Metal/Metalloid Levels in Electronic Cigarette Liquids, Aerosols, and Human Biosamples: A Systematic Review

Abstract

BACKGROUND: Electronic cigarettes (e-cigarettes) have become popular, in part because they are perceived as a safer alternative to tobacco cigarettes. An increasing number of studies, however, have found toxic metals/metalloids in e-cigarette emissions. OBJECTIVE: We summarized the evidence on metal/metalloid levels in e-cigarette liquid (e-liquid), aerosols, and biosamples of e-cigarette users across e-cigarette device systems to evaluate metal/metalloid exposure levels for e-cigarette users and the potential implications on health outcomes. METHODS: We searched PubMed/TOXLINE, Embase ® , and Web of Science for studies on metals/metalloids in e-liquid, e-cigarette aerosols, and biosamples of e-cigarette users. For metal/metalloid levels in e-liquid and aerosol samples, we collected the mean and standard deviation (SD) if these values were reported, derived mean and SD by using automated software to infer them if data were reported in a figure, or calculated the overall mean (mean ± SD) if data were reported only for separate groups. Metal/metalloid levels in e-liquids and aerosols were converted and reported in micrograms per kilogram and nanograms per puff, respectively, for easy comparison. RESULTS: We identified 24 studies on metals/metalloids in e-liquid, e-cigarette aerosols, and human biosamples of e-cigarette users. Metal/metalloid levels, including aluminum, antimony, arsenic, cadmium, cobalt, chromium, copper, iron, lead, manganese, nickel, selenium, tin, and zinc, were present in e-cigarette samples in the studies reviewed. Twelve studies reported metal/metalloid levels in e-liquids (bottles, cartridges, open wick, and tank), 12 studies reported metal/metalloid levels in e-cigarette aerosols (from cig-a-like and tank devices), and 4 studies reported metal/metalloid levels in human biosamples (urine, saliva, serum, and blood) of e-cigarette users. Metal/metalloid levels showed substantial heterogeneity depending on sample type, source of e-liquid, and device type. Metal/metalloid levels in e-liquid from cartridges or tank/open wicks were higher than those from bottles, possibly due to coil contact. Most metal/metalloid levels found in biosamples of e-cigarette users were similar or higher than levels found in biosamples of conventional cigarette users, and even higher than those found in biosamples of cigar users. CONCLUSION: E-cigarettes are a potential source of exposure to metals/metalloids. Differences in collection methods and puffing regimes likely contribute to the variability in metal/metalloid levels across studies, making comparison across studies difficult. Standardized protocols for the quantification of metal/metalloid levels from e-cigarette samples are needed. https://doi.

Chemical Elements in Electronic Cigarette Solvents and Aerosols Inhibit Mitochondrial Reductases and Induce Oxidative Stress

“…The literature on this has been thoroughly reviewed recently. 28 Atomizing units in EC have metal components that pass into aerosols upon heating. 9 , 11 , 12 However, relatively little is known about the contribution of EC fluids to the elements in aerosols.…”

“…14 Element transfer efficiencies were higher in clearomizer/mod devices than in cartomizers, in agreement with other studies using second-/third-generation ECs. 28 Transfer efficiency varied among elements, with tin and selenium transferring efficiently in most products including cartomizers. Iron, zinc, copper, chromium, lead, and nickel, which accumulated in fluid during use, also transferred to the aerosols of clearomizer/mods.…”

“…Most work on EC elements has involved their identification and quantification. 28 While the toxicity of the individual elements in EC has been studied extensively in other contexts, their toxicity in EC users has not been well characterized. As a preliminary step to understanding toxicity of the major elements in EC aerosols, we conducted several experiments using mitochondrial (MTT) and oxidative stress assays.…”

“…However, controlled combinatorial testing at the ALI would be possible in future studies using information in this and other recent publications. 21 , 22 , 28 , 46 …”

“…Third, while there are numerous publications on elements in EC aerosol, 28 very little is known about their speciation. However, speciation is important as it affects toxicity.…”