A Critical Review of Recent Literature on Metal Contents in E-Cigarette Aerosol

Abstract

The inhalation of metallic compounds in e-cigarette (EC) aerosol emissions presents legitimate concerns of potential harms for users. We provide a critical review of laboratory studies published after 2017 on metal contents in EC aerosol, focusing on the consistency between their experimental design, real life device usage and appropriate evaluation of exposure risks. All experiments reporting levels above toxicological markers for some metals (e.g., nickel, lead, copper, manganese) exhibited the following experimental flaws: (i) high powered sub-ohm tank devices tested by means of puffing protocols whose airflows and puff volumes are conceived and appropriate for low powered devices; this testing necessarily involves overheating conditions that favor the production of toxicants and generate aerosols that are likely repellent to human users; (ii) miscalculation of exposure levels from experimental outcomes; (iii) pods and tank devices acquired months and years before the experiments, so that corrosion effects cannot be ruled out; (iv) failure to disclose important information on the characteristics of pods and tank devices, on the experimental methodology and on the resulting outcomes, thus hindering the interpretation of results and the possibility of replication. In general, low powered devices tested without these shortcomings produced metal exposure levels well below strict reference toxicological markers. We believe this review provides useful guidelines for a more objective risk assessment of EC aerosol emissions and signals the necessity to upgrade current laboratory testing standards.

Critical Review of the Recent Literature on Organic Byproducts in E-Cigarette Aerosol Emissions

“…While the overwhelming majority of the 32 reviewed studies tested the low-powered devices that were of common usage before 2017 (first and second generation ciga-likes and clearomizers), all of which are today either obsolete or of marginal use, the methodological critique by Farsalinos and Gillman is still relevant in assessing present day lack of proper standards in laboratory testing. As we argue in Sections 2-4 (and in our recent review of metal studies [5]), unrealistic testing, overheating and possible dry hits can also occur in emission studies testing devices available today (specially, but not only, third generation sub-ohm models).…”

“…The review contains 92 articles selected through the PRISMA search protocol. All studies we are revising in the present review (and in our review on metals in [5]) are also cited by Ward et al…”

“…In a previous paper [5], we reviewed 12 studies targeting metal content in EC emissions. We found that all studies reporting high metal levels (e.g., nickel, lead, chromium and manganese) surpassing toxicological markers suffered from serious methodological shortcomings, especially (but not only) testing high-powered sub-ohm devices at high wattages with the puffing parameters of the CORESTA Method 81 [6,7]) or slight variations of it (i.e., "CORESTA-like").…”

“…Pending on the wattage range, the combination of high power with low airflow rate and puff volume is either prone or certain to lead to a device testing under overheating conditions. It is also unrepresentative of consumer usage, as sub-ohm devices are mostly used and widely recommended (by manufacturers, vaping magazines and forums) for the 'direct to lung' (DTL) vaping style that involves much larger airflows and puff volumes [8,9] (see also [5]).…”

“…To assess current laboratory testing of EC emissions, we apply in the present review a critical analysis of experimental methodology analogous to the one undertaken in [5] but now focusing on laboratory studies detecting nonmetallic byproducts. We provide an extensive review of a literature consisting of 38 articles published since 2018, listed and classified by subject in Table 1 below:…”