Teneurs en métaux dans les liquides des cigarettes électroniques: potentiel de relargage des métaux à partir des éléments chauffants et évaluation de l'exposition

Smoking is still the main reason for the risk of disability and premature death in Canada. Nowadays, various smoking cessation methods, such as pharmacological treatments and acupuncture, are offered. Unfortunately, these have a low success rate either because they are expensive or not entirely satisfactory, hampering quitting for smokers. Electronic cigarettes (ECs) were introduced in the American market in 2007 as alternative devices to conventional cigarettes. The traditional cigarette combustion process involves burning tobacco, while ECs generate aerosols obtained from liquids containing nicotine, propylene glycol, vegetable glycerine and flavoring (e-liquids). Today, ECs are well accepted among adults looking to leave the conventional cigarette. However, even though it’s accepted that vaping is much less harmful than smoking, there is not enough information available about the potential health risks associated with the long-term use of ECs. It’s known that the heating elements of atomizers in e-cigarettes can contribute to the presence of various metal analytes in e-liquids (e.g., nickel, chromium), which reach the aerosols inhaled by EC users. However, the mechanisms for releasing metals into e-liquids and the subsequent human exposure magnitude to metals are still poorly understood. To rigorously assess the process of releasing metals into e-liquids, metal analyte concentrations were measured in the e-liquids before and after coming into contact with the heating elements. The influence of specific parameters was considered, such as pH, nicotine concentrations, the flavoring role, the molecular form of nicotine (free-base or nicotine salt), the ratio of the organic phase between propylene glycol (PG) and vegetable glycerin (VG) and the composition of the heating elements. In addition, a screening level risk characterization of metals contained in the liquid and the aerosol phases was carried out to determine if the vapers were exposed to concentrations of metals higher than the reference values (e.g., minimal risk levels, MRL, etc.) The target metals were cadmium (Cd), chromium (Cr), nickel (Ni), and lead (Pb) due to their potential toxicity by the inhalation route and their carcinogenic effect in humans. Inductively coupled plasma mass spectrometry (ICP-MS) was used to measure metal concentrations after treatment of samples by acid digestion. The metal content of 17 e-liquids from refill bottles sold in Canada and in five coils widely used among vapers (Nichrome 80, Kanthal A-1, Stainless steel 317L and disposable coils such as Juul and Aspire BVC) was determined. Metal levels expressed as mean (standard deviation) metal concentration, were below detection (Cd) to very low in bottle samples (Ni ≤ 76(18); Pb ≤ 16(1.5); and Cr ≤ 386(15.6) µg/kg). Regarding metal concentrations in coils (mg/kg), values were in the range of Cr = 4.3E+04 – 2.1E+05, Ni = 2.0E+03 – 9.6E+05, Pb = 1.5E-02 – 4.1 et Cd = 0.6 – 32.4. Based on their properties, 12 e-liquids were selected for the metal leaching tests with the five heating coils. The metal leaching experiment lasted three days, and it consisted of putting each coil in contact with each e-liquid in a temperature range of 200-250 °C for 1 h, according to certain vaping parameters (e.g., the volume of e-liquid consumed per day, number of inhalations per day, etc.) ICP-MS analysis showed the increasing metal concentrations as a trend due to the interaction with the coils. In addition, it was also observed that using the same e-liquid, the metal leaching concentrations varied depending on the coil. Cr and Ni were constantly present, followed by Pb to a lesser extent. While Cd was below DL for most samples. Coil + e-liquid combinations with the highest concentrations of Ni, Cr, and Pb, expressed as mean (SD), were: Aspire BVC + Melon 0 mg/mL: Ni = 1.22E+04(281); Aspire BVC + Hit Nicotine 40 mg/mL: Cr = 864(116); and Nichrome 80 + Melon 0 mg/mL: Pb = 56(5) µg/kg. Data from this test reported that nicotine salts at 40 mg/mL may enhance Cr and Ni leaching for some coils. For example, Hit Nicotine 40 mg/mL with Aspire BVC leached the highest concentrations of Cr (864(116 µg/kg)) and Ni (3.39E+03(4.1E+01 µg/kg)). Stainless steel 317L released very low metal concentrations. The non-parametric Wilcoxon signed rank test showed that leached Ni concentrations were significantly different (p < 0.05) between Aspire BVC (p = 0.001, n = 11), Juul (p = 0.001, n = 11), and Nichrome 80 (p = 0.007, n = 11) each compared to stainless steel 317L, except for Kanthal A-1 (p = 0.1, n = 11). Aspire BVC released the highest Ni concentrations, but the lowest concentrations were released by stainless steel 317L. Finally, a conservative screening level risk characterization using reference values for inhalation with non-cancer health effects (e.g., MRL, PEL, etc.) and considering 80% absorption of metals, 500 inhalations per day, and a daily volume of inhaled air equal to 20 m3 was conducted. The safe concentration (SC) of Cr, Ni, Pb, and Cd (mg/mL) were established in the liquid phase as threshold values. All metal concentrations in the refill bottles were always below the SCs. Among the metal leaching samples, Juul and Aspire BVC with Hit nicotine 40mg/mL were equal or above SC for Cr (III) (7.5E-04 mg/mL). They represented only 3.5% of the total (2 out of 57 samples). For Ni, 8.8% of samples (5 out of 57 samples) exceeded the SCsNi for intermediate (1.5E-03 mg/mL) and 10.5% of samples (6 out of 57 samples) exceeded SC for chronic exposure (6.7E-04 mg/mL). In the aerosol phase, the estimated daily dose (EDDILV) (mg/day) was calculated for Cr, Ni, and Pb, to establish limit values assuming a fraction of metals passage from the coil’s extracts to the aerosol (Cr = 15%, Ni= 30% and Pb= 40%). For intermediate exposure, only 1.8% of samples (1 out of 57 samples) exceeded EDDNi (4.0E-03mg/day), and 8.8% of samples (5 out of 57 samples) exceeded EDDNi for chronic exposure (1.8E-03 mg/day). Further studies regarding the different mechanism in metal leaching produced by the coils are needed, this could help to set regulations on the manufacturing of such essential parts of EC devices. Moreover, further research is required to carry out a more detailed risk assessment following exposure to metals (particularly Cr and Ni) from e-cigarettes.