Metal Concentrations in e-Cigarette Liquid and Aerosol Samples: The Contribution of Metallic Coils

Abstract

Background: Electronic cigarettes (e-cigarettes) generate an aerosol by heating a solution (e-liquid) with a metallic coil. Whether metals are transferred from the coil to the aerosol is unknown.

Objective: 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 26.7μg/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.

Figure 1.

Boxplots of metal concentrations in e-cigarette dispenser, aerosol, and tank samples. The dispenser sample has not had any contact with the e-cigarette device. The horizontal lines within boxes indicate medians; boxes, interquartile ranges; whiskers, values within 1.5 times the interquartile range from boxes; solid circles outside the boxes, outlier data values. Table 2 lists the raw data for all metals represented in this figure. All metals in Table 2 are represented in this figure except Cd and Sb, as their concentrations were below $1\mathrm{\mu }\mathrm{g}/\mathrm{kg}$ for most samples. Note: For samples with $\ge 25\%$ of the samples below the limit of detection, the minimum and the percentile 25th values are the same and therefore the lower whisker is missing.

Figure 2.

Correlations between metals in samples from e-cigarette devices: (A) aerosol samples, and (B) tank samples. All metals shown in Figure 1 are shown here. The diagonal panel shows the histograms of the ${\log }_{10}$-transformed distribution of each metal. The upper part of the panel represents the Spearman pairwise correlation coefficients between metals. The axes indicate the ${\log }_{10}$ metal concentrations values that are represented in the histograms. Correlations $\ge 0.50$ are bolded.

Figure 2.

Correlations between metals in samples from e-cigarette devices: (A) aerosol samples, and (B) tank samples. All metals shown in Figure 1 are shown here. The diagonal panel shows the histograms of the ${\log }_{10}$-transformed distribution of each metal. The upper part of the panel represents the Spearman pairwise correlation coefficients between metals. The axes indicate the ${\log }_{10}$ metal concentrations values that are represented in the histograms. Correlations $\ge 0.50$ are bolded.