E-cigarettes operate at higher voltage and power – resulting in higher concentrations of metals

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A team of scientists at the University of California, Riverside, has found the concentration of metals in electronic cigarette aerosols – or vapor – has increased since tank-style electronic cigarettes were introduced in 2013.

Electronic cigarettes, which consist of a battery, atomizing unit, and refill fluid, are now available in new tank-style designs, equipped with more powerful batteries and larger capacity reservoirs for storing more refill fluid.

But the high-power batteries and atomizers used in these new styles can alter the metal concentrations that transfer into the aerosol.

“These tank-style e-cigarettes operate at higher voltage and power, resulting in higher concentrations of metals, such as lead, nickel, iron, and copper, in their aerosols,” said Monique Williams, a postdoctoral researcher in the Department of Molecular, Cell, and Systems Biology, and the first author of the research paper that appears today in Scientific Reports.

“Most of the metals in e-cigarette aerosols likely come from the nichrome wire, tin solder joints, brass clamps, insulating sheaths, and wicks—components of the atomizer unit.”

The researchers examined six tank-style electronic cigarettes and found all the aerosols had metals that appeared to originate in the atomizers.

Further, they found the model with fewest metal parts in its atomizer had the fewest metals in its aerosol.

Of the 19 metals they screened, aluminum, calcium, chromium, copper, iron, lead, magnesium, nickel, silicon, tin, and zinc were from components in the atomizing units.

“Concentrations of the metals, such as lead, in the aerosols increased with more voltage,” Williams said.

“Concentrations of some elements – chromium, lead, and nickel – were high enough to be a health concern.

We found the concentrations of chromium, copper, lead, nickel, and zinc exceeded the proposed permissible exposure limit from the Occupational Safety and Health Administration.”

Chromium, lead, and nickel are known carcinogens.

Prolonged exposure to chromium could cause gastrointestinal effects, nasal and lung cancer, respiratory irritation, and lung function impairment.

Prolonged exposure to lead could produce vomiting, diarrhea, cardiovascular effects, and lung cancer.

Nickel inhalation could cause lung disease, damage to the nasal cavity, lung irritation, lung inflammation, hyperplasia in pulmonary cells, and fibrosis.

The researchers analyzed the following six tanks and their atomizers:

Kangertech Protank, Aspire Nautilus tank, Kanger T3S tank, Tsunami 2.4, Smok tank, and Clone.

They collected aerosols from these brands using two methods and found the total concentrations of metals varied, ranging from 43 to 3,138 micrograms per liter with the “impinger method” of collection and 226 to 6,767 micrograms per liter with the “cold trap method.”

“When batteries with more power are used in these tank-style e-cigarettes, their atomizing units can heat to temperatures greater than 300 C, which could produce harmful byproducts,” said Prue Talbot, a professor of cell biology, who led the research team.

“The presence of heavy metals, including some known carcinogens, in e-cigarette aerosols is concerning because with prolonged exposure they could cause adverse health effects.

“Our data on tank-style e-cigarettes and the concentrations of metals they deliver may be useful to regulatory agencies, health care providers, and consumers,” she added.


Our goal was to investigate the transfer of metals from the heating coil to the e-liquid in the e-cigarette tank and the generated aerosol.

Methods:

We sampled 56 e-cigarette devices from daily e-cigarette users and obtained samples from the refilling dispenser, aerosol, and remaining e-liquid in the tank. Aerosol liquid was collected via deposition of aerosol droplets in a series of conical pipette tips. Metals were reported as mass fractions (μg/kg) in liquids and converted to mass concentrations (mg/m3) for aerosols.

Results:

Median metal concentrations (μg/kg) were higher in samples from the aerosol and tank vs. the dispenser (all p < 0.001): 16.3 and 31.2 vs. 10.9 for Al; 8.38 and 55.4 vs.  < 0.5 for Cr; 68.4 and 233 vs. 2.03 for Ni; 14.8 and 40.2 vs. 0.476 for Pb; and 515 and 426 vs. 13.1 for Zn. Mn, Fe, Cu, Sb, and Sn were detectable in most samples.

Cd was detected in 0.0, 30.4, and 55.1% of the dispenser, aerosol, and tank samples respectively. Arsenic was detected in 10.7% of dispenser samples (median 26.7 μg/kg) and these concentrations were similar in aerosol and tank samples. Aerosol mass concentrations (mg/m3) for the detected metals spanned several orders of magnitude and exceeded current health-based limits in close to 50% or more of the samples for Cr, Mn, Ni, and Pb.

Conclusions:

Our findings indicate that e-cigarettes are a potential source of exposure to toxic metals (Cr, Ni, and Pb), and to metals that are toxic when inhaled (Mn and Zn).

Markedly higher concentrations in the aerosol and tank samples versus the dispenser demonstrate that coil contact induced e-liquid contamination. https://doi.org/10.1289/EHP2175

The use of electronic cigarettes (e-cigarettes) is increasing despite uncertainties about their toxicity and health effects (Giovenco et al. 2015McCarthy 2015Schoenborn and Gindi 2015McQueen et al. 2015Orr and Asal 2014Ambrose et al. 2014). e-Cigarettes generate nicotine and non-nicotine containing aerosols by resistance heating a solution (e-liquid) through a metallic coil (Williams et al. 2013Fuoco et al. 2014). Commonly used coils include Kanthal, made of iron, chromium, and aluminum, and Nichrome, made of nickel and chromium (Farsalinos et al. 2015). Other metals such as tin are used in the joints (Williams et al. 2015). A few studies have detected toxic metals such as chromium, nickel, and lead in e-liquid and in the aerosol produced by e-cigarettes (Williams et al. 2013Saffari et al. 2014Goniewicz et al. 2014Hess et al., 2017). Concern for metal exposure is derived from the serious health effects of metals, including neurotoxicity (Garza et al. 2006) and cardiovascular disease (Navas-Acien et al. 2007) for lead, and respiratory disease and lung cancer for chromium (chromium VI) and nickel (IARC 2012a2012bJaishankar et al. 2014).

Studies on metals in e-cigarettes have focused on cigalikes (Hess et al., 2017Mikheev et al. 2016Williams et al. 2013), which are first generation devices with the shape of conventional tobacco cigarettes.

These cigalikes contain a disposable cartomizer that contains the coil and comes preloaded with e-liquid.

Daily e-cigarette users, however, often utilize reusable modified devices, known as mods or tank-style devices, which come with a box or cylindrical-shaped battery and a mouthpiece with a tank to refill the e-liquid from a bottle dispenser (Cooper et al. 2016).

Tank-style devices are highly diverse in voltage and coil composition, as they can be assembled and manipulated by the user. Direct sampling from e-cigarette consumers rather than purchasing e-cigarettes from a store or company is thus needed to assess typically used devices.

Previous research is also lacking in comparisons between metal concentrations in e-liquid from the refilling dispenser (before contact with the device and the heating coil), e-liquid in the device itself (in contact with the heating coil), and the generated aerosol (inhaled by the user).

The goal of this study was to evaluate the potential contribution of the heating coil to metal exposure in e-cigarette users by analyzing a 15-metal panel in samples from different types of tank-style e-cigarettes collected from daily e-cigarette consumers from Maryland.

The samples included e-liquid from the refilling dispenser, the tank (after the device was used), and the generated aerosol.

We hypothesized higher metal concentrations in samples that have been in contact with the heating coil (aerosol and tank) compared with samples that have never been in contact with the coil (refilling dispenser). We also compared metal concentrations by the type of coil, device voltage, and frequency of coil change, as reported by the user.

Results

Metal Detection

Of the 15 elements analyzed, with results included in Table 1, four (As, Ti, U, and W) were excluded from further analyses shown in Tables 28 due to low detection in a majority of the samples. As, Ti, and U were detected in less than 20% of all sample types and W was detected in less than 20% of dispenser and aerosol samples. For the other 11 metals, the percentages of samples with detectable metal concentrations ranged from 0.0% for Cd to 92.9% for Zn in the dispenser samples; from 30.4% for Cd to 100% for Sn in the aerosol samples; and from 55.1% for Cd to 100% for Cr, Cu, Fe, Ni, Pb, Sn, and Zn in the tank samples.

Table 1

Number (percentage) of e-cigarette samples with detectable metal concentrations in each sample type.

MetalLOD (μg/kg)Dispenser (n = 56)Aerosol (n = 56)Tank (n = 49)
Al5.045 (80.4)55 (98.2)48 (98.0)
As1.06 (10.7)10 (17.9)6 (12.2)
Cd0.10 (0.0)17 (30.4)27 (55.1)
Cr0.526 (46.4)36 (64.3)49 (100)
Cu1.032 (57.1)46 (82.1)49 (100)
Fe5.044 (78.6)33 (58.9)49 (100)
Mn1.030 (53.6)36 (64.3)48 (98.0)
Ni1.031 (55.4)48 (85.7)49 (100)
Pb0.245 (80.4)53 (94.6)49 (100)
Sb0.117 (30.4)34 (60.7)35 (71.4)
Sn0.149 (87.5)56 (100)49 (100)
Ti5.01 (1.8)1 (1.8)4 (8.2)
U0.13 (5.4)0 (0.0)3 (6.1)
W0.14 (7.1)8 (14.3)21 (42.9)
Zn1.052 (92.9)53 (94.6)49 (100)

Note: Al, aluminum; As, arsenic; Cd, cadmium; Cr, chromium; Cu, copper; Fe, iron; LOD, limit of detection; Mn, manganese; Ni, nickel; Pb, lead; Sb, antimony; Sn, tin; Ti, titanium; U, uranium; W, tungsten; Zn zinc.

Table 2

Median (interquartile range) and limit of detection of metal concentrations (μg/kg) in e-cigarette samples from the dispenser (no previous contact with the device), the aerosol, and the tank (in contact with the device).

MetalDispenser (n = 56)Aerosol (n = 56)Tank (n = 49)
Al10.9 (7.22–20.2)16.3 (12.2–22.2)31.2 (17.5–128)
Cd < 0.1 ( < 0.1,  < 0.1) < 0.1 ( < 0.1, 0.134)0.126 ( < 0.1, 0.267)
Cr < 0.5 ( < 0.5–2.26)8.38 ( < 0.5–43.9)55.4 (17.4–217)
Cu5.14 ( < 1.0–16.1)15.1 (5.70–51.0)148 (42.0–543)
Fe26.9 (9.14–91.3)21.7 ( < 0.5–236)382 (127–1,360)
Mn1.09 ( < 1.0–2.74)2.42 ( < 1.0–9.56)31.9 (13.0–93.9)
Ni2.03 ( < 1.0–42.1)68.4 (6.19–289)233 (69.5–675)
Pb0.476 (0.243–1.05)14.8 (3.10–37.1)40.2 (13.6–189)
Sb < 0.1 ( < 0.1–0.219)0.553 ( < 0.1–1.93)0.563 ( < 0.1–2.57)
Sn1.33 (0.489–3.55)5.65 (2.38–19.4)20.3 (9.10–72.2)
Zn13.1 (6.74–23.0)515 (228–809)426 (152–1,540)

Note: Metals with  > 50% detection in at least one sample type. The number next to the symbol < corresponds to the limit of detection for each specific metal. For some samples the median, the 25th percentile and/or the 75th percentile were below the limit of detection. Al, aluminum; Cd, cadmium; Cr, chromium; Cu, copper; Fe, iron; Mn, manganese; Ni, nickel; Pb, lead; Sb, antimony; Sn, tin; Zn zinc.

Table 8

Median (range) of daily metal concentrations (mg/m3) in collected aerosol samples with regulatory and health-based limits for Ni, Cr, Pb, and Mn.

ValueNiCrPbMn
Median4.44 × 10−48.46 × 10−51.06 × 10−41.97 × 10−5
Range(4.35 × 10−6 to 1.12 × 10−1)(7.97 × 10−7 to 2.95 × 10−2)(1.49 × 10−6 to 2.75 × 10−2)(1.39 × 10−6 to 1.42 × 10−3)
Regulatory or health-based limitsa
(Percent exceeding limit [%])
2.00 × 10−4b
(57)
5.00 × 10−6c
(68)
1.50 × 10−4d
(48)
3.00 × 10−4e
(14)
 1.00 × 10−4f
(46)
1.50 × 10−3g
(11)
6.00 × 10−6h
(75)

Note: To convert results in mg/m3 to mg/puff, multiply by 6.67 × 10−5m3/puff. ATSDR, Agency for Toxic Substances and Disease Registry; Cr, chromium; Mn, manganese; MRL, minimum risk level; NAAQS, National Ambient Air Quality Standard; Ni, nickel; Pb, lead; RfC, cancer reference concentration.

aU.S. EPA NAAQS are regulatory, all other limits are health based.

bATSDR MRL for Ni (ATSDR 2005aU.S. EPA 2000a).

cMRL for Cr(VI) in mists (ATSDR 2012a). MRLs are daily averages.

dU.S. EPA NAAQS (rolling 3-month average) (U.S. EPA 2016).

eMRL for Mn (ATSDR 2012b). MRLs are daily averages.

fMRL for soluble Cr(III) (ATSDR 2012a). MRLs are daily averages.

gU.S. EPA NAAQS for non-attainment areas (U.S. EPA 2016).

hU.S. EPA RfC, daily values (U.S. EPA 2012).

Metal Concentrations

Compared with e-liquid from the dispenser, metal concentrations were higher in aerosol samples, and markedly higher in tank samples for most metals (Figure 1). For Al, Cr, and Ni, metals known to be part of the coil alloys, median concentrations increased from the dispenser sample to the aerosol and tank samples from 10.9 to 16.3, and 31.2 μg/kg respectively for Al, from  < 0.5 to 8.38, and 55.4 μg/kg respectively for Cr, and from 2.03 to 68.4, and 233 μg/kg respectively for Ni (Table 2). Metals for which the median (interquartile range) concentration increased between the dispenser and aerosol, but was similar between aerosol and tank samples, included Pb [from 0.476 (0.243, 1.05) to 14.8 (3.10, 37.1) and 40.2 (13.6, 189) μg/kg, respectively] and Zn [from 13.1 (6.74, 23.0) to 515 (228, 809) and 426 (152, 1,540) μg/kg, respectively]. In contrast, Cu, Mn, Sb, and Sn showed moderate increases in the aerosol samples, but much larger increases in the tank samples compared with dispenser samples. Cd was below the LOD in all dispenser samples and in 70% of aerosol samples, but was detected in 55% of tank samples, with a median value of 0.126 μg/kg (IQR < 0.1, 0.267) μg/kg. The median (IQR) concentrations among 22 samples with detectable arsenic were 26.7 (12.0–45.6) μg/kg for the dispenser (n = 6), 12.9 (9.33–55.2) μg/kg for the aerosol (n = 10), and 28.5 (12.6–47.6) μg/kg for the tank samples (n = 6) (data not shown).

In paired sample analyses within devices, the increases in metal concentrations in the aerosol and tank samples compared with the original e-liquid from the dispenser were all statistically significant (all p < 0.008), except for Fe in the aerosol (Table 3). The highest increases were for Zn (ratio 29.5), Pb (ratio 25.4), Ni (ratio 8.43), and Cr (6.78) in the aerosol, and for Pb (ratio 116), Cr (ratio 70.7), Ni (ratio 64.6), Cu (51.4), and Zn (36.7) in the tank. Only Cd (ratio 2.30), Al (ratio 3.79), and Sb (ratio 4.65) displayed ratios below 10 in tank compared with dispenser samples.

Table 3

Ratio (95% confidence interval) of metal concentrations in e-cigarette aerosol and tank samples compared with dispenser sample.

MetalAerosol vs. Dispenser (n = 56)Tank vs. Dispenser (n = 49)
Ratio (95% CI)p-ValueRatio (95% CI)p-Value
Al1.73 (1.27, 2.36) < 0.0013.79 (2.62, 5.50) < 0.001
Cd1.60 (1.26, 2.04) < 0.0012.30 (1.68, 3.15) < 0.001
Cr6.78 (3.46, 13.3) < 0.00170.7 (41.4, 121) < 0.001
Cu3.30 (1.54, 7.07)0.00351.4 (24.8, 106) < 0.001
Fe1.29 (0.69, 2.40)0.4117.6 (9.71, 31.9) < 0.001
Mn1.93 (1.20, 3.09)0.00719.6 (12.1, 32.0) < 0.001
Ni8.43 (3.17, 22.4) < 0.00164.6 (27.2, 153) < 0.001
Pb25.4 (14.0, 45.9) < 0.001116 (64.0, 211) < 0.001
Sb3.58 (2.26, 5.69) < 0.0014.65 (2.81, 7.71) < 0.001
Sn6.59 (4.16, 10.4) < 0.00124.2 (14.3, 40.7) < 0.001
Zn29.5 (17.4, 50.2) < 0.00136.7 (21.4, 62.7) < 0.001

Note: The ratio of the geometric mean of metal concentrations in e-cigarette aerosol and tank samples compared with the dispenser was obtained by exponentiating the corresponding mean difference (95% confidence interval) in log-transformed metal concentrations. The p-values were obtained with a paired t-test. All tests were two-sided. Al, aluminum; Cd, cadmium; CI, confidence interval; Cr, chromium; Cu, copper; Fe, iron; Mn, manganese; Ni, nickel; Pb, lead; Sb, antimony; Sn, tin; Zn zinc.

Metal Correlations

Across metals, Spearman correlations in e-liquid from the dispenser were generally low (well below 0.40) except for Al and Mn (r = 0.40), Fe and Mn (r = 0.49), Sn and Zn (r = 0.41), Mn and Zn (r = 0.43), and Ni and Cu (r = 0.69) (see Figure S1); they were higher in aerosol samples, with three correlations being above 0.70 (Cr and Fe, Cr and Mn, and Fe and Mn) and 24 above 0.40 (Figure 2A); and they were markedly higher in tank samples with 23 correlations above 0.40 and 5 above 0.80 (Figure 2B). Within-metal correlations between the dispenser and aerosol samples were statistically significant for Fe, Mn, Sb, and Sn (ranging from 0.28 for Fe to 0.42 for Sb) (Table 4); between the dispenser and tank samples, they were statistically significant for Al, Mn, and Sb (ranging between 0.29 for Al and 0.39 for Mn); and between the aerosol and tank samples, they were all statistically significant, except for Cd and Cu, and ranged between 0.37 for Mn and 0.52 for Al. For As, among the detectable samples, the within-metal correlation was 0.84, 0.97, and 0.81 between the dispenser and aerosol, dispenser and tank, and aerosol and tank samples, respectively (data not shown).

Table 4

Within-metal Spearman correlations in e-cigarette samples.

MetalDispenser vs. Aerosol (n = 56)Dispenser vs. Tank (n = 49)Aerosol vs. Tank (n = 49)
Correlationp-ValueCorrelationp-ValueCorrelationp-Value
Al0.130.330.290.0460.52 < 0.001
Cda0.170.26
Cr0.160.220.270.0640.48 < 0.001
Cu−0.140.320.200.160.190.19
Fe0.280.0380.160.280.420.003
Mn0.300.0250.390.0060.370.009
Ni−0.220.110.040.790.430.002
Pb0.230.0950.230.110.430.002
Sb0.420.0010.340.0160.440.002
Sn0.380.0040.250.0810.460.001
Zn0.250.0640.180.220.450.001

Note: The p-values were obtained from the Spearman correlation coefficient test. —, no data; Al, aluminum; Cd, cadmium; Cr, chromium; Cu, copper; Fe, iron; Mn, manganese; Ni, nickel; Pb, lead; Sb, antimony; Sn, tin; Zn zinc.

aCd was not detected in any of the dispenser samples; therefore, Dispenser vs. Aerosol and Dispenser vs. Tank correlations were not calculated.

Metal Concentrations by Voltage, Type of Coil, and Frequency of Coil Change

All metals in Table 2 are shown in these analyses except Cd and Sb, because their concentrations were below 1 μg/kg for most samples. Metal concentrations in dispenser and aerosol samples were not statistically different by voltage (Table 5). In tank samples we found statistically significant differences by voltage tertiles for Al, Fe, and Mn, with the intermediate tertile presenting the highest metal concentrations. For Ni, the difference by voltage was borderline significant (p = 0.05) with concentrations also higher at the intermediate tertile (4.00–4.40 V). When analyzed by type of coil, metal concentrations in dispenser samples were similar (Table 6). In aerosol samples, Cr, Fe, Mn, Ni, Pb, and Sn concentrations were higher in those from devices with a Kanthal coil compared with other coils. In tank samples, those from devices for which the user did not know the type of coil showed the highest concentrations for all metals. These differences of metal concentrations by type of coil were not significant (except for Cu in tank samples). There were no statistically significant differences in metal concentrations by frequency of coil change for dispenser and tank samples (Table 7). In aerosol samples, all metals were more concentrated in the aerosol from users who change the coils more than twice per month, with significant differences for Al, Cr, and Mn (Table 7). In tank samples, Al, Cr, Fe, Mn, Ni, and Sn concentrations were also higher for samples from devices for which the participants reported coil change more than twice per month.

Aerosol Metal Concentrations

Concentrations for each of the detected metals are estimated to be daily averages, and span several orders of magnitude (Table 8). We focus on Ni, Cr, Pb, Mn, and As because, due to their toxicity when found in aerosols, these compounds have health-based limit concentrations. Ni concentrations ranged from 4.35 × 10−6 to 1.12 × 10−1 (median 4.44 × 10−4) mg/m3, and 57% of e-cigarette aerosol samples exceeded the Agency for Toxic Substances Disease Registry (ATSDR 2016) daily chronic minimum risk level (MRL) for Ni of 2.00 × 10−4 mg/m3 (ATSDR 2005aU.S. EPA 2000a). Cr concentrations ranged from 7.97 × 10−7 to 2.95 × 10−2 (median 8.46 × 10−5) mg/m3. Because we did not determine the valence state of Cr in our samples, we do not know what proportion was Cr(VI) (hexavalent) and which was trivalent. If Cr in our samples were Cr(VI), 68% of the samples would exceed the daily MRL for Cr(VI) in mist (5.00 × 10−6 mg/m3), and 46% of the samples would exceed daily MRL for soluble Cr(III) (1.00 × 10−4 mg/m3) if Cr in our samples were Cr(III) (ATSDR 2012a). Pb concentrations ranged from 1.49 × 10−6 to 2.75 × 10−2 (median 1.06 × 10−4) mg/m3, with 48% of aerosol samples exceeding the U.S. EPA National Ambient Air Quality Standard (NAAQS) (U.S. EPA 2016) of 1.50 × 10−4 mg/m3 and 11% exceeding the standard in nonattainment areas of 1.50 × 10−3 mg/m3. Mn concentrations ranged from 1.39 × 10−6 to 1.42 × 10−3 (median 1.97 × 10−5) mg/m3; 14% of samples exceeded the daily Mn MRL of 3.00 × 10−4 mg/m3 (ATSDR 2012b) and 75% exceeded the U.S. EPA daily cancer reference concentration (RfC) of 6.00 × 10−6 mg/m3 (U.S. EPA 2012). Arsenic concentrations, calculated only among the 10 aerosol samples (17.9%) with detectable arsenic (data not shown) ranged from 7.72 × 10−6 to 1.04 × 10−3 (median 1.50 × 10−4) mg/m3. All other metals investigated were also found in concentrations spanning three to four orders of magnitude (Figure 1) in the condensed aerosol, which would translate to several orders of magnitude in the air using Equation 1.


More information: Monique Williams et al. Effects of Model, Method of Collection, and Topography on Chemical Elements and Metals in the Aerosol of Tank-Style Electronic Cigarettes, Scientific Reports (2019). DOI: 10.1038/s41598-019-50441-4

Journal information: Scientific Reports
Provided by University of California – Riverside

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