Particle size distributions according to various operating conditions

Coil resistance

The particle size distributions are documented in Fig. 2 and Table 1. Size distributions of generated particles were log-normally distributed with a GSD ranging from 1.2 to 1.6, depending upon the coil resistance. As the resistance of the test coils increased, the GMD of the particles, produced due to condensation of vaporized metal from the Kanthal A1 coil, increased from 40 nm, at 0.1 Ω, to 52 nm, at 1.0 Ω, 1 min into the sampling period (Fig. 2a). The GMD of the particles exhibited an approximate 10% increase in size as the coil resistance increased from 0.1 to 0.3 Ω. The GMD of generated particles then remained stable from 0.3 to 0.5 Ω. As the resistance of the coil increased from 0.5 to 1.0 Ω, the GMD increased approximated of 27%. Stabilization of GMD occurred approximately 10 min into the sampling period (Fig. 2c). The GMD was relatively stable throughout the sampling period at a resistance of 0.1 Ω. At higher resistances, the GMD increased 10 to 15 min into the sampling period before stabilizing.

Fig. 2 Particle size distributions by resistance a at 1 min and b at 3 min. c Geometric mean diameters and d total number concentrations over sampling time Full size image

Table 1 Particle size distribution and total number concentration under various operating conditions Full size table

The concentration of generated particles also varied by coil resistance. The average TNC generated over the sampling period increased over seven fold as the coil resistance was increased from 0.1 to 1.0 Ω. The magnitude of increase in TNC was not directly correlated with the level of increase in coil resistance (Fig. 2a). The TNC of particles showed an initial large increase of two and half fold from a 0.1 Ω resistance to a 0.3 Ω resistance setting. A more modest increase in TNC of 50% occurred from an increase in coil resistance from 0.3 to 0.5 Ω. As the resistance was increased from 0.5 to 1.0 Ω, the resulting TNC nearly doubled. The TNC exhibited a rapid rate of decay over the sampling period, decreasing to half the concentration every 3.6 min.

Applied power

Size distributions of generated particles were log-normally distributed with a GSD ranging from 1.2 up to 1.6, reached at the end of the sampling period of the highest power setting (Fig. 3a and Table 1). The GMD of generated particles increased from 35 to 55 nm, 1 min into the sampling period, as the applied power was increased from 10 to 30 W. This effect of a large increase in GMD with an increase of power did not hold for additional power settings. As the applied power was increased in 20 W increments, from 30 to 70 W, the GMD decreased from 30 to 50 W, and increased slightly from 50 to 70 W. The GMD exhibited relative stability throughout the sampling period from 10 to 70 W.

Fig. 3 Particle size distributions by applied power a at 1 min and b at 3 min. c Geometric mean diameters and d total number concentrations over sampling time Full size image

The concentration of generated particles showed an increase of over seven fold over the various power levels. An initial large increase of seven and a half fold in the TNC was observed from 10 to 30 W of applied power (Fig. 3d). The TNC remained stable with an increase in power to 50 W, decreased slightly by 6% as power increased to 70 W of applied power. The TNC showed a rapid decay rate in which the concentration decreased by one-half every 3.6 min. A study by Olmedo et al. (2018) investigated the effects of varying voltage applied to e-cigarette coils produced somewhat comparable results. Aerosol concentrations of Fe increased over 10 fold from the low voltage to intermediate voltage setting, but stabilized with a high voltage setting. Ni showed a similar trend, but less dramatic increase in concentration.

Duty cycle

A doubling of the duty cycle (5 to 10%) resulted in an increase in the GMD of the particles from 35 to 38 nm 1 min into the sampling period (Fig. 4). The GMD exhibited a larger increase, from 38 to 46 nm, as the duty cycle progressed from 10 to a 15% duty cycle. A further increase in the duty cycle time, from 15 to 50%, resulted in a modest decline in the GMD from 46 to 41 nm. The GMD was relatively stable throughout the sampling period with the 5% duty cycle. The GMD for the 10% and 15% duty cycle initially increased through the first 20 min of the sampling period, after which the GMD began to showing a decreasing trend.

Fig. 4 Particle size distributions by duty cycle a at 1 min and b at 3 min. c Geometric mean diameters and d total number concentrations over sampling time Full size image

The GMD showed a progressive increase throughout the sampling period for the 50% duty cycle (Fig. 4c). The TNC of particles increased eight and half fold as the duty cycle was increased from 5 to 10% (Fig. 4d). A further increase in duty cycle from 10 to 15% resulted in a more modest 14% increase in the TNC. The TNC remained stable as the duty cycle was increased from 15 to 50%, exhibiting a small 4% increase. A rapid rate of decline in TNC was noted over the sampling period, with the concentration decreasing by one-half each 3.5 min.

Composition analysis, TEM images, and EDX maps

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.

Fig. 5 TEM and EDX map images of sampled particles (these maps are for Fe, O, Al, Si, Mn, and Cr) Full size image

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.

Table 2 Metal contents in Kanthal wire and sampled particle Full size table

Discussion and limitations

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.