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.