A full guide to vapes: Post 1.
Understanding vape aerosols
Summary
This is the first Substack post of a series of posts describing how vapes work, the aerosol they generate, their properties, their optimal regime of operation, overheating conditions and dry puffs, as well as comparisons with tobacco smoke and other aerosols.
Understanding how vape aerosols form, operate and can be tested provides the knowledge to understand their pleasurable usage, their toxicity profile and relative safety with respect to tobacco smoke and other aerosols and pollutants.
This knowledge (presented without unnecesary technicisms) reassures our confidence on the role of vapes in harm reduction and serves us to counter ignorant and malicious disinformation.
Post 1:
The harms from smoking do not arise from nicotine, but from its harmful delivery vehicle: tobacco smoke (Michael Russell). Vaping replaces this harmful vehicle by what we colloquially call “vapor” (hence we “vape”), though we know it is not really vapor, but an “aerosol” to be contrasted with the “smoke” (another aerosol) that emerges from cigarettes.
However, the definition of “aerosol” covers many substances and phenomena, including tobacco smoke, with the differences between aerosols clearly reflected in their formation processes and in their properties.
In this post I explain vape aerosols as one of the type of “liquid phase” aerosols, formed from two phase changes (evaporation and condensation) as a liquid is heated under normal boiling. Vape aerosols are analogous to the familiar aerosol that forms in the kettle snout as we boil water for tea.
However, e-liquids are chemically more complex than water, so together with these phase changes the heating process forming vape aerosols involves low energy reactions that generate byproducts that (under normal conditions) appear in minute quantities.
What is an aerosol?
We need first the following quick definition :
AEROSOL (definition): a substrate of microscopic particles (particulate phase) suspended and transported by a gaseous medium (gas phase)
Both vape aerosols and tobacco smoke fully comply with this definition, though strictly speaking the term tobacco smoke denotes three related but distinct aerosols.
Besides vape emissions and tobacco smoke, there are many aerosols, in nature and also human made. A wide variety of phenomena comply with the definition of “aerosol”: clouds, fog, smog, air pollution, automobile exhaust, chimney smoke, sand storms, volcanic eruptions, household sprays, odorizers, candles, cooking, the “vapor” coming out of the tea pot, in the shower, sauna, etc.
What distinguishes one aerosol from another is the type of particles (solid/liquid, chemical composition, numbers, sizes) and the type of gas medium (typically gas mixtures). I describe first the formation of aerosols in a kettle as a simplified proxy for vape aerosols.
Vaporizing and boiling
Aerosols form through many physico-chemical processes. A simple example is the aerosol perceived as a visible cloud pouring from an electric kettle when boiling water for tea. This aerosol is produced by various processes, it is not instantaneous and does not involve initially the full body of water in the kettle.
The first process is “normal boiling”. As heat is supplied by the kettle heat element (a metallic resistance), small air bubbles form. As more heat is supplied the bubbling becomes more intense, water molecules in the liquid gain enough energy to form larger bubbles filled with water vapor (water in gaseous state) around the air bubbles. Evaporation occurs when bubbles in the liquid surface burst releasing water vapor.
As more heat (energy) is supplied water temperature stabilizes at 100°C, the boiling point temperature at sea level atmospheric pressure, with the supplied energy used for the vaporization of water (generation of water vapor) at a steady rate. At this point we can say that water is under normal boiling, with the water pressure balancing the pressure of the vapor contained in the surrounding air.
Typically kettles automatically turn off (or we turn them off) at the onset of normal boiling well before all water evaporates. For a cup of tea we only need water to reach its boiling point (we do not drink water vapor). Nevertheless, once water has reached its boiling point sufficient vapor mixed with air has been formed, whose pressure and temperature exceed pressure and temperature of the surrounding air.
If more heat is supplied, coil temperature eventually surpasses boiling temperature, passing to a stage known as “nucleated boiling”. More vapor is produced but no longer in a steady pace, with bubbles becoming unstable, merging and clustering into larger ones (bubble nucleation).
Further heat supply produces rapid increase of coil temperature and gradual coalescence of the bubbles until they form an isolating film surrounding the heating element with vapor that traps and retains heat. This stage is called “film boiling” and things can become messy and unstable.
When we prepare tea we turn power off to remain in “natural boiling” and avoid going into into nucleated boiling and beyond, producing only sufficient evaporation to reach the boiling temperature.
Pleasant vaping also requires remaining in “normal boiling” conditions, which strongly depend on a delicate balance between supplied power and the capacity to evacuate and cool sufficient e-liquid vapor to form the aerosol (we will see this further in the next post)
Liquid phase aerosols
As normal boiling initiates there will be sufficient vapor mixed with air inside the kettle, whose pressure and temperature are larger than outside pressure and temperature.
This pressure difference generates a driving force (a convective pressure gradient) in the air/vapor mix close to the kettle snout, spouting the hot water vapor mixed with air into a colder exterior air.
As the gas (water vapor mixed with air) is driven outside, it cools and condenses. The term “condensation” means the opposite of evaporation: the same substance (water) passing from gas to liquid.
In the case we are describing it is tiny parts of the water vapor (gas molecules) clustering and forming microscopic liquid droplets that move along the same vapor. An aerosol has been formed. !!!
This is the process of formation of an aerosol is known as “nucleated condensation”. The cloud seen emerging from the kettle spout is the visible manifestation of an aerosol whose “particles” are liquid (the water droplets) and the “gaseous medium” the water vapor mixed with air.
How come we see the cloud, but we normally do not see air (which is transparent)? Pure water vapor (or any pure gas) is invisible, i.e. it is fully transparent, but we now have water vapor mixed with air transporting millions of tiny microscopic water droplets.
For this liquid based aerosol, the droplets are spherical and have diameters of about 1/1000 millimeters. These millions of water droplets are sufficient in numbers and are sufficiently small to deflect and disperse light, hence we see them collectively as a cloud.
We can say that a vape is some sort of mini-kettle, since:
Both vape and kettle heat and vaporize liquids through electric energy supplied as heat to a coil.
Vaping aerosol forms in a very similar way as the kettle aerosol and has similar properties as the water aerosol formed in the kettle,
However, instead of water what boils and vaporizes in vapes is the e-liquid mixture, made of two solvents: propylene glycol (PG) and glycerol or vegetable glycerin (VG), nicotine and flavoring chemicals (plus pollutants at trace levels).
Boiling temperature is different for each e-liquid mixture, ranging from 180 °C for pure PG to 288°C for pure VG, with intermediate values in the range 180 °C to 288°C for different mixture components. The spouting of the e-liquid vapor is also driven by a pressure gradient, but now it originates from and is driven by the user’s inhalation.
As with the kettle:
the vapor (now a vapor of the e-liquid) is almost instantaneously transported by the airflow of the user inhalation, mixing with air, cooling and condensing
liquid droplets form by nucleated condensation and are transported by the inhaled flow: the aerosol is formed
the “particles” are liquid droplets made of same compounds as the liquid and the gaseous medium is a mixture of e-liquid vapor and air.
the vapor condensation to form the aerosol occurs locally, involving the e-liquid around the cotton wick in the coil, just as the kettle aerosol only forms locally in the kettle snout
However,
we do not see a cloud because the aerosol is completely inhaled, we see the cloud once it is exhaled
The analogy with the kettle aerosol is more accurate with exhaled environmental vape aerosol, which is a diluted version of the inhaled aerosol because users retain most of the inhaled aerosol mass (including the nicotine).
The term “phase” in the definition of aerosol refers to one of the three states in which thermal substances exist: liquid, vapor (or gas) and solid. The vape and kettle aerosols, are then substances that contain in themselves two phases: the gas phase and a liquid phase, since the particles are liquid droplets.
Phases of a substance can change with changes of temperature and pressure. The formation of vape aerosol involved only these two phase changes:
Evaporation: liquid to gas.
Condensation: gas to liquid.
together with very low energetic chemical reactions triggered by the heating process that generate trace levels of byproducts. The formation of many other aerosols (like smokes) is far more complicated, involving several phase changes like sublimation (solid to gas), as well as numerous very energetic chemical reactions.
Vape aerosols and their byproducts
The analogy between kettle and vaping aerosols is conceptually useful, but it has obvious limitations, mostly because e-liquid mixtures are chemically more complex than water, consisting of at least three main compounds (PG, VG, nicotine) plus dozens of flavor chemicals and traces of pollutants.
Once the aerosol forms, the distribution of each compound in the gaseous or particulate phases (the phase partition) depends on the conditions for evaporation and condensation, which are more complex than for individual compounds. In particular, it is the volatility (the capacity for evaporation) that mostly determines the phase partition of each compound.
At its generation the aerosol achieves a phase partition for each chemical. Being PG a lighter and very volatile molecule, it tends to be in the gas phase, with the heavier and less volatile VG tending to be in the droplets, while nicotine volatility varies between nicotine salts and base and also depends on the liquid PH, hence its phase partition can be very varied and complicated.
Some flavoring compounds transfer unchanged from the liquid to the aerosol, but most decompose as byproducts that join the byproducts that form from PG and VG from the heating process.
As the aerosol evolves from its formation into the respiratory tracts, there is no longer heat supply, hence only phase changes that modify the phase partition and droplet sizes through other physical aerosol processes whose effects depend on particle sizes and chemical composition (coagulation, nucleation, impaction, gravitational settling, diffusion).
The chemical composition of the aerosol is almost the same as that of the liquid, but the heating process adds new compounds (between 80 and 150), as byproducts formed by temperature dependent chemical reactions known as thermal degradation (or low energetic pyrolysis), in which larger molecules decompose into smaller ones.
These reactions mostly decompose PG, VG and flavor chemicals into chemicals known as aldehydes that are quite volatile and thus appear mostly in the gas phase. But since these reactions are not efficient at aerosol formation temperatures under normal conditions close to the boiling point of the e-liquid (180-288 °C), their resulting byproducts appear in the aerosol at minute trace level concentrations.
Under normal vaping conditions (which will be discussed in the next post), most of the 80-150 byproducts of the thermal degradation reactions appear in negligible amounts barely above the detection or quantification limits.
Among this byproduct jungle the most abundant (or rather the less negligible) and most commonly detected aldehydes are formaldehyde, acetaldehyde, acrolein, with typically formaldehyde making the largest yield.
Most emission studies (under normal conditions) report values for each of these aldehydes below 1 micrograms or 0.001 milligrams per puff (1 microgram is one part in one million of a gram), which are considerably lower yields than those of same compounds found in the smoke of the cigarettes used in laboratory tests: 7.5–12.5 mg/ puff (formaldehyde), 50–150 mg/puff (acetaldehyde), 7.5–15 mg/ puff (acrolein).
The detection of 10 micrograms of formaldehyde per puff in laboratory emission tests already signals problematic testing, either the obsolete “top coil” devices or machine puffing bordering on excessive supplied power.
Around 2015 several studies reported aldehyde levels above 300 mg/puff, far exceeding levels in tobacco smoke. However, the replication of these studies by Farsalinos and coworkers showed that such high levels were produced under extreme abnormal vaping conditions (the “dry puff”), generating an aerosol that would be repellent to users.
What’s next? Prelude to Post 2
We need to explain what are the “normal conditions” for vaping. The generation of vape aerosols involves various thermal processes of energy exchange and balance. Hence, the devices must be operated between specific ranges of parameters (supplied power, inhalation airflow, coil resistance, PG/VG ratio, nicotine type and concentration) that allow for these processes to unfold efficiently. These parameters define an Optimal Regime that can be tested in the laboratory, with a series of problatic outcomes taking place when operating the devices outside these ranges.
Look forward to reading more Roberto. Thanks.