Inclusion criteria

The review included articles reporting on (i.e., listing ENDS characteristics) or evaluating (i.e., assessing how design differences impact output) ENDS design characteristics and aerosol constituents, with a specific focus on the measurement of chemical species in aerosols produced by heated-coil ENDS.

Exclusion criteria

We excluded articles first by duplicates (n = 287), then a title and abstract screening, and finally by a full-text screening. Irrelevant studies included (n = 1,835), but were not limited to, articles not assessing ENDS aerosol, letters, commentaries, or other reviews, second-hand or third-hand aerosol/residue assessments from ENDS, and environmental exposure studies. Additionally, studies retrieved that were not available in English or were published anonymously were also eliminated at this stage. At the full-text screening, studies were eliminated upon the basis of 4 criteria: wrong study design (n = 32; e.g., indirect measures of toxicity or cytotoxicity studies, exhaled breath or saliva studies, or only analyzed e-liquid), no primary data (n = 28; e.g., perspective, comments, or responses to previous literature), no full text (n = 17), and assessed major aerosol constituents only (n = 14; studies exclusively analyzing the known major and expected aerosol constituents nicotine, PG, and VG) were also excluded as these are expected aerosol constituents when liquids containing them are aerosolized.

We defined risk of bias a priori as the utilization of analytical techniques employed by research groups externally (Group A), internally (Group B), or non-replicated (Group C) for a specific major chemical category. Because we did not quantitatively combine study results, no articles were excluded based on risk of bias.

Search strategy

We searched Ovid MEDLINE, Epub Ahead of Print, In-Process & Other Non-Indexed Citations, Daily and Versions electronic databases, selecting articles published from 1946 until May 07, 2020. We employed the following keywords: "electronic nicotine device," “vape,” “vaping,” "electronic nicotine delivery system," "electronic cigarette," "e-cigarette," “ecig,” and "e-cigs." These terms were combined with the following terms: “acetaldehyde,” “acrolein,” “aerosol,” “air pollutant,” “aldehyde,” "aromatic amine," “atomizer,” “carbonyl,” “coil wire,” “e-liquid,” “exp flavoring agents,” “exp metals, heavy,” “exp temperature,” “formaldehyde,” "free radical," “glycerol,” "heavy metal," “nitrosamine,” “phenol,” "polycyclic aromatic hydrocarbon," “propylene glycol,” “thermal degradation,” “toxic,” “toxicity,” “vocs,” “volatile,” and "volatile organic compounds".

Associations between ENDS characteristics and aerosol toxicants

We further categorized articles based on their reporting of ENDS attributes and how differences in devices and operating conditions influence the presence of aerosol toxicants (Table 3); the absence of a correlation did not prevent a particular study from being included in Table 3. Below, the studies are grouped by the toxicants examined, and their main findings are summarized.

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Table 3. Number of studies reporting associations between varying ENDS^a characteristics and aerosol toxicants.

https://doi.org/10.1371/journal.pone.0234189.t003

Carbonyls/VOCs

Numerous studies have evaluated the relationship between power supplied to the atomizer heating element and aerosol carbonyl concentrations. Salamanca et al. [31] reported carbonyl levels of 1.20 μg/mg e-liquid consumed when an ENDS was operated at a power of 10 watts (W) and 4.43 μg/mg e-liquid with the power at 15 W. Beauval et al. [22] observed that carbonyl concentrations in aerosol were higher when the ENDS was operated at 30 W than at 18 W. Korzun et al. [27] marked propionaldehyde, acetaldehyde, acrolein, glycoaldehyde, hydroxyacetone below the limits of detection and quantification at 11, 13, and 17 W, but all carbonyls were above the limit of quantification at 24 W. El-Hellani et al. [40] concluded that power significantly correlated with total aldehydes in ENDS aerosol. In a particular ENDS recommended for use at 15–60 W, Uchiyama et al. [48] detected carbonyls at high levels at 50 W (4400 μg/15 puffs) and even higher at 60 W (6200 μg/15 puffs). Jensen et al. [44] identified carbonyls and VOCs in ENDS aerosol in single-puff experiments and observed increases in the measured compounds when power increased from 4 W to 6 W; the aerosol produced had a unique fingerprint that distinguished it from the unaerosolized e-liquid. Ogunwale et al. [29] observed that increasing ENDS power from 11.7 W to 16.6 W dramatically increased the aerosol concentrations of acetaldehyde, acrolein, formaldehyde, propionaldehyde, and butyraldehyde. Geiss et al. [25] observed that carbonyl production significantly increased with increasing power, with a sharp rise at 15 W and 20 W. Talih et al. [32] concluded that carbonyl concentrations in ENDS aerosols are correlated with both power applied per surface area of coil and liquid consumed. Vreeke et al. [35] observed that acrolein and acetaldehyde remained below the limit of detection when power increased from 55 W to 65 W on a sub-ohm (<1 ohm resistance) heating coil, but these toxicants increased when power increased from 9 W to 11 W on a supra-ohm (>1 ohm resistance) heating coil. Vreeke et al. [34] found that device temperature increases with increasing power and corresponds directly to dihydroxyacetone (DHA) production. Pankow et al. [50] observed that benzene concentrations in aerosol increased with increasing wattage from 6 W to 13 W on one device, but remained undetectable when wattage increased from 6 W to 20 W on a different device. Gillman et al. [26] observed that an increase in the efficiency of aerosol production, defined as (milligrams aerosol per puff) per watt of power applied to the coil, was associated with lower levels of aldehydes in the aerosol. Farsalinos et al. [24] observed that aerosol formaldehyde concentrations increased with increasing power applied to the heating coil, but also concluded that daily exposure to formaldehyde from consuming 3 grams of liquid would be 32% lower than from smoking cigarettes. Overall, the findings suggest that increased power is associated with increased carbonyl production.

The available literature suggests a similar relationship between voltage and the amount of carbonyls in ENDS aerosol. Uchiyama et al. [33] observed that carbonyls increased when applied voltage increased from 3.2 V to 4.4 V. Kośmider et al. [62] observed that PG-containing fluids were associated with significantly higher levels of formaldehyde and acetone when an ENDS was operated at 4.8 V than at 3.2 V. Sleiman et al. [64] observed that increasing the voltage applied to a single-coil device from 3.3 V to 4.8 V tripled total aldehyde emission rates from 53 to 165 μg/puff, and reusing the device several times increased aldehydes by more than 60%. Behar et al. [43] observed hydroxyacetone in ENDS aerosols at concentrations ranging from ~100 to ~10,000 μg/mL at 5 V but not at 3 V. Zhao et al. [71] observed that as voltage increased from 2.2 to 5.7 V, benzene increased 11-fold and toluene doubled. Similarly to increasing power, an increase in voltage has been observed to be associated with increased carbonyls in aerosol at a fixed resistance.

Suboptimal ENDS operating conditions result in what is commonly referred to as a “dry puff”, during which coil heating occurs in the absence of sufficient e-liquid to produce aerosol. Suboptimal ENDS operation can result in higher concentrations of carbonyls in aerosol. Farsalinos et al. [37] observed aldehydes under non–dry puff conditions but at substantially lower levels than with conventional cigarettes; however, dry puff conditions raised ENDS aerosol concentrations of formaldehyde, acetaldehyde, and acrolein by 30 to 250 times, potentially higher levels than a conventional cigarette. Available evidence seems to suggest that maintenance of optimal operation conditions (e.g., adequate wick saturation and avoiding excessive coil heating) is associated with lower levels of carbonyls.

Coil location, orientation, and resistance may also affect carbonyl production. Farsalinos et al. [23] observed that top-coil atomizers produce more carbonyls at higher voltages than at lower voltages, and bottom-coil atomizers produce extremely low levels of carbonyls at both high and low voltages. Gillman et al. [26] also observed that top-coil atomizers produce more carbonyls than bottom-coil atomizers at the same wattage. Pankow et al. [50] reported that benzene concentrations were largely undetectable in an ENDS with a single vertical coil with a cotton wick, but were more readily detected with a single horizontal coil with a silica wick. Vreeke et al. [34] observed that aerosol concentrations of DHA were directly proportional to the mass of aerosolized VG:PG, but a vertical coil produced less DHA than a horizontal coil. Stephens et al. [45] similarly observed more carbonyls with a top coil than a bottom coil; insufficient wick saturation was associated with coil build-up and greater carbonyl output compared to a stainless steel organic cotton coil which provided ample wick saturation with minimal coil deposits and fewer carbonyls. Son et al. [47] identified a top coil device generated increased concentrations of formaldehyde (4.80 μg/puff) compared to a mod device with a Clapton-style coil (0.47 μg/puff) Top-coil horizontal atomizers have the potential to generate more carbonyls compared to bottom-coil horizontal atomizers, while vertical coils produced fewer carbonyls than horizontal coils.

Several studies, but not all, have observed that carbonyl and VOC production varies by device type. Uchiyama et al. [33] reported significant differences in carbonyl production between different brands, whereas Flora et al. [83] observed low formaldehyde concentrations with low variability (0.09 to 0.33 μg/puff) across 4 of the same ENDS products. Gillman et al. [46] examined the formaldehyde, acetaldehyde, and acrolein output of 10 identical ENDS products noting the inherent variability observed from the devices. Goniewicz et al. [79] observed 4 carbonyls (among 15 analyzed) and 2 VOCs (among 11 analyzed) that were present in aerosols generated from almost all 12 ENDS brands. Bitzer et al. [14] observed some significant differences between several non-refillable closed-system devices and 4 carbonyls. Wagner et al. [85] did not detect 1,3 butadiene; isoprene; acrylonitrile; benzene; or toluene in the aerosol of 6 top-selling, commercially available ENDS. Laugesen et al. [80] observed across 9 ENDS brands an average formaldehyde concentration of 1.07 μg/L of vapor, 0.81 μg/L of acetaldehyde, and 1.06 μg/L of acrolein; however, the concentrations of aldehydes on average have decreased over time. El-Hellani et al. [40] concluded that ENDS brand significantly correlated with total aldehydes in ENDS aerosol. Vreeke et al. [35] reported fewer carbonyl compounds produced from a sub-ohm tank than from a supra-ohm clearomizer (atomizer and cartridge). Gillman et al. [26] reported that sub-ohm devices produced lower levels of carbonyls than supra-ohm devices at similar wattages. Device type has been shown to potentially impact carbonyl and VOC output.

Numerous articles have reported that e-liquid constituents impact carbonyl aerosol concentrations. Kim et al. [51] observed that carbonyls were present in aerosol associated with major flavoring groups but not in un-aerosolized liquid. Herrington et al. [94] observed that acetaldehyde and acrolein exist in ENDS aerosol but not in e-liquid solutions. Conklin et al. [42] demonstrated that VG generates higher levels of formaldehyde and acrolein and PG generates higher levels of acetaldehyde. Klager et al. [97] observed median formaldehyde concentrations of 626 μg/m³ in aerosol produced with 24 e-liquid flavors. Beauval et al. [60] observed formaldehyde and acetaldehyde close to 1 ng/mL of emission with no substantial differences between flavorings, with the exception of acrolein concentrations from aerosol produced with “blond tobacco” flavored e-liquid. Papoušek et al. [93] observed that acrolein was present in higher concentrations in tobacco-flavored than in fruit-flavored e-liquid. Zhao et al. [71] observed that menthol flavor generated 330% more benzene and 120% more toluene than in tobacco flavor. Qu et al. [74] observed that the carbonyl compounds emission factors increased linearly 1.0- to 92-fold when flavor content of the e-liquid increased from 5% to 50% and concluded that most carbonyls in e-liquids were from flavorings. Allen et al. [92] detected diacetyl in aerosols in 39 of 51 e-liquids. Kośmider et al. [67] detected benzaldehyde in 108 of 145 flavored e-liquids and observed the highest levels (5.1–141.2 μg/30 puffs) in cherry-flavored products. Pankow et al. [50] observed that benzene concentrations in ENDS aerosol increased with the addition of benzoic acid and benzaldehyde to the e-liquid solution. Vreeke et al. [35] observed that e-liquid containing 10% flavoring compound was associated with increased carbonyls in aerosol compared with e-liquids containing only VG:PG. Duell et al. [39] observed that the concentrations of aldehydes (propionaldehyde, acetaldehyde, glycolaldehyde, and acrolein) increased as sucralose concentration in the e-liquid increased. The relative concentrations of carbonyls in the aerosol have varied from study to study [36, 38]. Reilly et al. [84] observed no differences in formaldehyde and acetone across the aerosols of 4 different JUUL flavors. Additionally, Mallock et al. [75] observed that European and American versions of JUUL pods produced similar levels of carbonyls. Available evidence suggests that the chemical constituency of aerosol is dependent upon the matrix of the e-liquid as carbonyl compounds are present in the aerosol as both flavoring agents (i.e., diacetyl) and products of thermal degradation (i.e., aldehydes).

E-liquid composition also impacts the transfer of chemicals to the aerosol. Erythropel et al. [68] observed that benzaldehyde carryover (i.e., percentage of chemical transferred from e-liquid to aerosol) significantly increased when PG content increased from 0% PG to 100% PG. This suggests that the concentration of toxicants in aerosol can be influenced by the bulk e-liquid solution.

A common technique for carbonyl analysis is to bubble aerosol through 2,4-dinitrophenylhydrazine (DNPH). This allows carbonyls to form carbonyl-DNPH adducts that can be detected with high performance liquid chromatography. Hemiacetals are formed through the reaction of a carbonyl with a hydroxyl, such as one on PG or VG, and become unreactive to DNPH. The stability of hemiacetal adducts formed when reactive carbonyls, such as formaldehyde, interact with VG or PG is controversial, as are the implications of the results. Though not directly observed, some investigators suggest that the stability of the carbonyl-DNPH adduct drives equilibria of hemiacetals, acetals, and free carbonyls towards that of the carbonyl-DNPH adduct and the formation of hemiacetals is reversed in DNPH solution [26]; others maintain that the formation of hemiacetals leads to a severe underestimation of the absolute amount of carbonyls [31, 99]. Since an ENDS user would inhale the hemiacetal or acetal [68], both are excluded as carbonyls in the tables.

Various ENDS operating parameters increase the concentrations of hemiacetals in aerosol. Jensen et al. [99] reported a significant change in the presence of formaldehyde hemiacetals as voltage increased from 3.3 V (not detected) to 5.0 V (380 ± 90 μg/10 puffs). Salamanca et al. [31] observed approximately 4 times the amount of formaldehyde and formaldehyde hemiacetals when a device was powered at 15 W than at 10 W. However, both power settings had the same ratio of formaldehyde to formaldehyde hemiacetals. Vreeke et al. [35] increased power from 55 W to 65 W for a newer generation device and identified increased formaldehyde hemiacetals. Similarly, power increases from 9 W to 11 W significantly increased formaldehyde hemiacetals. Additionally, Vreeke et al. [35] reported that the addition of triacetin, a common flavor additive, increased levels of formaldehyde hemiacetals. Duell et al. [39] observed that formaldehyde hemiacetals increased with increasing concentrations of sucralose added to the e-liquid. Erythropel et al. [68] reported carryover rates for both aldehydes and corresponding acetals to be 50–80% of the original concentration in e-liquid. Power and voltage have been demonstrated to increase formaldehyde hemiacetals in aerosol which can be further influenced by e-liquid composition.

Trace elements

Important relationships have been observed between trace elements in ENDS aerosols and variations in power, device, air-flow, coil material, and e-liquid. Zhao et al. [52] observed that when power was increased on an open-system device from 20 W to 40 W, median arsenic (As), chromium (Cr), Cu, iron (Fe), manganese (Mn), nickel (Ni), Pb, antimony (Sb), tin (Sn), and Zn concentrations increased 14, 54, 17, 30, 41, 96, 14, 81, 631, and 7-fold, respectively. Zhao et al. [52] observed that open-system ENDS with kanthal (Fe, Cr, and aluminum [Al] alloy) and stainless steel coils had consistently higher Fe and Ni levels. Goniewicz et al. [79] identified Cd, Ni, and Pb in all vapors generated from 12 different first-generation ENDS models. Prokopowicz et al. [72] detected Pb in the aerosol of 2-tank-system ENDS. Ting et al. [54] identified 5% of ENDS and e-liquid combinations tested emitted Cr at levels that exceeded permissible daily exposure limits. Williams et al. [87] analyzed disposable ENDS and observed that aerosols generated at low and high air-flow rates produced the same pattern of elements, although the total element concentration decreased at the higher air-flow rate. In this study, at least 35 trace elements were identified in ENDS aerosol; Si was the dominant element, but other elements such as calcium (Ca), sodium (Na), Cu, magnesium (Mg), Sn, Pb, Zn, boron (B), selenium (Se), Al, Fe, germanium (Ge), Sb, Ni, and strontium (Sr) were present in most ENDS aerosols, with relative concentrations varying by ENDS model. Beauval et al. [60] evaluated 6 e-liquid refills and observed that only Cd, Cr, and Sb were present in aerosol, with concentrations up to 0.14, 3.4, and 0.47 pg/mL puff, respectively. The concentrations of metals varied minimally with the presence or absence of flavoring and nicotine. In contrast, Mikheev et al. [86] observed that that As, Cr, Ni, Cu, Sb, Sn, and Zn levels in ENDS aerosol varied widely across nicotine- and non-nicotine-containing flavors. Liu et al. [98] observed certain As species significantly increased in concentration from e-liquids to aerosol. Not all published articles have detected trace elements. Flora et al. [83] did not detect As and cadmium (Cd) in ENDS aerosol across 4 commercial e-cigarette non-refillable closed-system products. The ten most examined elements in descending order were Cu, Ni, Zn, Pb, Cd, As, Fe, Cr, Sn, and Al.

ROS and free radicals

The concentration of free radicals in aerosol has been reported to vary by power and associated coil temperature. Bitzer et al. [56] observed increases in free radicals with increases in coil temperature from 100 to 300 °C and power from 10 W to 50 W. Haddad et al. [55] observed that ROS flux increased 3-fold when the power increased from 5 W to 11 W on a single coil device, and increased approximately 7-fold when the power increased from 100 W to 200 W on an octuple coil device. Son et al. [57] observed that a device formed more hydroxyl radicals at 31.3 W than at 6.4 W. Free radicals have been demonstrated to increase with an increase in power.

The effects of device type on free radicals have been examined. Bitzer et al. [14] observed that free radicals are produced by non-refillable closed-system ENDS. Shein et al. [89] observed similar amounts of free radicals in aerosols using standard wicking and heating, a mesh coil, and an air control. Hasan et al. [90] found free radical concentration from two closed-system ENDS were significantly lower than conventional tobacco cigarettes, but the oxidative potency from the free radicals originating from the ENDS was much higher. No clear relationship between free radicals and device design has been observed.

The composition of e-liquid affects the concentrations of free radicals in aerosol. Bitzer et al. [56] observed increases in free radicals with changing VG:PG ratios from 100:0 to 0:100. Son et al. [57] observed that VG- and VG:PG-based e-liquids formed twice as many radicals than exclusively PG-based e-liquids; specifically, 90- and 170-mL puffs (3.8 s each) generated 4.1- and 10.3-fold more hydroxyl radicals than a 35-mL puff (also 3.8 s). Reilly et al. [84] analyzed aerosol from JUUL and observed that free radicals were 2.95 ± 0.81 nmol/g for tobacco flavor at a 70:30 VG:PG ratio; the concentration was higher (3.01 ± 0.28 nmol/g) when the e-liquid composition was 70:30 VG:PG without flavoring. Additionally significance was reached when the e-liquid was 40:60 VG:PG without flavoring (4.68 ± 0.58 nmol/g), and increased further to 4.78 ± 0.73 nmol/g when citral was added to the 40:60 VG:PG base. Goel et al. [73] generated aerosols from 3 different e-liquids generated by a tank system and estimated free-radical production to be 10.3, 4.0 and 2.5 × 10¹³ radicals per puff of menthol, citrus, and tobacco flavors, respectively. Bitzer et al. [58] analyzed the free radicals generated from 49 commercially available e-liquids and observed that nearly 43% of the flavors resulted in significant increases in radical production compared with the base VG:PG (40:60) mixture, but the amount varied greatly among the flavors; the flavorings dipentene, ethyl maltol, citral, linalool, and piperonal promoted radical formation in a concentration-dependent manner, whereas ethyl vanillin inhibited radical formation. Overall, ROS and free radicals in ENDS aerosol vary by e-liquid composition, but the precise relationship has yet to be determined.

PAHs

PAH concentrations in aerosol vary with device and e-liquid but are generally low if detected. Flora et al. [83] analyzed aerosol from 4 commercially available e-cigarette products and observed that benzo[a]pyrene was below the limit of detection of 10 ng/device. Similarly, Wagner et al. [85] did not detect benzo[a]pyrene in the aerosol of 6 top-selling commercially available ENDS. Beauval et al. [60] systematically detected naphthalene and acenaphthylene in ENDS aerosol at low concentrations (up to 4.10 and 0.37 pg/mL puff, respectively). Eddingsaas et al. [59] analyzed aerosols created from vanilla, cinnamon, and mango e-liquids and observed cadalene in the mango flavor. Available evidence suggests that PAH concentrations are low which is consistent with the knowledge that PAHs are primarily products of combustion [100] and ENDS operate at lower temperatures than combusted tobacco products.

TSNAs

Studies of different ENDS products have observed low or undetectable levels of TSNAs. Flora et al. [83] did not detect TSNAs (NNN and NNK) in aerosols from 4 commercially available ENDS products. Goniewicz et al. [79] analyzed 12 ENDS products and observed NNN concentrations ranging from not detected to 4.3 ± 2.4 ng and NNK from not detected to 28.3 ± 13.2 per 150 puffs. Farsalinos et al. [91] observed that no TSNAs were above the level of detection for aerosols generated from 3 tobacco-flavored e-liquids aerosolized by a second-generation ENDS. Though TSNAs are carcinogenic and their concentrations eclipse many other carcinogenic compounds in conventional cigarettes [101], they are less prevalent in ENDS.

Additional articles

Garcia-Gomez et al. [102] analyzed the aerosol of an ENDS through secondary electrospray ionization mass spectrometry, which identified more than 250 chemical species, including alkaloids and various flavoring compounds. The authors noted that some substances increased with increasing power of the device. Behar et al. [103] found cinnamaldehyde in 20 (51%) of 39 refill fluids, and aerosols contained several different constituents at 5 V than at 3 V. Soussy et al. [104] reported that emissions of 5-hydroxymethylfurfural and furfural were correlated with device power and sweetener concentration in the e-liquid. These observations support the notion that power and voltage influence aerosol toxicant concentration.

Lee et al. [105] detected more carbonyls in ENDS aerosol generated from tobacco flavors than from menthol. Kośmider et al. [28, 70] observed that aerosol carbonyls varied by puffing protocol when different e-liquid nicotine concentrations were assessed. Uchiyama et al. [106] observed large variations in carbonyl concentrations between and within unnamed devices. El-Hage et al. [107] reported the transfer of pyrazines, a class of flavoring compounds, from e-liquid to aerosol. Rawlinson et al. [108] identified the chemical profiles of aerosols generated from 4 e-liquids contained between 30–90 compounds mostly characterized as “unknown” exceeding the 5 ng/puff threshold which may include PAHs.

El-Hellani et al. [109] focused on how device modifications such as power, coil material, and coil geometry may affect the emission of small gasses, notably carbon monoxide; they reported that power and coil material significantly affect analyzed chemical species, with nickel wire being the most reactive. Wang et al. [110] reported a device-independent study wherein various ratios of VG and PG were aerosolized at a range of temperatures, yielding carbonyl compounds. VG generated much higher concentrations of formaldehyde than PG; acrolein was detected only when VG was present in the solution and the temperature was ≥270 °C. Saliba et al. [111] explored the device-independent effect of the metal coil on subsequent carbonyl production. The wires had a catalytic impact on carbonyls, producing species at 250 °C, as opposed to 460 °C without wire. Moreover, the investigators found the material and age of the wire had a strong correlation with carbonyl production, with new nichrome (Ni and Cr alloy) being the least reactive and old nichrome being the most reactive. Williams et al. [112, 113] reported a comprehensive metal analysis of ENDS aerosol the sources of the metals to be the metal components of the heating elements such as coils, wires, or solder joints.

Belushkin et al. [114] completed a comprehensive toxicant panel of 34 ENDS and 57 e-liquids. Benzene was identified in some high power mod systems, but was not as prevalent in non-refillable closed systems. Carbonyl emissions were lower in closed systems than open systems. Particularly noted, formaldehyde increased with the depletion of e-liquid in the reservoir, starting at approximately 50% capacity. Trace elements, TSNAs, and PAHs were seldom above the limit of quantification.

Risk of bias assessment

Table 4 demonstrates that the analytical techniques used to assess major chemical groups of aerosol toxicants have been largely replicated.

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Table 4. Risk of bias determined by analytical technique utilized by studies and number of studies of measured major chemical groups in aerosol.

https://doi.org/10.1371/journal.pone.0234189.t004