Technical specifications and device evolvement
E-cigarettes different models share a basic operating mode. The
mouthpiece allows the vaper to draw air, whose flow activates a sensor,
causing the heating of a filament inside the atomizer. A capillary action
brings the liquid to the filament. The warmed filament vaporizes the fluid;
then, the condensation of the produced gas with atmospheric water
generates an inhalable aerosol. Products lacking the air-flow sensor are
provided with a button whose pressure closes a circuit that activates
the battery31.
Throughout the years, e-cigarettes design and technical features
evolved, providing the market with updated products meeting consumers’
different demands. First generation ECs, usually referred to as
cig-a-likes, have been conceived to resemble the design and feeling of
traditional cigarettes. They are equipped with low voltage batteries,
available in different versions: the 3-piece style, composed of
independent atomizing unit, battery, and fluid reservoir; the 2-piece
style, with the battery being the only separable element; the 1-piece
disposable, to be discarded after one use. Since the fragile atomizers
can be easily damaged, variations in performance aren’t
unlikely32,33. In second generation ECs, known as
“clearomizers”, larger batteries of variable voltage are provided with
a removable atomizing unit enclosed in a shell which is screwed into the
fluid reservoir and the battery. Their larger fluid reservoirs are
fillable34–36. Using a third generation EC
(“mods”), in which the reservoirs disassemble, the consumer can
regulate battery voltage and power. The atomizing units exists in three
versions (various styled, replaceable dripping, sub-ohm). Despite some
metal components being absent, the overall amount of metal is greater.
The concurrent increase in battery power makes third generation products
able to release higher concentrations of metals into the aerosol.
Furthermore, the presence of two filaments in some atomizers enhances
heat distribution, resulting in a more abundant production of
aerosol37. For the replaceable dripping atomizers,
vapers build their own coils and drip the fluid directly onto them;
otherwise a fluid thank encases the atomizer37,38.
E-cigarettes modernisation process helped contain health implications.
In early models, tin solder joints tied the filament to a thicker wire.
These joints could be friable, eventually releasing tin in aerosols, a
flaw remedied by coating the thick wire with silver, using stable tin
solder joints outside of the atomizer, or joining wires by clamping or
brazing rather than soldering. The thick wire, made of nickel or copper
coated with either tin (associated with stannosis and
pneumoconiosis39) or silver, was not included in
second generation products and later40. By removing
the silicon sheath from second and third generation products, its
presence in aerosols drastically decreased41.
Nonetheless, the empowerment in 2nd and
3rd generation batteries, accompanied by the increase
in atomizer size and mass of metal, allowed to generate larger amounts
of aerosol37, resulting at the same time in a greater
transfer of particles, metals, toxicants35,36.
Furthermore, as voltage/power ratio increased, new potentially toxic
by-products could emerge from the liquid41. Likewise,
in larger reservoirs such as those of second and third generation ECs,
fluid stagnation could enrich aerosols with additional toxicants through
repeated use36.
Fourth generation ECs, referred to as pod mod devices and equipped with
fix voltage batteries, have become popular among teenagers as a socially
acceptable alternative to conventional cigarettes due to their stylish
design (e.g. USB or teardrop shape), wide selection of flavours and
user-friendly functions42,43. Their likeness to an USB
memory stick allows them to be discretely used in no smoking areas and
easily concealed from parents, contributing to a new widespread
phenomenon, known as “stealth vaping”19,44,45. A
distinctive feature of fourth generations devices is the use of nicotine
in its protonated form, which reduces the irritating effect on throat
mucosa while increasing the amount of nicotine delivered in
aerosols46.
The heterogeneity outlined above complicates research on potential
health effects, since the variability in design and technical features
prevents us from discussing e-cigarettes as a single device. For
instance, power output affects yield and aerosols content: in order to
resemble cigarettes, closed systems are provided with lower-voltage
batteries and/or higher resistance heating elements47,
whose thermal breakdown can produce toxicants normally absent in
traditional cigarettes48. Devices allowing the user to
drip liquid onto the heating element can generate an amount of aldehyde
equal or higher than tobacco cigarettes, due to the high temperatures
reached49. Furthermore, besides type and age of the
device, e-cigarette health impact depends on multiple variables
including ambient factors (e.g. climate conditions, room size and
density of people) and user’s habits (puff length and
frequency)50.