Characteristics of metallic nanoparticles emitted from heated Kanthal e-cigarette coils

As shown in Fig. 1, the experimental setup consisted of an air supply system, an e-cigarette generation system, and a measurement system. The air supply system was composed of an oil trap, a diffusion dryer, and a high-efficiency particulate air (HEPA) filter to remove oil contaminants, humidity, and particles, respectively. The dry, particle-free air, controlled with a mass flow controller (MC-10SLPM-TFT, Alicat Scientific, Inc., USA), was delivered to a glass chamber at the flow rate of 5 L/min to create sheath air surrounding atomizer. The e-cigarette generation system consisted of an atomizer, a controller, a diaphragm pump, and the glass chamber. Kanthal A1 coil (diameter of 0.405 mm (26 Ga.), inner diameter of 3 mm) was installed in the atomizer. A box modified (MOD) type controller was used in the system. The diaphragm pump supplied the air from the atomizer to the sampling chamber (9 L, 0.18 m × 0.18 m × 0.28 m) at the flow rate of 2.5 L/min.

The size distributions of the particles emitted from the atomizer were measured using a scanning mobility particle sizer (SMPS; 3938, TSI Inc., USA), consisting of a classifier controller (3082, TSI Inc., USA), a differential mobility analyzer (DMA; 3081A, TSI Inc. USA), a condensation particle counter (CPC; 3776, TSI Inc., USA), and a soft X-ray aerosol neutralizer (3088; TSI Inc., USA). The size distributions of emitted particles were measured every 1 min during a 30-min sampling run. Geometric mean diameter (GMD), geometric standard deviation (GSD), and total number concentration (TNC) were calculated using an aerosol instrument manager (Version 10.2.0.11, TSI Inc., USA).

In order to examine the morphology and chemical composition of the generated particles, a sample was collected onto a transmission electron microscope (TEM) grid (300-mesh, Au grid, Ultrathin Carbon Film on Lacey Carbon Support Film, 01824G, Ted Pella, Inc., USA) using an ESPnano (Model 100, DASH Connector Technology, Inc., USA). Morphology of collected particle was examined using a TEM (Talos 200X, ThermoFisher Scientific, USA). Elemental composition of particles was obtained using an energy dispersive X-ray spectroscopy (Talos 200X, ThermoFisher Scientific, USA). The spectral peaks obtained were compared against standard peaks for the elements of interest.

Test settings were adjusted to mimic various operation conditions. In the first test condition, the atomizer was turned on and off at a consistent rate (0.1 Hz frequency and 50% duty cycle), with 10 W of applied power, and tested with different coil resistance settings (0.1, 0.3, 0.5, and 1.0 Ω). The second test condition involved applying increasing amounts of power to the coil, starting with 10 W and increasing the power in increments of 20 W, with a 5% duty cycle and a resistance of 0.5 Ω. The final test condition involved varying the duty cycle (5%, 10%, 15%, and 50%), with a consistent resistance of 0.5 Ω and applied power of 10 W. Each test condition ran for a period of 30 min. Three trials were conducted for each test condition, with the average of the trials used in the data analysis. A new coil was tested for each trial.

Test Kanthal A1 coil material was analyzed using an inductively coupled plasma (ICP)–mass spectrometry (MS). Test coil was digested using a Discover SP-D Microwave Digester (CEM, NC, USA) and diluted 45 times to get a final HNO₃ concentration of 2% (v/v). An ICAP-Q ICP-MS (Thermo Fisher Scientific, MA USA) paired with an ESI SC-4DXS Autosampler (Elemental Scientific, NE, USA) was used to measure 39 elements (Ag, Al, As, B, Ba, Be, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Hg, Ho, La, Lu, Mn, Nd, Ni, Pb, Pr, Rb, Se, Sm, Sr, Th, Ti, Tl, Tm, U, V, Yb, and Zn). All samples were run in kinetic energy discrimination (KED) mode, using high purity helium (99.999%) as the collision gas. Online internal standard addition was performed using 50 μg/L scandium and an SC FAST Valve (Elemental Scientific, NE, USA). The ICP-MS was run with a dwell time of 0.01 s for all analytes, with 25 sweeps and three main runs per sample.

The first image in Fig. 5 illustrates the shape of the particle generated from a resistance of 0.5 Ω at the applied power of 10 W. The collected particle appears as aggregated formed from much smaller primary particles. However, it was difficult to identify individual primary particles since they were coagulated and then might be sintered (partially melted) by heat from coil.

The TEM image of the particle is combined with EDX maps. These maps correspond to Fe, O, Al, Si, Mn, and Cr, respectively. The dots in these images indicate the positions of each element in the first image. For example, Fe and O are concentrated in the regions corresponding to the particles in the first image, which shows that the particles contain Fe and O. The composition of the collected particle was dominated by Fe and O as documented in the Table 2. The fractions of Fe and O were 0.56 and 0.40 in mass, respectively. Fe oxide is an expected finding, as filtered air rather than an inert gas was used as the carrier gas, in which case metallic particles would have been generated (Khan et al. 2014). Analysis of the coil material revealed that the Kanthal wire is primarily comprised of Fe and Al, along with lesser amount of Cr and Mn (Table 2). However, the Al content of generated particles was much less than the content of the Kanthal coil.

Metallic nanoparticles were generated from e-cigarette coils in the range of 32 to 57 nm, depending upon the test operating conditions. Large increases in TNC were also noted with increases resistance, applied power, and duty cycle. The pattern on TNC increase, however, differed by operating conditions. Coil resistance exhibited consistent increases in TNC with increasing wire resistance, while applied power showed an initial large increase in TNC, followed by a stabilization of concentration with further increases in power. Duty cycle showed a pattern intermediate of resistance and power, with increases of TNC with the first two changes in duty cycle length, followed by a stabilization.

Nanoparticles generated from e-cigarette coils may exert adverse health effects on individuals who inhale aerosols from the e-cigarettes. These nanoparticles are able to penetrate into the interstitial space and elicit an inflammatory response (Oberdörster et al. 1994; Srinivas et al. 2012). Nanoparticles also translocate from the lungs to the lymphatic system, where they accumulate in the tracheobronchial lymph nodes (Srinivas et al. 2012; Takenaka et al. 1986), and the circulatory system (Takenaka et al. 1986; Miller et al. 2013), where they accumulate at the site of atherosclerotic plaque build-up (Miller et al. 2013). However, it is difficult to conclude that nanoparticles produced from e-cigarette coils can cause severe toxic effects because of limited information. To consider the potential toxicity of metallic nanoparticles in humans, more detailed information such as mass based dose, chemical form, solubility, and surface area of particles should be analyzed. After characterization of nanoparticles, further research is needed to assess the effects of inhaled nanoparticles from e-cigarettes on the respiratory and cardiovascular systems of e-cigarette users and others who may be exposed to e-cigarette emissions.

We developed and validated our e-cigarette generation system by characterizing the size and number of nanoparticles produced and their chemical compositions. One finding of interest was the steep decrease in the TNC over the course of the sampling period. This effect was consistent across the various operating conditions (power, resistance, and duty cycle). A possible explanation for this observation may be the formation of a metal oxide layer on the surface of the heating coil (Boggs 1971; Prescott and Graham 1992; Sauer et al. 1982; Sundberg et al. 2004). When Al alloys undergo oxidation, they develop layers of protective alumina on the surface (Sundberg et al. 2004; Boggs 1971). Any breach in this layer allows for oxygen to contact the metal underneath that layer, resulting in the formation of Fe oxide (Boggs 1971). As this layer is formed, further metal evaporation, and particle generation, would be decreased. Utilization of an inert carrier gas, such as argon, would prevent the phenomenon from occurring, resulting in a stable production of particles over an extended period of time (Peineke et al. 2006). The rate at which oxidation occurs is dependent upon several factors. The amount of Al in the alloy is one of these factors. The oxidation rate of the material increases with a greater percentage of Al present, up to a certain amount, after which increasing Al content results in decreasing oxidation rate, as the alloy contains enough Al to resist Fe nodule formation (Boggs 1971; Prescott and Graham 1992). Exposure to high temperature (over 1,000 °C) will result in an increased rate of oxidation (Sauer et al. 1982; Boggs 1971; Prescott and Graham 1992). Oxidation also proceeds more rapidly with water vapor present in the atmosphere (Boggs 1971). In order to reduce exposure to metallic nanoparticles from an e-cigarette system, we recommend utilizing used or ceramic-coated metal coil, which will generate fewer particles than a new coil.

The results of this study need to be viewed in the presence of several limitations. Particles were generated using a modified e-cigarette system, under controlled conditions. The air supplied to the system was filtered and dehumidified. Since humidity affects the oxidation rate of the metal, e-cigarette use under normal operating conditions may exhibit a different particle generation rate. Additionally, the duty cycle set for the e-cigarette system in this study may not correspond with actual e-cigarette usage patterns. Only one type of coil was used in this study. Future studies should investigate the particle generation characteristics of other materials used in e-cigarette coils.

The e-cigarette test was also conducted without the nicotine solution and the wick used in an e-cigarette system. The metallic nanoparticles generated from the coil may act as a seed for the growth of fumes from e-cigarette solution and result in differences in the diameter of particles. The possibility also exists for particles to be generated from sources other than the e-cigarette coils. Additional sources of particles, generated from heating of the circuitry and housing material of the e-cigarette system, may also contribute to the concentration of particles measured.

During the experiments, particles were generated intermittently based on duty cycle time, rather than a continuous process. For sampling steady-state particles, a sampling chamber was connected to the atomizer chamber (Fig. 1). The size of the nanoparticles generated may be increased due to coagulation occurring in the sampling chamber. Smaller particles would be expected if sampling occurred closer to the source of generation (Khan et al. 2014).

The test conditions of the e-cigarette generating system may not be representative of actual use patterns of an e-cigarette. The duration of a puff from an e-cigarette may vary by use, with inexperienced users taking shorter puff on the order of 2 s and experienced users taking longer puff of an average 4 s (Talih et al. 2015). Generally, the metal coil is surrounded by a wick and immersed in a nicotine solution. Exposure of the metal coil to the atmosphere, however, may occur once the reservoir containing the nicotine solution runs dry.