Levels of carbonyl compounds in e-cigarette aerosols are mainly depends on the e-liquid solvent and battery output voltage. Both VG and PG could undergo decomposition at high temperatures to generate low molecular weight carbonyl compounds with established toxic properties, including formaldehyde, acetaldehyde, acrolein, and acetone (Kosmider et al., 2014). CS is the primary source of exposure to several VOC carcinogens and toxicants, whereas the VOCs in e-cigarette aerosols are rarely reported (Marco et al., 2015). TSNAs are the most prevalent strong carcinogens in smokeless tobacco products and CS, and widely believed to play a significant role as causes of oral cancer in people who use these products (Stepanov et al., 2005). PAHs are also a class of environmental pollutants created primarily from incomplete combustion of various organic materials including tobacco products, which is one significant source of PAHs exposure for smokers and secondhand smokers (Ding et al., 2005). The metals in CS are mainly from the pollution of the growing environment, processing and transportation process. The heating element of e-cigarettes is usually a metallic coil, which always contains Kanthal, made of iron, chromium, and aluminum, and Nichrome, made of nickel and chromium (Olmedo et al, 2018). Thus, the detection of these toxic metals such as chromium, nickel, and lead are very necessary for the risk monitoring of e-cigarettes.
Public Health England reported that e-cigarettes were likely to be 95% less harmful than tobacco products and could be a means for smokers to quit or reduce their consumption of cigarettes (McNeill et al., 2015), based on the evidence of the composition and quantities of constituents found in e-cigarette aerosols and the relative risk associated with the main components. Although some studies have shown that a few adverse potential effects of e-cigarettes on cell models can be shown in vitro (Romagna et al., 2013; Humidah et al., 2018), and low levels of chronic irritation of the respiratory tract also can be anticipated at certain levels of vaping (Czekala et al., 2019), but the negative effects are much less than those of conventional cigarettes (Nutt et al., 2014).
In present study, the selected HPHCs in the two commercially available e-cigarette aerosols and a typically traditional CS from Chinese market were systematic comparatively tested, including eight carbonyls (formaldehyde, acetaldehyde, acrolein, propionaldehyde, butenal, butyraldehyde, acetone, and butanone), five VOCs (1,3-butadiene, acrylonitrile, isoprene, benzene, and methylbenzene), four TSNAs (NNN, NNK, NAB, and NAT), sixteen PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-c,d]pyrene, di-benzo[a,h]anthracene, and benzo[g,h,i]perylene), and fifteen metal elements (Al, As, Cd, Cr, Cu, Fe, Zn Mn, Ni, Pb, Sb, Sn, Hg, Co, and Se). A comparative study was also deduced to evaluate the toxicity of TPM extracts (ECM, ECT. and CS) by an in vitro model using one immune cell line Jukart, and three tumor cell lines (CNE-1, HONE1, Hep-2).
The carbonyls assaying indicated that the carbonyl compounds in aerosols of ECM and ECT were at the nanogram level per puff, which were far below the limited safety value. However, most of the toxic carbonyls in CS were at the microgram level (Table 1). None of the tested VOCs, TSNAs and PAHs had been detected in ECM and ECT aerosols at the current instrument detection limits, whereas most of the VOCs, TSNAs and PAHs in CS were at high levels (Tables 1–2). The concentrations of As, Cd, Sb, and Pb in CS were also much higher than those in e-cigarette aerosols, especially for Cd and Pb, which were ranked in the seventh and second place in the priority list of dangerous substances published by the US Agency for Toxic Substances and Disease Registry (U.S. Public and Health Service, 1992). The data also indicated that almost all the metals in e-cigarette aerosols were less than one percent of the limit safety value except for Ni, which was close to a quarter of the limit in ECM aerosol and could be mainly from the metallic coil in heating element (Table 3).
The potential cytotoxicity evaluation of TPM extracts of two e-cigarette aerosols and CS on cell levels also exhibited the significant differences. As shown in Fig. 1, the mild cytotoxicity could be found in the two e-cigarette groups at dose of 100 µg/mL, which were significantly weaker compared to CS. The effect of the TPM extracts on cell colony formation also certified that the CS exhibited intense inhibition on the four cell lines (Fig. 2). What’s more, the inhibition of the ECT and ECM on cell colony formation were also much weaker than CS at the same dose.