Metal concentration in e-vapor is comparable to atmospheric air, except for Ni
Researchers at Lincoln Memorial University (Harrogate, TN, USA) and at William Carey University (Hattiesburg, MS, USA) carried out a comparative analysis of trace metals in e-cigarette aerosol and cigarette smoke.
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The contents of Al (aluminium), As (arsenic), Cd (cadmium), Cu (copper), Fe (iron), Mn (manganese), Ni (nickel), Pb (lead), and Zn (zinc) were determined from both sources, in the e-liquid and tobacco. The authors also conducted a conspicuous elemental analysis of all the parts of the cartomizers and analysed carbon monoxide (CO) emitted from the electronic and combustible cigarettes. Finally, the matter deposited on membrane filters exposed to smoke or aerosol was submitted to elemental analysis of C (carbone), O (oxygen) and N (nitrogen).
Analysis of Trace Metals
The major finding of this study is that none of the analyzed trace metals on aerosol-exposed membrane filters were significantly different from control filters, except Ni, which was nearly five times higher than on control filters.
Toxicity: Is Ni a source of concern?
The authors show that, except for Ni, the trace metals found in e-vapor were at levels comparable with ambient air and in much lesser concentration than in the smoke of Marlboro cigarettes. Except for Ni, it is unlikely that the aerosol produced by e-cigarettes contains enough of the other trace metals to induce significant pathology. However, we will see later that it is not to be considered as a general ruling.
Ni is first responsible for allergies. Ni can be found in many everyday items, such as coins, belt buckles, zippers, eyeglass frames, and causes an itchy rash where the skin is in contact with the metal. It is what you have to be careful with, when using an e-cigarette because a similar allergic reaction can also be expected in the respiratory tract, especially if you ever had a skin reaction in the past.
It is explained by the authors that the potential carcinogenicity of Ni is related to its ability to form nickel carbonyl (Ni(CO)4) with excess of CO. Hence, the toxicity of Ni is higher for combustible products and in case of dual use than for e-cigarettes only. This is confirmed by their analysis of CO that showed 831 ± 166 μM/L in smoke while its value in e-vapor was lower than 0.010 ± 0.003 μM/L, close to air samples. It must be noticed that Ni was undetectable in Marlboro smoke during this study.
A device-related issue
In their discussion, the researchers point out the fact that the presence of trace metals in e-vapor is strongly related to the device that is tested. Here, the e-cigarette was a Triple3 eGo that was used with a e-liquid tobacco flavor, very high nicotine (7 s Electronic Cigarette company, South Lake, TX).
The atomizing chamber and the cartridge are combined into a single unit, the clearomizer, a plastic tank of 1.6 ml e-liquid capacity. The device also includes a 650 mAh lithium ion battery (3.7 V, unregulated), a silicon ring at the base of the drip-tip. The resistance coils varies between 2.2 and 2.6 Ω for an average power output of ~5.7 W.
7 s Electronic Cigarette company Juice:
80% propylene glycol and 20% vegetable glycerin, 24 mg/ml of nicotine or ~3.4 mg nicotine/15 puffs. A trace of flavoring is added to the blend to provide the tobacco taste.Technical features of the e-cigarette and the e-liquid used in the test
Scanning electron microscopy (SEM) coupled to energy-dispersive X-ray spectrometery (EDS) allowed the scientists to determine with precision the composition of each part of the cartomizer after careful dissection. They identified the Ni-rich parts that were in contact with the e-liquid and that could have been a source for this metal. Analysis of the e-liquid before filling the tank revealed that no trace metal was present at detectable levels.
Their results indicate that most of the Ni was concentrated at the level of the core, the coil, the thick wire and the weld joint of the core assembly. Hence, the heating element of the eGo was very likely the source of Ni to the aerosol.
The authors demonstrate, here, that the issue it not the quality of the e-liquid but rather the hardware itself, and especially its resistance coil. In other studies, aerosols of four different brands of cartridge type e-cigarettes showed that Sn, Cu and Zn, in addition to Ni could also be emitted in the aerosol. In this case, the source of trace metal contamination was found to be the metallic parts in contact with the e-liquid stored in the cartridge. The transfer from the solid phase to the e-liquid may depend on several factors among which the corrosiveness of the e-liquid, its pH and the presence of organic acceptors like proteins, for example.
Other interesting findings
Volume of e-liquid vaporized per 5 s puff
The volume of e-liquid vaporized per 5 s puff was calculated by the authors and averaged 9.3 μl in this study (7 s e-liquid/eGo). Such a value is specific to the cartomizer used in the experimental design and may be used for the comparison with other datasets and the interconversion between puffs and volume.
Visual inspection of filters
The authors demonstrate the performance of the experimental design to trap particle with the visual inspection of filters.
A blank membrane filter is white. The pinkish appearance of the filters exposed to e-vapor is uniform and consistent with the brownish color of the 7 s e-liquid. In contrast, smoke stains the membranes with a color gradient ranging from light beige to dark brown, with increasing puff number.
Fundamental differences between vapor and smoke
The authors show fundamental differences between the physical natures of the aerosol (made up of liquid droplets) and the smoke (which is made up mostly of gas and particulate matter).This difference lead to a lower percent recovery of aerosol on membranes filters than percent recovery of smoke.
Another difference concerns the temperature: The aerosol was warmer than smoke although smoke is generated at higher temperature (800°C) than vapor (350°C).
Estimated smoke and aerosol trace metal content per 20 pack equivalent
More interestingly, the researchers determined estimated contents of trace metals from the vaporization of e-liquid equivalent to 20 cigarettes or from the combustion of 20 Marlboro cigarettes. The comparison with recommended exposure limits (REL) published by the National Institute for Occupational Safety and Health (NIOSH) and the permissible exposure limits (PEL) published by the Occupational Safety and Health Administration (OSHA) is tricky because, conceptually, the use of those products is first recreational and, second, intermittent which contrasts to occupational exposure in workspaces. This has been discussed in length by K. Farsalinos.
A simple and effective experimental design has been imagined to collect and trap aerosol as smoke. Various designs exist, used by other research teams and ranging from the simplest syringe-pumping protocol to the most advances vaping/smoking machines that can reproduce different vaping or smoking topographies. In this debate, the authors point out the need for standardizing the experimental approach so that experiments may be reproduced with the same settings and results compared.
The researchers recommend to manufacturers to minimize the use of excessive Ni in devices. But at the user’s level, the first recommendation is to avoid using NiChrome or Ni200 resistance coils in electronic reconstructible mods.
They notice that with temperature controlled devices, Ni toxicity becomes even more critical because most of these last-generation devices use coils made of quasi pure Ni. Ni-Chrome (alloy of 80% Ni and 20% Cr) is another popular material that should be avoided because it contains high concentration of Ni. In contrast, Kanthal (alloy of Fe, Cr and Al) represents the opposite extreme since it doesn’t contain Ni. But the latter also cannot be used in TC mode because its resistance doesn’t change with the electric current and cannot be used to predict coil temperature.
Palazzolo, D. L., Crow, A. P., Nelson, J. M., & Johnson, R. A. (2016). Trace metals derived from electronic cigarette (ECIG) generated aerosol: potential problem of ECIG devices that contain nickel. Frontiers in Physiology, 7, 663.
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