Literature Review
Electronic cigarettes are nicotine delivery devices that appear similar to conventional cigarettes. According to Palazzolo (2013), the ECs have been promoted as tools that can aid addicted smokers to quit their habit. Despite this, Palazzolo (2013) claims that although they are marketed as a safe alternative to the CC, they still contain harmful elements.
Before being launched in the United States and Europe in 2006, they were developed and patented in China in 2003 (Almeida-da-Silva et al., 2021). The producers shared that they were cheaper and safer than the usual or conventional products, as suggested by Almeida-da-Silva et al. (2021). In 2014, the United States National Health Interview Survey stated that among the 146 million working grownups, 5.5 million smoked the ECs (Almeida-da-Silva et al., 2021). The usage was highest among non-Hispanic white men between 18 and 24 years old (Almeida-da-Silva et al., 2021). These individuals came from households with an average income below $35000 annually.
Analysis Through Comparison of Information Found in Articles
The following discusses the results and information in Table 1 in the evidence provided. The Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines guided the identification of research publications, such as Willershausen et al. (2014). Medline (PubMed), Scopus, and Web of Science were other major databases (Willershausen et al., 2014). As Willershausen et al. (2014) suggested, multiple keywords were utilized to generate the studies.
Willershausen et al. (2014) reveal that the data were screened to check for identical data, and the action led to the realization of over sixty articles. The eligibility criteria consisted of free full-text original studies associated with the trace metals in electronic cigarettes and their effect on an individual’s health. It was challenging to include articles in a language that was not English.
Table 1: Metals and their concentration level in electronic cigarettes
Recommendations, reviews, non-original papers, expert statements, and technical reports were excluded. Yamin et al. (2010) showed that the procedure led to the discovery of sixteen original articles, of which a few were removed and about thirteen were evaluated. One study was excluded as it failed to mention the trace metals scrutinized in e-liquid or its aerosol. Complete information on twelve studies was included in the review.
Yamin et al. (2010) reveal that a review was done of metals evaluated and their concentrations determined (see Table 1). The level of metals in electronic cigarette aerosols was separately recorded. The concentrations were changed into nanograms/10 puffs for easier comparison of results between researchers in Table 1.
Results
All the studies included in the procedure suggested above analyzed various metals in EC aerosols. Sosa et al. (2019) share that the analyzed elements’ toxicity ranged from high to low. The metals Sosa et al. (2019) highlight include Cd, Ni, Cr, Mn, Pb, Al, Zr, Ti, Ag, and Li. In addition to this, Sosa et al. (2019) reveal that they discovered Si.
In various studies, the sampling techniques for electronic cigarette aerosols were not similar (Sosa et al., 2019). For instance, the Cooperation Centre for Scientific Research Relative to Tobacco method was used to simulate vaping while e-vapors were produced using a smoking machine (Goniewicz et al., 2013). Ninety-six e-vapors were produced by 96 electronic cigarette puffs (Goniewicz et al., 2013). Goniewicz et al. (2013) used similar conditions, revealing how the electronic devices produce aerosol. Results were obtained from ten e-smokers and utilized in the study.
The testing processes utilized averaged puffing conditions, the duration, and intervals between puffs, volume, and the number of puffs taken in a single session. One hundred fifty puffs were given to each electronic cigarette in 10 series of 15, with a 5-minute break in between each series (Mishra et al., 2017). After the batteries were recharged at night, each was tested three times over the next three days—every test item generated vapors during 150 inhaled puffs (Mishra et al., 2017). Mishra et al. (2017) record that the absorption process was used to collect the metals. Their quantization was conducted with inductively coupled plasma with a mass spectrometry mechanism.
Method of Detection
It is reported that there are studies that used ICP-MS or inductively coupled plasma optical emission spectrometry to quantify the metals released in the aerosols. Such include Mishra et al. (2017) and Williams et al. (2017). Atomic absorptiometry was used in the study’s evaluation procedure (Williams et al., 2017). Most research found Al, Cu, Pb, Cr, and Ni (Williams et al., 2017). They also reported relatively low quantities of elements like Zn, Fe, and As.
Nine investigations revealed the presence of Ni in the aerosol of electronic cigarettes (Mishra et al., 2017). The measured concentrations varied from 5 to 7.33 ng/10 puffs (Mishra et al., 2017). Cr was mentioned in six investigations, with values in 2 studies ranging from 7 to 200 ng/10 puffs (Mishra et al., 2017). Pb was reported in 6 studies with levels ranging from 2 to 38 ng/10 puffs.
Similarly, Al was reported in around five studies in concentrations ranging from 266 to 394 ng/10 puffs.
In four investigations, Cd levels ranging from 0.66 to 14.6 ng/10 puffs were found (Schober et al., 2014). Schober et al. (2014) note that Sn was found in several studies to have a concentration higher than 36 ng/10 puffs but below 6000 ng/10 puffs. Additionally, copper was discovered in eight studies, ranging from 11 to 2247 ng/10 puffs (Schober et al., 2014). Mn was observed in 4 studies at a concentration of 2 to 35 ng/10 puffs in two studies.
Source of Metals in Electronic Cigarettes
Several studies investigated the sources of the elements found in aerosols from ECs. One of them studied metal substrates in various components of the atomizer. Creatinine and Nickel were discovered in the nichrome filament, silver and copper in the thick wire, and zinc and copper in the brass clamp, according to Beauval et al. (2017). Si, Ca, oxygen, Al, and Mg from the wick, sheath, and Pb and Sn from solder junctions. The core assembly’s elemental analysis revealed the presence of trace metals, namely Zn, Al, Ni, and Fe (Olmedo Palma et al., 2018).
In one investigation by Dunbar et al. (2018), sources of Cu were identified as EC batteries and cartomizers. High levels of Sn were reported to be released from friable solder joints and Cu wires coated with Sn (Dunbar et al., 2018). It is essential to utilize a procedure called centrifugation to detect other elements.
Sn whiskers were on the solder joints and wires close to the joints. According to Dunbar et al. (2018), the ones that were dark in color included tin oxide, which is frequently produced when metal is heated. As a result, its deposition close to the filament helped us identify its origin (Dunbar et al., 2018).
Furthermore, Dunbar et al. (2018) note that a few cartomizer fibers had a green color. These were primarily Cu particles that shifted from the larger wire or solder. Ratajczak et al. (2018) concluded that Sn in the pellets migrated from the solder joints or from the solder that escaped into the cartomizers during presale testing or manufacturing.
Additionally, Ratajczak et al. (2018) implied that as the solder joints were free of Pb, they were more fragile and prone to cyclic temperature changes. Small quantities of Cr and Ni from the nichrome wire, Ag from the coatings on the Cu wire, and Fe from the mouthpiece were reported (Ratajczak et al., 2018). Silicate beads from the fiberglass wick were also discovered in the aerosol.
The elemental analysis of the wick revealed the presence of silicon, Mg, Ca, and Al. Moreover, boron, which is utilized to manufacture glass wicks, was also reported. An earlier analysis by Ratajczak et al. (2018) showed that the outer and inner surfaces of the casing consisted of Mn and Fe. The core tip comprised Cu, Ni, and Zn (Ratajczak et al., 2018). The upper core comprised Si, whereas Pb and Zn were available in the gasket.
The fabric material possessed a high percentage of Ni and Cu. The inner and outer surfaces of the woven tube were mainly comprised of Sn, Si, and Al. The lower and upper halves of the core were coated with Ag, with underlying metal compositions of Cu and Ni. The wick fibers within the surrounding resistance coil are primarily comprised of Si. In contrast, the coil filament around the wick fibers contained high amounts of Ni, with fewer quantities of Mn and Si.
The weld joint linking the coil with the thick extension wire consisted of high amounts of Ni and some Si. The wire is composed of Ni with minimal amounts of Cu. Its juncture, coil, and weld joint contained Ni and Cr primarily. The latter’s levels exceeded the allowed five percent threshold (Ratajczak et al., 2018).
Some investigations compared the metals in various elements of different electronic cigarette brands. For example, Ratajczak et al. (2018) reported that most brands’ filaments consisted of Cr and Ni. In a single brand, the filament consisted mainly of Cr, Al, Fe, Ti, Cu, and Mo (Ratajczak et al., 2018).
Others like Starbuzz and Vype had Cr, Ni, and Fe in their filaments. The thick wire was copper-coated with Ag in the brand BluCig, whereas in NJOY King, Starbuzz, and Tsunami, the wires consisted of Ni and Cu coated with Ag. The brand Smooth surrounded the copper wire with Sn (Walley et al., 2019). The thick wire, as well as filaments, were joined with either solder or clamps.
In five brands, Zn and Cu clamps were used in joining, whereas in other brands, they were joined with a solder of Sn. The electronic hookahs possessed Pb and Sn in their solder joints; the former was found in their aerosols. The joints between the battery and thick wire consisted of Sn solder in every brand, except in Luxury Lites and Imperial Hookah, where Pb was present (Walley et al., 2019). The sheaths comprise Si, Al, Ca, Mg, and oxygen in all the brands (Walley et al., 2019). A similar study was conducted in which they compared four brands of electronic cigarettes.
Even though the brand names were not shown, three brands had a cartomizer, whereas one brand was a disposable device. Aerosol from brand A had significant quantities of Sn (Walley et al., 2019). This brand’s further examination by Walley et al. (2019) showed that Sn concentrations varied approximately 30-fold between its cartomizers. Thus, two cartomizers depicting Sn’s low and high-end ranges were analyzed. The thin wires in the cartomizers consisted of Cr and Ni, whereas the thick cu wire was coated with Ag.
The clamps are joining the two wires in cartomizer Al, mainly including Cu, while in A2, they contained Ni with some Zn and Cu. The thick wire was attached to the air tube through Sn solder joints in brand A. The fibers in cartomizer Al were coated with Cu and Sn. It was implied that unstable joints were a source of Sn in the cartomizer of brand A.
The basic design of the cartomizers in brands B, C, and D was similar to that of brand A. As Walley et al. (2019) suggested, the thin wire consisted of nichrome. It coiled around a loop of thick wire made of Cu, coated with Ag in B and Sn in C.
In each brand, the wires of Cr and Ni were crimped together inside a metal casing (Pappas 2011). The thick Cu wire was coated with Ag (Walley et al., 2019). The joints between the thick and thin wires in B are composed of Sn solder, and those in C are made of Cr and Cu, whereas in D, they were joined by a Cu-Zn alloy clamp. In cartomizers from B, the Sn solder joins the thick wire to the air tube and mouthpiece. Nevertheless, in D, they were directly attached to the battery by the Sn soldiers.
Concentrations of Metals in Conventional Cigarette Smoke and Electronic Cigarette Aerosol
Around ten studies compared metals among various brands of electronic cigarettes, whereas five included a comparison of their levels between conventional cigarette smoke and electronic cigarette aerosol. CC smoke is a significant source of harmful metals to smokers and passive non-smokers (Walley et al., 2019). Smoking changes mental homeostasis in the human body, resulting in numerous health conditions.
Metals such as Cr, A1, Pb, Se, and vanadium have been reported in cigarette smoke. It is generally accepted that Pb and Cd concentrations in conventional cigarettes range from 1 to 3 μg/g and from 1 to 2 μg/g, respectively. The Pb and Cd concentrations in filter cigarettes were around 1.7 and 2.4 μg/g, respectively. Cigarettes, on average, contain 1-2 μg of Cd, and an individual smoking 20 cigarettes per day may inhale close to 1 μg of Cd per day.
As suggested earlier, tobacco grown in soils with greater available Pb and Cd may indicate increased levels in the tobacco lamina. Thus, cigarette brands with similar tar measures could result in different levels of heavy metals, depending on the growth conditions of tobacco. This was viewed in research analyzing levels of heavy metals in 20 popular cigarette brands in Saudi Arabia (Walley et al., 2019).
The levels of Cd consumed from smoking a pack of various brands were about 1.40– 2.70 μg. The value was close to that of cigarettes from Korea and the United Kingdom. Nevertheless, the quantity of Pb inhaled from the brands was four times greater than that of Korean and UK cigarettes.
It was implied that the variations could be attributed to the metal content in the soil, growth conditions, type of tobacco, and tobacco treatment process. A different study also approximates the amounts of Cd, As, Ni, Cr, and Pb in cigarettes obtained from adult smokers participating in the 2009 International Tobacco Control United States Survey wave. (Kaisar et al., 2017). Differences in the metal concentrations between various brands were attributed to the source of tobacco (Walley et al., 2019).
For Ni, significant pairwise variations were viewed between Philip Morris USA and R.J. Reynolds brands (Beauval et al., 2017). More Cr was present in the RJR brand than in any other. However, there were only minor variations in Cd, As, and Pb levels amongst the brands (Suter et al., 2015). Brands varied in the amount of metal in their electronic cigarette aerosols (Beauval et al., 2017). Moreover, differences were present when EC aerosols were contrasted with conventional cigarette smoke.
Health Effects of Metal on the Human Body
Once within the human body, metals, including lead, copper, and iron, have been discovered to have negative consequences. Metals mentioned above have adverse effects on people’s health. They occasionally act as a fictitious component of the body, but they may rarely interfere with metabolic functions (Farsalinos & Polosa, 2014).
They directly influence vital organs such as the liver, lungs, brain, and kidneys. They indirectly result in neurologic, immunologic, developmental, reproductive, and carcinogenic effects. These may be chronic or acute, depending upon the exposure duration. Unlike conventional tobacco smoking, research reporting the direct health effects of trace metals in electronic cigarette aerosols is negligible.
Lead (Pb)
Exposure to Pb may happen through inhalation of tobacco smoke, comprising the electronic cigarette aerosol. Lead is a significant neurocognitive and kidney toxicant for children at a relatively low concentration. It is present in mainstream and sidestream smoke, including the gas phase.
According to Kumar (2021), Americans exposed to secondhand smoke had higher blood lead levels between 1988 and 1994. The latter was correlated directly with serum cotinine levels and the number of smokers at home. Moreover, children in urban areas or young adults of low socioeconomic status had high Pb exposure (Mason et al, 2014). Secondhand smoke exposure is an unidentified source of relatively small particles of Pb, which are more easily absorbed in the bronchial-alveolar area. The health effects of lead in tobacco smoke can be seen in Table 2.
Table 2: Lead (Pb) and iron (Fe) metals and their health effects
The neurotoxic effects of lead may range from alteration of nerve conduction velocity to encephalopathy. The signs and symptoms may become so severe that someone develops convulsions, paralysis, coma, or even death. The nervous system is impacted through various direct or indirect mechanisms. Lead may change the development of the system, which involves disrupting vital molecules during neuronal migration and differentiation, interfering with synapse formation, or prematurely differentiating glial cells.
Iron (Fe)
Iron can cause choroiditis, conjunctivitis, and retinitis if it contacts and stays in the tissues. Long-term inhalation of excessive iron oxide fumes or dust concentrations may lead to benign pneumoconiosis, observable as an X-ray change. No physical impairment of lung function has been related to siderosis (Kumar, 2021). Inhaling excessive concentrations of iron oxide may increase the danger of lung cancer development in employees exposed to pulmonary carcinogens.
Mild to moderate lung scarring has been discovered in unusual cases of pulmonary siderosis. Patients experience persistent coughing, breathlessness, and decreased lung function. Nevertheless, individuals in occupations exposed to iron dust are often exposed to other types of dust, such as silica. Repeated inhalation results in silicosis, as suggested by Kumar (2021). Due to this, it is unknown whether the inhalation of pure iron or rust can cause detrimental scarring that has been seen in some instances of pulmonary siderosis.
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