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All content in this area was uploaded by Viktoriia Vyshneva on Sep 06, 2022
Content may be subject to copyright.
Submitted | 13 April 2022 Accepted | 15 May 2022
Keywords | E-Liquid – Propylene Glycol – Vegetable Glycerine – Electronic Nicotine Delivery Systems
– Aerosol – Nicotine
VYSHNEVA, V.
Hokord Limited, Sheung Wan, Hong Kong Citation | Vyshneva, V. (2022). Effect of propylene glycol and vegetable
glycerine ratio in e-liquid on aerosol formation: Overview of relevant
properties. Adiktologie, 22 (2), 118–125.
https://doi.org/10.35198/01-2022-002-0005
Effect of Propylene Glycol and
Vegetable Glycerine Ratio in
E-Liquid on Aerosol Formation:
Overview of Relevant Properties
BACKGROUND: Electronic nicotine delivery systems
(ENDS) generate an aerosol by vaporising e-liquids
that usually consist of propylene glycol (PG),
vegetable glycerine (VG), and other ingredients
(water, nicotine, and flavours). The chemical and
physical properties of these components have a
signicant effect on aerosol formation and must be
identied in order to improve product attractiveness
and assess the degree of health risks. AIM: The
aim of this article is to provide a description of the
composition of the e-liquid base and its impact on
the physical properties of the liquid used and the
behaviour of the aerosol generated and particles
separately. METHODS: For this purpose, 46 articles
were selected using a series of keywords. English-
language publications were chosen. RESULTS: The
impact of the PG/VG ratio on the physical properties
of the e-liquid (boiling point, viscosity, volatility,
hygroscopicity), aerosol emission characteristics
(refractive index, light scattering coefcient, particle
size distribution, concentration, emission of harmful
compounds), vape attractiveness (taste, “throat-hit”,
“cloud effect”), nicotine flux, coil temperature, and
puff topography is presented. CONCLUSIONS: The
PG/VG ratio is strongly correlated with the emission
of carbonyls, which has adverse health effects and
should be optimised. Furthermore, PG and VG also
affect the other important characteristics of the
aerosol generated by ENDS, which impact on both
attractiveness and the consumption of harmful
compounds. These ndings could be considered for
further research with the aim of improving electronic
nicotine delivery systems as this can reduce levels
of toxicants. This can be achieved by optimising the
geometry of the components with respect to heating
power and e-liquid.
Corresponding author | Viktoriia Vyshneva, Hokord Limited, Ofce 212, Kwong Kee Building, 54–56
Jervois Street, Sheung Wan, Hong Kong
viktoriia.vyshneva@hokord.com
118ADDICTOLOGYadiktologie-journal.eu
ADIKTOLOGIE
https://doi.org/10.35198/01-2022-002-0005
B1 INTRODUCTION
The popularity of electronic nicotine delivery systems (ENDS)
is growing rapidly worldwide as they produce fewer harmful
aerosols than conventional cigarettes do, since they do not
burn tobacco and fewer chemicals are delivered to the users’
bodies (Park & Choi, 2019).
The comparison between traditional tobacco cigarettes and
ENDS has been explored in a plethora of studies (Glantz &
Bareham, 2018; Glasser et al., 2017; Ward et al., 2020) which
conrm that electronic nicotine delivery systems are popularly
considered a less harmful alternative to traditional smoking,
which is represented as a product with a substantial health risk.
Increased attention to those systems has contributed to the rapid
progress of the design of the devices. The rst- generation elec-
tronic cigarettes are called “cigalikes”; they were designed like
traditional cigarettes, while the second- and third-generation
systems have changeable parts and their shape and design can
vary widely (Keamy-Minor et al., 2019; Pepper & Brewer, 2014).
Numerous studies have been performed to evaluate the toxi-
cological characterisation of vaping emission products, most of
which remark that the chemical composition of ENDS is less
harmful than traditional tobacco smoke (Belka et al., 2017;
Dutra & Glantz, 2014; Son et al., 2020; Ward et al., 2020).
However, vaped e-liquids can also generate substances that
may lead to the formation of toxic components, especially
carbonyls that are similar to those seen in conventional ciga-
rettes (Son et al., 2020). Such observations argue in favour of
continuing the study of the emission products of vape devices
(El-Hellani et al., 2018, Jiang et al., 2020; Kosmider et al., 2014;
Son et al., 2020). Additionally, a signicant part of the ndings
has been focused on the consideration of differences in the
impact on the human body of both ways of smoking (Park &
Choi, 2019).
The emissions produced by different kinds of smoking sys-
tems depend on the design features of the devices’ compo-
nents, which is why the understanding of the dependence of
some parameters of vape devices on others remains unclear
and must be further examined. This scientic interest is clearly
linked to the wide popularity of electronic nicotine delivery
systems, which, in turn, raises concerns about the possibil-
ity of adverse health effects in users and environmental haz-
ards (Marques et al., 2021). In addition, a preference for such
devices is questionable because of the use of vape devices by
nicotine-naive adolescents (Brett et al., 2021; Camenga et al.,
2014; Dutra & Glantz, 2014).
In general, vape devices utilise a battery-powered coil to aero-
solise an e-liquid that often contains nicotine and/or avouring
into an inhalable aerosol. This device is usually composed of
electrical components that include a battery, chip, wire, button,
and atomising components (a coil, wick, tank, and mouthpiece;
Gao et al., 2021).
First, the heater coil is activated during the puff and the liq-
uid surrounding the coil heats up and vaporises. The vapour
that has been formed comes in contact with the cold air and
is condensed to form an aerosol of ultrane particles that car-
ries nicotine deep into the lungs. Aerosol particles are rapidly
absorbed, then travel through the left heart, translocating to
the brain in a few seconds. They can coagulate and evaporate,
changing their size and behaviour suitably (Nides et al., 2014;
Yingst et al., 2019).
Certainly, the design of these components may inuence aero-
sol formation and delivery, which is one of the key parameters
in aerosol formation. Understanding the impact these charac-
teristics have on aerosol delivery at each stage may contrib-
ute to deepening our knowledge of the public health effects of
vape devices.
Another equally important parameter is the composition of the
e-liquid. The e-liquid usually contains a base that includes pro-
pylene glycol (PG) or vegetable glycerine (VG; or a mixture of
both) with or without the addition of avours, water, and nico-
tine (Uryupin et al., 2013). Variations in the ratios of the com-
ponents of the e-liquid can affect aerosol formation in different
ways, particularly in terms of particle size, visible effects, nic-
otine ux, etc. via the different chemical and thermodynamic
properties of the substances used.
Consequently, the aerosols generated from vape devices have
a signicant impact on the human respiratory system. This
is primarily related to the difference in the absorption of par-
ticles inside the human body with various sizes as a result of
the inuence of gravity, inertia, and Brownian motion. Hence,
small particles are affected by Brownian motion with the abil-
ity to penetrate into lungs, whereas large particles can only
settle in the upper respiratory tract via gravity (Manigrasso
et al., 2015). Probably, the medium-sized particles move with
air ow: inhale and exhale. Gaining a further understanding
of the properties of e-liquids, features of device components,
thermodynamic characteristics of the system, and other rele-
vant parameters may assist in answering questions about the
assimilation of aerosol particles pathing through airways and
their health effect (Oldham et al., 2018).
This review seeks to improve our understanding of the general
properties of PG and VG, features of their chemical structure,
and its effect on physical and chemical properties.
B2 METHODS
A literature review was performed to highlight the main
properties of e-liquids containing PG/VG and their effect
on various features. The research process was investigated
using Google Scholar as this database ensures a wide range
of articles. The research criteria were a combination of the
keywords “e-liquid”, “vegetable glycerine”, “propylene gly-
col”, “ENDS”, “aerosol”, and “nicotine”. Moreover, only articles
written in English were chosen. In general, 46 research arti-
cles were selected so as to highlight the main properties and
combine them in order to provide a broad understanding of
the characteristics of the substances used. Additionally, two
safety sheets were included to provide information about the
physical properties of PG and VG.
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It has been observed that glycerine is not a harmful com-
pound. Therefore, the permissible dose of glycerol and pro-
pylene glycol in an aerosol is 10 mg/m3 for an exposure time of
eight hours, according to the Occupational Safety and Health
Administration (Jacob et al., 2018; Segur, 1953).
PG and VG have different properties, the combination of which
has an effect on the thermodynamic properties of the e-liquid.
The PG/VG ratio as a solvent system may have a signicant
impact on the behaviour of the aerosol particles and their inu-
ence on taste, nicotine ux, and other key parameters that are
determined by the viscosity, boiling point, volatility, and chem-
ical structure of VG and PG. The molecular weight and viscosity
of VG are greater than those of PG, meaning that the addition
of VG to the base contributes to increasing the viscosity of the
e-liquid (Talih et al., 2017). Therefore, the particles generated
with a high VG content of the e-liquid may aggregate into large-
sized particles because of their viscosity (Wu et al., 2021).
The situation related to boiling point is similar: the more VG
there is in the e-liquid, the higher the boiling point is. Much
attention should be paid to this because reaching boiling point
by means of the coil leads to evaporation of the e-liquid and phe-
nomena that occur during the vaporisation process and over-
heating cause the degradation of products, with the formation
of harmful compounds (Talih et al., 2017; Wright et al., 2016).
The main difference between PG and VG is their hygroscopicity
– the ability to absorb moisture from the ambient air or human
respiratory system. For VG this property is expressed in the
ability to accumulate or supply moisture until equilibrium with
the ambient relative humidity is established. The hygroscopic-
ity of the substances increases with the number of hydro-
philic groups in their molecular structure. For that reason the
hygroscopicity of VG is stronger than that of PG (the number of
hydrophilic groups is three and two, respectively). The molec-
ular structures of PG (C3H8O2) and glycerine (C3H8O3) are shown
in Figure 1 (a) and Figure 1 (b) respectively. Hygroscopicity could
lead to growing particle sizes and changing the conditions
under which the experiment is performed if test samples are
exposed to the air for some time (Segur, 1953; Wu et al., 2021).
B3 RESULTS
An e-liquid consists of a solvent system (PG and VG) in which
other additives are dissolved in various mixture ratios. The
main qualities of those substances can be summarised as
shown in Table 1. The characteristics of each component of
the electronic liquids play a signicant role in the formation
of the aerosol; therefore, determination of the physical and
chemical properties is important in studying electronic nic-
otine delivery systems.
Propylene glycol is a colourless, odourless, non-turbid, vis-
cous liquid with a slightly sweet taste, the physical properties
of which are introduced in Table 1. It is easily miscible with a
range of solvents (water, acetone, chloroform, etc.; Jacob et al.,
2018; Technical Product Information, 2018). PG is a widely
used compound in many industries, such as the food and
tobacco industry, cosmetics, chemical intermediates, pharma-
ceuticals, paints and coatings, etc. In general, PG is classied as
a safe product according to the Food and Drug Administration
but it can irritate the respiratory tract and cause allergic reac-
tions (Kulhánek & Baptistová, 2020).
Glycerol is a clear, viscous, odourless, sweet-tasting hygroscopic
liquid that can be extracted from natural oils and therefore it
is called “vegetable glycerine” (Kulhánek & Baptistová, 2020).
Glycerol is a chemical compound, whereas glycerine refers
to commercial products that contain a percentage of water (a
glycerol-water solution). The physical properties of glycerol are
presented in Table 1. Glycerol is used in various applications,
including the pharmaceutical, plastics, and tobacco and food
industries, especially because of its useful solvent properties,
which allow it to combine solutions with water, methanol, etha-
nol, and glycol (Wernke, 2014).
Glycerine and its aqueous solutions have the following essen-
tial characteristics: low saturated vapour pressure; high hygro-
scopicity and boiling point; a decrease in density and viscosity
with an increase in temperature; less surface tension and spe-
cic heat of pure glycerine than water (Kulhánek & Baptistová,
2020; Segur, 1953).
Table 1 | General physical properties of glycerol and propylene glycol
Properties Units Glycerol Propylene Glycol
Density kg × m-3 1.261 1.036
Molecular weight g × mol -1 92.09 76.09
Viscosity Pa × s 1.49 0.0499
Boiling point ◦ C 290 187
Refractive index 1.47399 1.4329
Heat of vaporization kJ × kg -1 830 711
Specic heat capacity J × kg -1 × C -1 2350 2481
Surface Tension mN × m -1 63.4 26
Vapor pressure (20 ◦C) kPa < 0.00033 0.0131
Effect of Propylene Glycol and Vegetable Glycerine Ratio in E-Liquid on Aerosol Formation: Overview of ... 120ADDICTOLOGY
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The PG/VG ratio may affect the delivery of nicotine and actually
the taste. First of all, a larger ratio of PG can cause a “throat-hit”
effect associated with more nicotine delivery (Li et al., 2016). In
contrast, a larger ratio of VG leads to “cloud effects”. The “cloud
effects” phenomenon inuences product attractiveness, which,
therefore, leads to consumers buying products with more artis-
tic-looking exhaled aerosols more frequently (Brett et al., 2021).
Meanwhile, the “throat-hit” is associated with satisfaction and
the dependence on vaping because of the high nicotine ux in
the e-liquid containing PG (Etter, 2016; Goldenson et al., 2016).
This behaviour is the cause of the physical properties of the
components, in particular, optical properties, particle size, and
aerosol concentration.
One of the most signicant characteristics that should be taken
into account in aerosol particle analyses is the optical prop-
erties of the aerosol generated from the e-liquid. One of the
important parameters is the refractive index, which includes
real and imaginary parts reecting the light-scattering and light
absorption properties of the aerosol particles (Ingebrethsen et
al., 2012; Liu et al., 2015).
Neither PG nor VG absorbs in the visible spectrum wavelength
(400-780 nm), and thus they have a small imaginary part of the
refractive index (n→0) (Baassiri et al., 2017). It must be noted
that the real part of the refractive index is larger for VG than PG
for the visible wavelength range, inuencing more foggy “cloud
effects” of the aerosol generated from e-liquids with a higher
VG content (Wu et al., 2021). Baassiri et al. (2017) measured
the refractive index of a PG/VG solution with various PG/VG
ratios. They investigated the accretion of refractive index with
an increasing VG content. Moreover, they explored the effect
of the composition of the liquid on the light-scattering coef-
cient, and established that for 100:0 VG:PG this coefcient is
less than in the case of a 0:100 VG/PG ratio.
In contrast, the mass concentration of the measured total par-
ticulate matter (TPM) in the lters increases with an increasing
PG/VG ratio. It means that the more PG there is in the e-liquid,
the more concentrated the aerosol is. In that case, it is expected
that a more evident “cloud effect” will be seen, which contra-
dicts the preliminary conclusions. The aerosol generated by
an e-liquid containing more propylene glycol consists of par-
ticles smaller than that generated by an e-liquid containing
more glycerol. Nevertheless, there are two explanations for
that effect. Firstly, the lower vapour pressure of VG affects the
slower evaporation process of particles with a high VG content,
which leads to “long-lived” aerosol clouds. Secondly, an aerosol
with a higher PG content has a lower light-scattering coefcient
(Baassiri et al., 2017).
The reason for most of the effects is the volatility of the sub-
stances under study. The volatility of substances has been
associated with vapour pressure: the low vapour pressure of
VG causes less volatility. Such observations help draw the con-
clusion that a liquid with a greater PG ratio evaporates more
rapidly than a liquid with a greater VG content. Of course, it is
also affected by the enthalpy of vaporisation and specic heat
but since these parameters are almost equal for PG and VG, the
volatility could be only determined by vapour pressure. The
main reason for this behaviour is the difference in molecular
structure. The point is that VG has one more OH group than PG,
which leads to stronger hydrogen bond intermolecular forces
in the e-liquid solution (Li et al., 2021). E-liquid could contain
water, which characteristics must also be taken into account.
Water is more volatile than PG but has greater specic heat
Figure 1 | Molecular structure of propylene glycol (C3H8O2) Figure 2 | Molecular structure of glycerine (C3H8O3)
REVIEW PAPER121 ADDICTOLOGY
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capacity than PG at a given temperature. Hence, more energy
must be input to evaporate water with the same rate as PG
does. Consequently, PG evaporates faster than water anyway
(Baassiri et al., 2017).
The composition of the e-liquid also has an inuence on the
aerosol dynamics. Li et al. (2020) illustrated the correlation
between the particle loss rate and PG/VG ratio. In general,
the particle loss rate increases with a rise in the PG/VG ratio
from 0:100 to 100:0 without nicotine content, while adding
nicotine promotes the reverse behaviour. The reason for this
behaviour may be the saturated vapour pressure of nicotine,
which could impact on the overall e-liquid saturation vapour
pressure (Li et al., 2020).
The PG/VG ratio affects the nicotine ux. The nicotine ux
is the rate at which the aerosol is emitted from a vape device
per unit of time (Eissenberg & Shihadeh, 2015). That param-
eter is used as a potential regulatory value, being an essential
factor that can inuence the perception of aerosol emission
by users. Separately, Talih et al. (2017) previously showed
that an e-liquid with a higher PG content transports more
nicotine than an e-liquid with a higher VG content because of
its threshold for evaporation and therefore greater volatility.
Thus, PG may be called a vehicle of nicotine (Lechasseur et al.,
2019). That phenomenon is related to the “throat-hit” effect
that was discussed above (Baassiri et al., 2017). Spindel et al.
(2018) conrmed that a pure PG liquid or 55PG:VG delivered
more nicotine than an e-liquid containing VG. In that nding
users took shorter puffs using devices with PG content than
VG content but they obtained a higher plasma nicotine con-
centration (Spindle et al., 2018). Talih et al. (2017) described
a transport model that clearly explains the observation of
greater TPM emissions and nicotine in an e-liquid with a
high PG content and therefore conrmed the assumption of
greater nicotine transfer by PG (Talih et al., 2017).
Another important characteristic that is inuenced by the PG/
VG ratio is the particle size distribution. The particle size dis-
tribution may be measured by various techniques (Belka et al.,
2017; Fuoco et al., 2014; Ingebrethsen et al., 2012; Oldham et
al., 2018). Baassiri et al. (2017) showed that a PG-dominant
e-liquid emits smaller particles than a VG-dominant e-liquid
but, at the same time, the number of particles is greater in the
rst case. Identical results were obtained by Zhang et al. (2013).
Pourchez et al. (2018) also found that a high-PG e-liquid gen-
erates smaller particles than a high-VG e-liquid at low wattage,
while reaching 22 W leads to their convergence. Moreover,
Zervas et al. (2018) noticed that the particle size distribution
of propylene glycol gives a linear size distribution. In contrast,
the distributions of VG and a mixture of PG and VG have some
peaks that are connected to the high boiling point of glycerine
compared to PG.
VG and PG undergo decomposition to molecular carbonyl
compounds at high temperatures (Jiang et al., 2020; Y. Li et al.,
2021). El-Hellani et al. (2018) showed that there is no corre-
lation between the PG/VG ratio and the formation of carbon-
yls. But Kosmider et al. (2014) found that high-PG-level liquids
generate more carbonyls than VG-based e-liquids. The levels
of formaldehyde, acetaldehyde, and acetone are dramatically
increased with a higher PG content for a certain range of bat-
tery output voltage. This is particularly clear for a high bat-
tery output voltage, which indicates that the level of carbonyls
increases with increased battery output voltage, as it leads to
a higher coil temperature (Kosmider et al., 2014). Y. Li et al.
(2021) also found that some carbonyl degradation products
dramatically decreased with an increasing VG fraction in the
e-liquid but the fraction of acrolein rose in that case. Conklin et
al. (2018) conrmed that PG generates a higher level of acetal-
dehyde and crotonaldehyde, whereas VG leads to higher levels
of formaldehyde and the formation of the unsaturated aldehyde
acrolein. Moreover, they indicated that lower levels of carbonyls
were emitted by a mixture with a 25PG:75VG ratio, which indi-
cates that some proportions of PG and VG can reduce the level
of carbonyls. Such observations may be related to heat trans-
fer, which is strongly correlated with the chemical and physi-
cal properties of the components of e-liquids discussed above
(Talih et al., 2017). The emission of carbonyls is one of the key
parameters as it has adverse health effects. For instance, for-
maldehyde and acetaldehyde are classied as potential car-
cinogenic substances. Acrolein has a strong irritating effect
on the mucous membranes of the eyes and respiratory tract.
Overall, these compounds are potentially hazardous and their
consumption may be regulated by the PG/VG ratio or optimis-
ing the device’s characteristics.
Thus, the thermal degradation of products is the effect of the
coil temperature, which is determined by the composition of
the e-liquid. The coil temperature may also be inuenced by
the PG/VG ratio and the addition of avours, nicotine, and
water, which could change the heat capacity, boiling point, vis-
cosity, airow, and other relevant characteristics. For instance,
Korzun et al. (2018) noticed that the solvent consumption of
an e-cigarette increases signicantly with increased airow,
which could be a reason for the faster cooling effect. It may also
be assumed that the delivery of the e-liquid to the heating zone
occurs faster with a high PG content. In general, the charac-
teristics of coils and their dependence on devices’ parameters
have been examined in a number of studies (Saleh et al., 2020;
Y. Li et al., 2021; Zhao et al., 2016) but further research is still
needed, especially regarding the inuence of the composition
of the liquid on the coil temperature.
It is also noteworthy that the PG/VG ratio was found to have
an impact on the puff topography via taking larger and longer
puffs using a VG-based e-liquid in order to bring more nicotine
(Y. Li et al., 2021) However, the puff volume and puff duration
were not controlled, thus bringing the results obtained into
question. Therefore, that research study should be repeated in
order to gain accurate results.
A number of the properties that were discussed above may
have a huge impact on the taste experienced by the user.
Undoubtedly, the PG/VG ratio determines the particle size and
therefore its absorption. To be exact, the particles generated
by a VG-based e-liquid are mainly large, sweet, and with the
property of adhesion. On the other hand, e-liquids with a high
PG ratio emit particles with a clearly “true” taste because of the
small particle size, which penetrates into the alveoli.
Effect of Propylene Glycol and Vegetable Glycerine Ratio in E-Liquid on Aerosol Formation: Overview of ... 122ADDICTOLOGY
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B4 CONCLUSIONS
Overall, the composition of the e-liquid is the essential param-
eter in vape emission characteristics. This study highlighted
the features that have been determined by the VG and PG ratios
and their impact on the properties of the aerosol that is formed.
The attractiveness of vape devices (“cloud effect” and “throat-
hit”) is inuenced by the PG/VG ratio. The “throat-hit” is linked
to the possibility of PG evaporating rapidly compared to VG,
while the chemical structure and capacity to generate particles
with a greater refractive index and light-scattering coefcient
of VG leads to clear visual effects that are preferred by users.
The properties of PG and VG described in this study help to
establish a link between the thermodynamic properties of the
e-liquid and transport phenomena during vaping that regu-
late the quantity of thermal degradation products in general.
On the basis of this fact, the most important eld of the impact
of the PG/VG ratio is the emission of carbonyls, as their action
is directly related to the user’s health. Formaldehyde, acetal-
dehyde, acrolein, and other compounds have adverse health
effects such as causing damage to the lining of the lungs and
irritation of the mucous membrane and can also cause cancer,
etc. The regulation of the PG/VG ratio may reduce the emission
of these compounds and thereby minimise the risk to users’
health. The selection of the safest PG/VG ratio could be investi-
gated with deep knowledge of the chemical and physical prop-
erties of PG, VG, and heat transfer parameters.
Further study of the dependencies of the base components
of e-liquids and other device characteristics is important in
research into electronic nicotine delivery systems. The attrac-
tiveness and health safety of ENDS are essential aspects of
the vaping industry and both of them are regulated by the PG/
VG ratio. In addition, other features play a signicant role in
aerosol formation and its composition, for instance, heating
element characteristics. The optimisation of heating element
characteristics and the composition of the e-liquid is a way of
improving vaping devices.
Broadly speaking, the properties described above are deter-
mined not only by the PG/VG ratio, but also by the addition
of avours and nicotine. Consequently, studying aspects of
their impact is also important in studying the emissions from
vape devices.
Declaration of interest: No conflict of interest.
REVIEW PAPER123 ADDICTOLOGY
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