nickel chromium (nichrome) wires of 30
AWG (254.6 mm), 32 AWG (201.9 mm), and 34 AWG
(160.1 mm) were used to wrap all coils in two configurations:
contact and non-contact (Figure 1). The coils in each gauge
and configuration were tested under two types of conditions:
(i) ‘dry medium’containing no silica wick and no e-liquid
and (ii) ‘wet medium,’which contained the 2 mm silica wick
and e-liquid. Each coil was constructed using the 1.5 mm
diameter rod from the Coil Master 5-in-1 coiling kit and
were wrapped to 1.8 X. For non-contact configuration, the
coils were pulled apart so that the wraps were no longer in
contact. The coils were housed in a Kayfun Lite Styled RBA
and a Fluke multimeter was used to ensure that every coil
type, configuration, and gauge was wrapped to 1.8 X. In the
contact configuration, the coils were wrapped and placed in
the atomizer and then annealed to ensure even heating across
the coil before the resistance was tested. For all ‘wet medium’
conditions in the temperature studies, a 2 mm silica wick was
threaded through the coil and the atomizer was filled with
the e-liquid. E-liquid refill formulations used for the tempera-
ture studies were 100% VG, 100% PG, and a 50:50 PG:VG
made in-house with no nicotine.
Coil temperature studies
The atomizers were heated using a Sorensen power supply,
which controlled the voltage output. The coils were heated at
increasing voltages of 3.5, 3.7, 4.2, and 5.0 V (6.8, 7.6, 9.8,
13.9 W) . The trials were completed in triplicate, resulting in
three trials for each coil under each voltage, configuration,
and dry or wet medium (100% PG and 100% VG). Both coil
materials were then tested under contact and non-contact
configurations at 32 gauge in a 50:50 PG:VG solution. Two
Figure 1. Electronic cigarette coil wrapped in a contact (A) and non-contact (B)
configuration.
2 H. A. MULDER ET AL.
dual-laser infrared temperature sensors by Micro-Epsilon
were used to measure the temperature outputs of the coils. A
thermoMETER CT Laser M3 with a measuring range of
100–650 C and spectral range of 2.3 mm was used for all ‘wet
medium’studies. A thermoMETER CT Laser M1/M2 with a
measuring range of 650–1800 C and a spectral range of 1/
1.6 mm was used for all ‘dry medium’studies. The coils were
heated for eight seconds, and the maximum temperature at
both ends near the pole and at the center of the coil was
recorded. The temperature at the three positions was meas-
ured ten times. Compact Connect version 1.9.8.6 was used to
record the temperatures of the coils as they were heated.
Coil SEM analysis
The coils analyzed for SEM were heated from the Sorensen
power supply at their maximum operating voltage for ten
seconds under dry conditions. The coils were heated for
one, 50, and 150 times at their maximum operating temper-
atures and stored in a trace metal free microcentrifuge
tubes. The max operating temperatures were determined by
the voltage and coil combination that could withstand a ten
second burn without melting the coil. The analysis of the
coil composition pre and post-heating was conducted using
a Hitachi SU-70 Scanning Electron Microscope coupled
with Energy Dispersive X-Ray Spectroscopy (SEM-EDX).
Samples were mounted on an aluminum disk coated with
carbon tape, with the ends taped down and the wraps pulled
apart and pressed into the carbon tape. An EDX spectrum
was obtained simultaneously with the SEM images. The
SEM-EDX was used with an accelerating voltage of 20 keV
and the center of the coil was identified and a measurement
was made in that center and to the immediate left and right,
totaling 12 measurements for each gauge at each configur-
ation at 2000magnification. The EDX spectrum and
weight percent of each metal present in the coil were also
obtained. SEM-EDX data for coils at 32 gauge for both coil
types and configurations were recorded at the center of the
coil for new, one, and50 heat cycles in the presence of a wet
media of 50:50 PG to VG.
The Brown–Forsythe test was used to test for equality of
variances. A significance level of 0.05 was used. The data
that was determined to exhibit equal variance was analyzed
using ANOVA. Data with unequal variance were analyzed
using Tukey-HSD. Data were analyzed using JMP pro 12
statistical software.
Results
Coil temperature studies
In the dry heat studies across the length of the coil, the cen-
ter of the coils was visibly brighter than the ends in both
configurations suggesting that the coils produced the most
heat in the center of the coil, and was confirmed with
5–10% higher temperature measurements in the center.
Kanthal in a contact configuration with the lowest gauge
(thickest wire) had the most significant increase in
temperature output, from 1134 C to 1436 C, with increas-
ing voltage. Wire gauge only significantly impacted tempera-
ture at the lower voltages in contact configuration, with the
thicker wire exhibiting the lowest starting temperature
(1134 C) at the lowest voltage (3.5 V) (Figure 2). The thin-
nest wire, 34 gauge, in contact configuration was unable to
withstand the heat produced at 5.0 V and burnt out/melted
before the maximum temperature could be recorded.
Kanthal in non-contact configuration had lower operational
temperatures compared to the contact configuration with
increasing voltages and wire gauges. Initial temperatures of
30 and 32 gauge wires at 3.5 V, dry, non-contact configura-
tions were 975 C and 1016 C versus 1134C and 1354 C
in dry, contact configurations, respectively. Temperatures of
34 gauge at 4.2 and 5.0 V dry, non-contact configurations
could not be recorded. For nichrome wire with dry heat in
a contact configuration, the operational temperature range
of 1051–1234 C was lower compared to Kanthal across the
voltages and wire gauges (Figure 2). In a contact configur-
ation, the temperature profile of 34 gauge nichrome could
not be recorded even at the lowest voltage due to burnout.
In a non-contact configuration, the wire had a temperature
range of 911–1234 C across the voltages and wire gauges.
Similar to non-contact Kanthal, the non-contact nichrome
wire at 34 gauge could not be recorded at 4.2 and 5.0 V.
For the wet heat studies with 100% vegetable glycerin
(VG), the temperature output was greatly reduced compared
to the dry heats. Kanthal wire in a contact configuration
had a temperature range of 266–298 C with increasing volt-
age and wire gauge (Figure 2). In a non-contact configur-
ation, the Kanthal wire had a temperature range of
241–275 C. Statistical analysis of the two coil configurations
determined that the temperature output in a contact verses
a non-contact configuration for Kanthal wire was not statis-
tically different (p¼0.453). For nichrome wire in a contact
configuration, the temperature of the coils ranged from
253–283 C across all voltages and gauges. In a non-contact
configuration, the nichrome wire had a temperature range
of 245–293 C. Statistical analysis of the two coil configura-
tions determined that the temperature output in a contact
versus non-contact configurations for nichrome wire was
not statistically different (p¼0.447). Comparing the two
coil types and the temperature output in Kanthal versus
nichrome wire was determined to not be statistically differ-
ent with varying wire gauge and voltage (p¼0.198).
For the wet heat studies with 100% propylene glycol
(PG), the coil temperature output was also greatly reduced
compared to the dry heats. Kanthal wire in a contact config-
uration had a temperature range of 182–386 C with
increasing voltage and wire gauge (Figure 2). In a non-con-
tact configuration, the Kanthal wire had a temperature
range of 148–186 C across all voltages and gauges.
Statistical analysis demonstrated that the differences in tem-
perature output of Kanthal in contact and non-contact con-
figurations was statistically significant (p<0.001). For
nichrome wire in a contact configuration, the temperature
of the coils ranged from 171–241 C across all voltages and
wire gauges. In a non-contact configuration, the nichrome
INHALATION TOXICOLOGY 3
wire had a temperature output range of 160–184 C.
Statistical analysis demonstrated that the difference in tem-
perature output of nichrome coils in contact and non-con-
tact configurations was not statistically significant
(p¼0.138). The temperature output between coil configura-
tions combined with coil metal was statistically significantly
different. Comparing the two coil configurations and the
temperature output in Kanthal versus nichrome wire was
statistically significant when varying wire gauge and volt-
age (p¼0.001).
In a 50:50 PG:VG solution, the 32-gauge Kanthal in a
contact configuration had a temperature of 197–240 C, and,
in a non-contact configuration, the range was 190–227 C
(Figure 3). In a 50:50 PG:VG medium, the Kanthal wire was
significantly different from coils heated in 100% vegetable
glycerin (p¼0.001), but not significantly different from coils
heated in 100% propylene glycol (p>0.05). For nichrome
wire in a contact configuration, the temperatures ranged
from 204–240 C and for a non-contact configuration,
212–240 C. For both coil types, the temperatures of the
nichrome coils heated in a 50:50 PG:VG medium was sig-
nificantly different than those heated in either 100% VG or
100% PG (p<0.05).
Coil SEM metal analysis
When heated dry, the metals that showed the most appre-
ciable loss following heating were iron and chromium for
Kanthal A-1 wire and iron and nickel for nichrome wire.
The oxygen content on each coil increased with each burn
cycle. Metal analysis performed on new unheated coils
yielded Kanthal baseline compositions to be 59.29-66.26%
iron, 19.72–21.03% chromium, and 3.60–4.79% oxygen. The
baseline composition of the nichrome wire was
18.81–19.69% iron, 51.50–54.34% chromium, and
3.44–4.78% oxygen for the three gauges.
With dry heating, chromium composition in the Kanthal
wire in contact configuration decreased from 19.72–21.03%
to 13.47–15.53% after one heating cycle and to 5.76–8.54%
after 50 burn cycles across the three gauges (Figure 4). After
150 heating cycles, the final chromium composition was
4.55%, 2.54%, and 1.86% for 30, 32, and 34-gauge wire,
respectively. The iron composition decreased from
59.59–66.62% to 33.52–43.17% after one heating cycle and
to 14.85–23.50% after 50 heating cycles. After 150 heating
cycles, the metal content for nickel was 23.50%, 14.85%, and
22.15% for the three gauges, respectively (Figure 4). The
total loss of metal in a contact configuration was between
15.27–19.17% for chromium and 52.48–58.26% for iron. For
each metal, a significant difference in metal composition at
each heating cycle increment was observed across the three
gauges (p<0.05). When comparing the total metal loss
between each wire gauge, for both chromium and iron,
there was a significant difference between 30-gauge and the
32 and 34-gauge wires (p<0.05). There was not a signifi-
cant difference in 32 and 34-gauge wire for total metal loss
for metal for chromium and iron (p¼0.4468, 0.5003).
When heated dry, Kanthal wire in a non-contact config-
uration, after 1 heat the metal content for chromium
dropped from 19.72–21.03% to 15.86–17.00% across the
three gauges and after 50 heats, to 8.97–11.45% (Figure 4).
After 150 heats, the final metal content for chromium was
10.34%, 6.98%, and 5.54% for 30, 32, and 34-gauge wire,
respectively (Figure 3). For iron, the metal content dropped
from 59.59–66.62% to 38.64–48.33% after one heat cycle
and to 22.98–32.11% after 50 heat cycles. After 150 heat
cycles, the metal content for nickel was 26.66%, 18.02%, and
16.64% for the three gauges, respectively (Figure 4). The
total loss of metal in a non-contact configuration was
between 9.38–15.49% for chromium and 39.96–46.10% for
Figure 2. Maximum temperature outputs of Kanthal in contact (C) and non-contact (NC) configurations in three mediums; Dry (A), 100% VG (B), and 100% PG (C).
Maximum temperature outputs of nichrome in contact and non-contact configurations in three mediums: Dry (E), 100% VG (F), and 100% PG (G).
4 H. A. MULDER ET AL.
iron. For each metal, there was a significant difference in
metal composition at each burn stage across the three
gauges (p<0.05). When comparing the total metal loss
between each wire gauge, for both chromium and iron,
there was a significant difference between 30 and 32 gauge
and 30 and 34 gauge (p<0.05). There was not a significant
difference in 32- and 34-gauge wire for total metal loss for
metal for chromium and iron (p¼0.3923, 0.8797). For
Kanthal wire, there was a significant difference between the
two configurations (contact and non-contact) at each gauge
for both chromium and iron (p<0.005).
Oxygen composition in a contact configuration for the
Kanthal wire increased from 3.60–4.74% to 20.15–24.04%
after one heating cycle and to 30.64–32.79% after 50 heat
cycles across the three gauges (Figure 4). After 150 heating
cycles, the final oxygen composition was 34.46%, 32.79%,
and 31.29% for 30, 32, and 34 G respectively. The total gain
of oxygen in the contact configuration was 30.85–35.09%.
Metal composition increased significantly at each heat stage
across the three gauges (p<0.05). When comparing the
total oxygen gain between each wire gauge, there was no
significant difference (p¼0.6261). In a non-contact
configuration, oxygen content increased to 16.39–23.03%
after one heat cycle and to 25.30–27.79% after 50 heat
cycles. After 150 heat cycles, the final oxygen composition
was 26.17%, 28.53%, and 31.06% for 30, 32, and 34 G
respectively. The total increase of oxygen in the non-contact
configuration was 22.57–26.32%. With all three gauges,
there was not a significant difference in the oxygen levels
from 50 to 150 heat cycles (p¼0.9393, 0.0819, and 0.2824
for 30, 32, and 34 G respectively). For 34 gauge, there was
also no significant difference between the oxygen levels
from one heat cycle to 50 heat cycles (p¼0.0573). When
comparing the total oxygen increase between each wire
gauge, there was no significant difference (p¼0.2848). For
Kanthal wire, there was a significant difference (p<0.05)
between the two configurations (contact and non-contact) at
each gauge at 150 heat cycles. For 30 and 32 gauge, there
was also a significant difference in oxygen accumulation at
50 heat cycles.
For nichrome wire in a contact configuration, after one -
dry heat cycle, the metal content for iron dropped from
18.81–19.09% to 11.22–16.91% across the three gauges and
after 50 dry heat cycles, to 5.32–8.19% (Figure 4). After
Figure 3. Maximum temperature outputs of 32 G Kanthal and nichrome wire in contact (C) and non-contact (NC) configurations in three mediums; 100% VG (A),
50:50 PG:VG (B), and 100% PG (C).
Figure 4. Metal loss of Kanthal (A, B, and C) and nichrome wire (D, E, and F) in contact (C) and non-contact (NC) configurations after one, 50, and 150 burns in a
dry medium.
INHALATION TOXICOLOGY 5
150 dry heat cycles, the final metal content for chromium
was 5.57%, 7.01%, and 5.75% for 30-, 32-, and 34-gauge
wire, respectively (Figure 3). For nickel, the metal content
dropped from 51.50–54.30% to 21.03–37.41% after 1 heat
and to 9.16–14.41% after 50 heat cycles. After 150 heat
cycles, the metal content for nickel was 10.84%, 11.51%, and
9.88% for the three gauges, respectively (Figure 4). The total
loss of metal in a contact configuration was between
12.05–14.12% for iron and 40.77-43.50% for nickel. For each
metal, there was a significant difference in metal compos-
ition after the first and 50th heat cycles across the three
gauges (p<0.05). However, there was not a significant dif-
ference in the loss of metal for nickel and iron from 50 to
150 heat cycles. When comparing the total metal loss
between each wire gauge, for both iron and nickel, there
was no significant difference between the three
gauges (p>0.05).
For nichrome wire in a non-contact configuration, after
1 heat cycle, the metal content for iron dropped from
18.81–19.09% to 13.04–15.77% across the three gauges and
after 50 burns, to 5.70–8.37% (Figure 2). After 150 heat
cycles, the final metal content for chromium was 12.13%,
12.87%, and 14.11% for 30-, 32-, and 34-gauge wire, respect-
ively (Figure 4). For nickel, the metal content dropped from
51.50–54.30% to 21.03–36.99% after one heat cycle and to
17.55–9.16% after 50 heat cycles (Figure 4). After 150 heat
cycles, the metal content for nickel was 14.76%, 7.59%, and
9.88% for the three gauges, respectively (Figure 3). The total
loss of metal in a non-contact configuration was between
12.13–14.11% for iron and 39.58–44.69% for nickel. For
each metal, there was a significant difference in metal com-
position after the first and 50th heat cycle across the three
gauges (p<0.05). However, a significant difference in the
loss of metal for nickel and iron from 50 to 150 heats was
not observed. When comparing the total metal loss between
each wire gauge, for nickel there was not a significant differ-
ence between 30- and 32-gauge wire (p¼0.5632), but a sig-
nificant difference between 30- and 34-gauge and 32- and
34-gauge was observed (p<0.05). For iron, there was a sig-
nificant difference when comparing 30-gauge wire to 32-
and 34-gauge wire (p<0.05), but not between 32- and 34-
gauge wire (p¼0.8105). When comparing contact and non-
contact configurations of nichrome wire, a significant differ-
ence in metal loss between the two configurations for both
iron and nickel was observed. However, for 32- and 34-
gauge wire, there was a significant difference for iron loss
(p<0.05), but there was no significant difference for nickel
metal loss (p>0.05).
Oxygen composition in a contact configuration for the
nichrome wire increased from 3.44–4.78% to 18.04–27.01%
after one heat cycle and to 29.12–30.14% after 50 heat cycles
(Figure 4). After 150 heating cycles, the final oxygen com-
position was 29.99%, 27.36%, and 27.95% at 30, 32, and 34
gauges, respectively. The total gain of oxygen in a contact
configuration was 23.41–25.21%. With 30 and 32 gauge,
there was a significant difference in oxygen levels between
new, one heat cycle, and 50 heat cycles. (p<0.05). For 34
gauge contact, there was only a significant difference in
oxygen levels between new and one heat cycle. There was
not a significant difference in the amount of oxygen accu-
mulated between the three gauges (p¼0.5682). For a non-
contact configuration, oxygen levels increased to
20.91–25.60% after one heat cycle and 24.12–34.03% after
50 heat cycles. After 150 heat cycles, the oxygen levels were
22.19%, 24.48%, and 24.06% for 30, 32, and 34 gauge
respectively. The total increase of oxygen in a non-contact
configuration was 17.41–20.62%. For 30 and 34 gauges,
there was no significant difference in oxygen levels after
one, 50 and 150 heat cycles (p>0.05). For 32 gauge wire,
there was a significant difference between new, one, and
fifty heat cycles, but no significant difference between oxy-
gen levels after one heat cycle and 150 heat cycles
(p¼0.6427). There was not a significant difference in the
amount of oxygen accumulated between the three gauges
(p¼0.2968). For nichrome, 30- and 34-gauge wires did not
have a significant difference between the two configurations
(contact and non-contact), but 32 gauge had a significant
difference in oxygen levels between the two configurations
at one heat cycle and 50 heat cycles (p<0.05).
The significance of the metal loss with increasing heat
cycles could be seen visually via SEM analysis. Figure 5
shows both 30-gauge Kanthal and nichrome wire at new/
pre-heated and after one, 50, and then 150 heats. Both wires
exhibit a smooth surface prior to burning and even after the
first burn, the metal begins to show signs of decay and fis-
sures. By the 150
th
heat cycle, both wires are pitted and
have rough surfaces. In some instances, charring could be
seen visually prior to SEM analysis. When all coils were
heated while with a 50:50 PG:VG e-liquid, no statistical dif-
ference was observed between contact and non-contact con-
figurations of either Kanthal or nichrome wires at either 1
or 50 heat cycles (p>0.05). Additionally, no statistical dif-
ference was observed between one and 50 heat
cycles (p>0.05).
Under a wet media of 50:50 PG:VG, the coils exhibited
less visible deterioration under SEM when compared with
the dry heat cycles. Statistical comparison of new, one, and
50 heat cycles were not statistically different in a wet media.
However, consecutive loops within coils displayed varying
degrees of deterioration (Figure 6). The difference in the
two medias (wet vs. dry) was significant after 50 heat
cycles (p<0.05).
Discussion
Coil temperature studies
Online forums for e-cigarette consumers who use rebuild-
able atomizers (RBA) have created numerous posts discus-
sing whether Kanthal A-1 and nichrome creates the best
experience. Most participants on those forums will claim
that the choice between wire type is a personal one.
However, it has been highlighted by consumers that Kanthal
wire can withstand higher temperatures and thus, lasts lon-
ger than nichrome (Reddit 2015). The other coil topic fre-
quently discussed in these forums is the configuration of the
coil. Most RBA users will say that contact configuration
6 H. A. MULDER ET AL.
coils will burn hotter than a non-contact coil, and that the
non-contact coil will create a more even burn across the
wick and coil (Contact Micro Coils Vs Spaced Coils 2014).
Consumers also say that vaping with a contact coil will lead
to a ‘dry burn’or heating the wick to the point where there
is no e-liquid left to aerosolize. Further, assessing the coil
temperature outputs while dry is critical given that a model
called the rebuildable dripper atomizer (RDA) enables users
to drip an e-liquid directly onto a coil or coil/wick system
(Poklis et al. 2017; Harrell and Eissenberg 2018).
When assessing the coil temperatures heated in the
absence of a liquid, Kanthal wire reached higher tempera-
tures than nichrome and could withstand greater voltages.
Heating coils in a dry medium and with increasing voltage
and wire gauge showed that contact coils would reach a
temperature where they would melt or ‘burn out’and cease
Figure 6. Coil surface morphology - SEM images of contact and non-contact configurations of Kanthal wire coils after one (A) 50 (B) wet heating cycles and
nichrome wire coils after one (C) and 50 (C) wet heating cycles.
Figure 5. Coil surface morphology - SEM images of Kanthal wire coils new (A & B), and after 1 (C), 50 (D), and 150 (E) dry heating cycles (2000x magnification) and
nichrome wire coils new (F & G), and after 1 (H), 50 (I), and 150 (J) dry heating cycles.
INHALATION TOXICOLOGY 7
to work above a certain voltage (Figure 1). For Kanthal
wire, 34-gauge was the only wire in a contact configuration
that could not withstand the highest voltage, 5 V. For
nichrome, only 30-gauge wire, the thickest wire, could with-
stand the temperatures produced from all four voltages.
However, in a wet medium, both coils were able to with-
stand the same amount of voltage applied in both 100% PG
and 100% VG e-liquids.
In 100% VG, no significant difference between the coil
type and configuration in regards to their observed tempera-
ture outputs was observed (p¼0.198). However, in 100%
PG, the two wire types had a statistically significant differ-
ence in temperature outputs. Kanthal also showed a differ-
ence in temperature output between the two coil
configurations. Propylene glycol is less viscous than vege-
table glycerin. As it was previously mentioned, Kanthal
reaches higher temperatures in a dry medium when com-
pared to nichrome wire and a higher temperature in its con-
tact configuration over non-contact. The less viscous
properties of propylene glycol allow Kanthal coils to reach a
higher temperature than nichrome during the eight second
heating period used in this study. At these higher tempera-
tures, a greater risk of ‘dry burning’the coil in 100% PG e-
liquids exits. Some consumers who have recently transi-
tioned to e-cigarettes from traditional cigarette usage prefer
100% PG-based e-liquids, citing that the vapor produced
emulates a traditional smoking experience (Legostar 2013).
The 50:50 PG:VG ratio data for Kanthal wire showed a
similar trend in comparison to 100% VG vs 100% PG
(Figure 2). No significant difference in the temperature out-
put of the coils when compared to 100% VG was observed,
but when compared to the 100% PG medium, Kanthal tem-
perature outputs were statistically different. Similar 100%
VG and 100% PG, the difference in media viscosity influen-
ces the temperature outputs of the coils and their ability to
conduct heat.
Coil SEM analysis
SEM data gathered in this study showed a significant metal
loss for iron, chromium, and nickel in the coils after being
heated while dry. The metal loss from the coils may be pre-
sent in aerosol inhaled by the e-cigarette user. The metals
that exhibited composition changes in this study were con-
sistent with metals identified and quantified in previous
studies (Williams et al. 2013; Goniewicz et al 2014; Hess
et al. 2017; Olmedo et al. 2018). When reporting studies
about metals in e-cigarette aerosols, it is important to
include the coil specifications. The coil type and configur-
ation will have an effect on which metals are present and
the amount of metal present in the aerosol. Previous studies
have highlighted the dangers of nickel and chromium being
inhaled via e-cigarette aerosol. Nickel and chromium are
both carcinogenic and have been linked to illnesses such as
chronic bronchitis and lung cancer (Hess et al. 2017).
Dry heating a coil is a common technique used with
rebuildable atomizers. It allows the user to shape the coil
and ensure even heating across it when building. It is also
used to ‘clean’the coil of e-liquid build-up when changing
or refilling the tank. Online forums have reported that the
build-up of e-liquids primarily happens on coils that are
built in a contact configuration. This study determined that
there was a significant metal loss from newly constructed
coils in both contact and non-contact configurations after
just one dry heat cycle. Further, a significant difference in
the amount of metal loss between the contact and non-con-
tact configurations was observed. Figure 3 shows that con-
tact configurations had a greater metal loss overtime when
compared to non-contact configurations. This metal loss
and differences between wire type and configuration is sup-
ported by the temperature differences in the coil tempera-
tures when dry (Figure 2). The dry burning method for
coils has caused members of the scientific community to
caution e-cigarette users from pursuing this method, citing
that the possible metal loss from the intense heat of the dry
burn could be inhaled by the users (Farasalinos 2015).
Changes to the coil from dry heating over time showed
not only metal loss, but visual changes to the coils. SEM
images showed increased surface pitting and the formation
of a white solid, indicating oxidation. SEM images of the
coils showed that pieces of the wire were missing, possibly
due to disintegration. Oxygen levels increased with each
heat cycle and an increase of aluminum in nichrome and
Kanthal, respectively, was also observed. Kanthal and
nichrome wire in a contact configuration both showed sig-
nificant oxygen changes after the first heat cycle. Nichrome
wire, however, showed that by the 50 and 150 heat cycles,
oxygen levels did not change significantly, which was also
seen in both wire types noncontact configurations. The
apparent loss of oxygen seen in nichrome wire between 50
and 150 heat cycles may be due to the metal degrading
over time.
The increase in oxygen content is possibly due to the oxi-
dation of the coils as they are heated. Nichrome wire will
form a chromium oxide on the outer side of the coil as it is
heated and Kanthal will form an aluminum oxide coating
(Kanthal 2018b). The formation of these oxides are intended
to protect the coil from further oxidation, however, the
metal oxides can be inhaled into deep lung tissue, where
injury is likely to occur. A particle size analysis of e-cigarette
aerosol shows that in the three e-liquid compositions pre-
sented in this study (100% VG, PG, and 50:50 PG:VG),
aerosol particles were less than 5 mm in size, demonstrating
that the aerosol deposits in the deep lung tissue (Mulder
et al. 2020). Further, other studies that assessed metal aero-
sol content from e-cigarettes using Kanthal and nichrome
wire also observed aluminum and chromium, suggesting
that oxidized metals can still be inhaled by the e-cigarette
user (Olmedo et al. 2018; Williams et al. 2019).
The SEM-EDX data from wet heat cycles demonstrated
no significant difference in metal deterioration between coil
types and configurations. This supports the temperature
data which also did not change significantly between the
coil types and configuration. In the end, user preferences
are likely a reflection of subconscious behavioral effects.
8 H. A. MULDER ET AL.
Conclusions
Despite enthusiastic conversations about e-cigarette coil
types and configurations, in the presence of e-liquids, there
is no significant difference between the two types of wire
and configurations on temperature output or as observed by
SEM-EDX. Dry burning has been shown to cause significant
metal loss from the coil as early as one heat cycle and can
potentially be inhaled by the e-cigarette consumer in the
process of building or cleaning their coil. The metals identi-
fied on the coils are known to cause negative health effects
when inhaled over time. It is also important that in future
studies that report the presence and concentrations of metal
in e-cigarette aerosol include all specifications about the
atomizer, including the heating element. This is because the
metal compositions will affect the presence and abundance
of certain metals in the aerosol.
Disclosure statement
The authors have no conflicts of interest to disclose.
Funding
This project was supported by the National Institute of Justice, Office
of Justice Programs, U.S. Department of Justice [Award No. 2014-R2-
CX-K010 and 2016-DN-BX-0150] and the National Institutes of Health
[Award No. P30DA033934]. The opinions, findings, and conclusions
or recommendations expressed in this publication/program/exhibition
are those of the author(s) and do not necessarily reflect those of the
Department of Justice.
ORCID
Justin L. Poklis http://orcid.org/0000-0001-5470-5717
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INHALATION TOXICOLOGY 9
