A full guide to vape aerosols: Post 8, environmental aerosols (part 1)

This is the eigth Substack post. It is the first one of a series of 3 posts dealing with environmental vape aerosols.

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This is the 8th Substack post, the first of a series of 3 posts describing the environmental vaping aerosols that users release to the surrounding air when they exhale. Since exhaled vape aerosols expose bystanders and non-users, it is important to understand and analyze their properties, to compare with inhaled aerosol, environmental tobacco smoke and other pollutants. Al this is needed to provide knowledge to asses objectively risks to bystanders. Without being “experts” this knowledge reassures our confidence on the role of vapes in harm reduction and serves us to counter ignorant and malicious disinformation

Summary of previous posts:

Post 1. Basics: what is an aerosol? Vapes and kettles. Byproducts of the heating process. Post 2. Physical processes in vaping. Essentials of laboratory testing. Optimal Regime. Post 3. Overheating: exponential production of toxic byproducts and the “dry puff”. Post 4. Laboratory testing. The CORESTA standard and the evolution of the vape market. Post 5 Metals. Post 6: Organic byproducts. Post 7: vape exposure in preclinic studies

Environmental vape aerosols.
Post 8: Part I, general properties.

Vapers exhale the aerosol they inhale (just as smokers exhale inhaled smoke). In both cases the exhaled stuff (visible clouds) are also aerosols, but they have substantially different physical and chemical properties from the inhaled ones. Since bystanders are exposed to these environmental aerosols, it is important to understand their properties to assess their potential risks.

NOTE ON NOMENCLATURE. I will not use the adjective “second hand” to denote aersols that originate from users exhalations (ie “second hand smoke” or “second hand aerosol”). Instead, I will refer to them as environmental tobacco smoke (ETS) and exhaled vape aerosol (EVA)

MOTIVATION. There is a broad consensus that non-smokers surrounding smokers face risks from exposure to the environmental tobacco smoke (ETS). Vapes emerged as harm reduction products that substitute a product (tobacco cigarette) that delivers nicotine through a toxic vehicle (tobacco smoke) with a product (vape) that delivers nicotine through a much safer vehicle (vape aerosol). The user (a vaper) does not inhale smoke, inhales an aerosol not produced by combustion, but by the condensation of the vapor of a liquid mixture (see aerosol basics in Post 1). However, vaping also exposes non-users to an exhaled aerosol. This poses an important question of public health interest: should exposure to Exhaled Vape Aerosol (EVA) be also a concern like exposure to ETS or to other environmental pollutants? In this post and the following ones I will address these issues.

Background: Environmental Tobacco Smoke (ETS)

Tobacco smoke is a highly complex mixture of three aerosols (normally called “emissions”):

• Inhaled Mainstream Smoke (IMS): the smoke that the smoker inhales

• Exhaled Mainstream Smoke (EMS): the smoke that the smoker exhales

• Sidestream Smoke (SS): the smoke emerging from the smoldering tip of the cigarette

• Environmental Tobacco Smoke (ETS): a fresh mixture of exhaled and sidestream smoke that ages as it interacts with environmental pollutants

Tobacco smoke is produced by combustion, but combustion involves several processes. Inhaled Mainstream smoke (IMS) originates in two processes: (1) lighting of the cigarette (flame ignition) at 700-800 °C. Flame ignition is an exothermic thermal process: energy in the form of heat is released from an external energy source (the flame) in the presence of oxygen (reactant) into the tobacco biomass (fuel), producing very energetic, self-sustainable, chemical oxidation reactions that generate hundreds of new organic compounds. (2) As the user inhales more oxygen is supplied, further activating these exothermic reactions and rising the temperature of the cigarette tip up to 950°C.

This vast amount of energy (heat) at 950°C released into the tip of the cigarette must be absorbed and transported by the cigarette, a cylindrical rod 10 cm long 1.5 cm wide made of tobacco leaves, paper and a filter. The final IMS forms by this energy transport and absorption, involving very complex endothermic (ie energy absorbing) chemical reactions that generate most of the 7000 detected compounds and decrease temperature (cooling) to 40°C to avoid harming the smoker’s mouth.

The main physicochemical processes that take place in the cigarette rod are: distillation (separation of gas and particles), pyrolysis (splitting large molecules into smaller ones), pyrosynthesis (when pyrolysis byproducts react to form new molecules), condensation (gases forming liquid particles), evaporation (gases from liquid particles).

Between puffs the sidestream smoke (SS) forms from the cigarette smoldering at the tip of the cigarette at 450 °C. There is no longer an external energy source (no flame) and no large oxygen supply (no inhalation), but oxygen is available from the surrounding air and the exothermic reactions in the tip of the cigarette are sufficiently self-sustainable (smoldeing) to require sufficiently long time (more than typically inter-puff times) for extinction. The SS is a different aerosol (a different smoke) from the IMS, as it was not affected by the endothermic reactions in the cigarette rod, so it has different (lower) density, temperature and chemical composition, it has larger share of solid particles and volatile gases (CO and nitrogen oxides).

The processes described above explain the complexity of ETS, which can be roughly described by two forms:

• Fresh ETS. It is a diluted mixture of (1) the SS that is continuously emitted (even when the smoker is not inhaling), it makes about 80% of the smoke freshly released into the environment (2) the exhaled mainstream smoke (EMS), the product of the interaction of IMS with the respiratory tracts (a sort of biological lab under very humid conditions), so it is a different and more diluted aerosol

• Aging ETS. The mixture of exhaled and sidestream smoke dilutes even further as it diffuses and interacts with air pollutants through various types of reactions (oxidation, photochemical), which further modify its chemical composition (it is a “secondary” aerosol).

There are important common characteristics (signatures) in these aerosols: most of the gases of fresh and aged ETS come from the EMS, while most particles come from the SS. The particulate phase of all these aerosols is made of ultra fine particles of very small diameters (< 300 nm), whose composition is mostly non-volatile and semi-volatile compounds (complex hydrocarbons with some inorganic compounds), while the gas phases are mixtures of volatile gases. These are common features of aerosols generated by combustion (air pollution, chimneys, etc).

Key properties of ETS: its very small sized particles and their non-volatile and semi-volatile composition implies very long airborne permanence in the environment. Their mean half life is 20-40 minutes (for each exhaled puff), but permanence time can be up to several days (depending on ventilation), until they deposit in surfaces (walls, furniture, clothes), settle into the ground or are removed by forced ventilation. Volatile gases with low molecular weight (CO, CO2) spread rapidly and interact with environmental pollutants and also remain long times in the environment.

Exposure to ETS is NOT intermittent. The physicochemical properties of ETS explain our widespread sense and smell of ETS lingering in indoor spaces well after smoking took place. These properties also show that indoor exposure to ETS is not intermittent, that is: it does not last only during the seconds/minutes while the smoker smokes, it lasts much longer times and spreads through larger distances in all directions. Evidently, exposure doses can reach high levels, specially long exposure times in poorly ventilated spaces. Smoke free policies in public indoor spaces are well justified.

Comparison with vape aerosols

Vape aerosols lack (by far) the physicochemical complexity of tobacco smoke. The first important difference is the absence of a sidestream emission SS: there is no smoldering tip releasing something into the environment. As opposed to tobacco smoke, we have basically only two aerosols

• Inhaled Mainstream Aerosol (IMA): the vape aerosol that the vaper inhales

• Exhaled Mainstream Aerosol (EMA): the vape aerosol that the vaper exhales, which is indeed the “Envirohmental” Vape Aerosol (EVA)

Not really aging. That’s it: we only have an inhaled and an exhaled aerosol, the one the vaper draws into the respiratory system and the one released to the environment (the EVA). But, doesn’t EVA age like ETS? is there an “aged” EVA?. It depends on what we mean by “aging”, which I discuss at the end of the post.

Vape aerosol is completely different from tobacco smoke. Vape aerosols are formed by condensation (see Post 1 ) of the vapor formed by heating at boiling temperatures (188-288 °C) a liquid mixture made of propylene glycol (PG), glycerol or vegetable glycerine (VG), nicotine, favoring compounds and water (plus negligible impurities). While generation of tobacco smoke involves extremely energetic oxidation reactions, the heating process to generate vape aerosol is basically only a change of phase (liquid → vapor → aerosol), accompanied by low energy pyrolysis reactions (heat degradation, see Post 1, Post 2, Post 6 for detail).

Phase partition. The “particles” are liquid droplets made almost 100% of PG, VG, nicotine and water, with negligible contribution of thermal degradation organic byproducts (Post 6) and an ultra negligible (one in 10 million) contribution of nanoparticles made of metallic oxides (Post 5). The gas phase is made of same compounds (PG, VG, nicotine, water), but contains the most abundant (or less negligible) organic byproducts (aldehydes) that are very volatile. Flavorings also tend to be in the gas phase. Since PG is much more volatile than VG, it tends to evaporate into the gas phase, while VG tends to remain in the droplets (this has been studied by Parmentier et al)

Vape aerosols as they are inhaled

Inhalation involves going into a very wet humid environment. Initially, as it is produced by condensation of the vapor in the wick and the inhalation conduct, the droplets are very small (typically average of d ~ 200 nm). However, as the aerosol is drawn by inhalation (forced convection) towards the vaper’s mouth several competing phenomena of aerosol physics take place: the droplets continue condensing/evaporating, they also collide (with themselves and with walls), coalesce, agglomerate and hygroscopically coagulate (absorb water) into larger droplets. As they enter the very wet and humid environment of the mouth, hygroscopic coagulation might defeat evaporation.

The diameter size and mass distribution of the droplets changes, with the peaks of the mass distribution shifting to large diameters, since the fewer big droplets can have masses thousands of times larger than much more numerous small ones. However, there is still a broad diameter distribution in the range 0 to 1000 nm, with possible peaks diameter around 200 nm, but mass distributions might show peaks at 5 or even 20 micrometers. The gas phase composition is affected by these phenomena, specially droplet evaporation/condensation. The phenomena that vape aerosols experience as they enter into the user’s mouth were modeled by Floyd et al

Once inside the respiratory tracts the PG/VG and nicotine content of the droplets and the gas phase varies because the phase partition (which compounds are in the droplets or in the gas) and the composition of each phase is sensitive to the rates of condensation/evaporation and hygroscopic coagulation, settling in the respiratory walls and conducts, all of which affect compounds differently: the very volatile PG is much likely than VG to be absorbed and retained, while with nicotine this depends on the aerosol PH.

Respiratory deposition models. Modeling the evolution of an aerosol as it goes from the mouth, to the pharinx, trachea and alveoli is very complex. It has been studied through deposition models of therapeutic aerosols and inhalable medication [see Hickey, A. J., & Mansour, H. M. (Eds.). (2019). Inhalation aerosols: physical and biological basis for therapy. CRC press.]. There are also deposition models for tobacco smoke, for bio-aerosols and for pharmaceutic aerosols. Typically these models find a “U-shaped” size dependent deposition pattern for droplets:

Figure 1. Total deposition vs particle size for a pharmaceutic aerosol. Source: Heyder et al J Aerosol Sci, 17, 811, (1986)

This is taken from one of the pioneering studies, notice that larger droplets (d = 10-15 micrometers) and the smallest ones (d = 0.01 - 0.05 micrometers) have high deposition rates, the former tend to impact the walls of the mouth, pharinx and trachea, while the latter reach the upper tracts and diffuse through the alveoli, with intermediate droplets (low part of the “U”) showing smaller deposition rates. The most comprehensive of such models for vaping was undertaken by Asgharian et al that showed a continuous growth of particle diameters

Figure 2. Droplet growthe from hygroscopic coagulation. Asgharian et al..

This study traced only the evolution (hygroscopic growth) of a single droplet of d = 500 nm. Growth is larger for initially larger droplets but occurs in all sizes. Asgharian et al also plotted the uptake (retention) fraction of nicotine, G and VG in the oral cavity, tracheobronchean (TB) and pulmonary (PUL) regions

Figure 3. Uptake (retention) fraction for PG, VG and nicotine Asgharian et al

Notice the lowest retention in the TB and higher in the oral cavity (large droplets) and pulmonary (small droplets) regions. Also, their model predicts nicotine and PG total retention over 90% and VG around 80%. These retentions of the main aerosol constituents (PG, VG, nicotine) is consistent with the observational study by St Helen et al Aldehydes also show very high retention rates (see Samburova et al and Hopstock et al).

The inhaled vape aerosol experiences mostly physical changes when inside respiratory tracts (a sort of biological lab). Inside the respiratory tracts we can consider the droplets as immersed in a vapor medium made by the PG dominated gas phase diluted in circulating air. Since evaporation/condensation are of secondary importance with respect to hygroscopic coagulation and other aerosol phenomena, there is a rough thermal equilibrium in which the droplet liquid pressure roughly balances the atmospheric pressure of the gas.

The thermal equilibrium in which evaporation and condensation cancel each other is called “saturation” and is evaluated by the saturation ratio S = Pa/Pd =1, where Pa is the air atmospheric pressure of the gas medium (gas phase of the aerosol diluted in air) and Pd the partial pressure of the liquid droplet (“partial” means the pressure it would have if it was without the gas). The notion of relative humidity (RH) is defined by associating RH = 100 with S = 1, so that RH = 110 corresponds to S = 1.1 (supersaturated, evaporation dominates) and RH = 70 to S =0.7 (unsaturated condensation dominates).

Vape aerosols as they are exhaled

Dilution and phase partition. As shown by lung models and observations > 90% of PG, > 90% of nicotine and >80% of VG are retained. Therefore, the vape aerosol that is exhaled is very diluted, its mass is about 10% of the inhaled mass (~ 90% was retained). It has been purged of most aldehydes content (also >90% retained). Since PG is the dominant compound in the gas phase and VG in the droplets, their large percentage of retention implies similar retention in both phases and thus the phase partition of exhaled aerosol should remain approximately similar to that of the inhaled aerosol.

Exhalation conditions favor droplet evaporation. The exhaled aerosol goes from a very humid saturated environment (almost 100% relative humidity) at practically constant temperature of 37 °C into a much colder and dry surrounding air (in normal environments vapers do not vape at ~100 RH and 37°C). In such colder environment, the droplet liquid pressure decreases while the gas medium remains at fixed atmospheric pressure. The saturation ratio increases to supersaturation S > 1, which is a common phenomenon when a saturated vapor is cooled by adiabatic expansion (no heat exchange from surrounding air). Cooled supersaturated vapors with adiabatic expansion can be generated in the laboratory by gas exiting from a nozzle (see Hinds W.C. Aerosol Technoloy), so it is a good approximation for exhaled vape aerosol. These conditions favor evaporation, as a decreased droplet liquid pressure facilitates more molecules in the droplet to migrate to the gas medium.

Droplets evaporate. The fact that physical conditions of exhaled vape aerosol favors droplet evaporation in the gas medium (gas phase of the aerosol diluted in surrounding air) has been experimentally verified. In fact, this is a generic characteristic of liquid droplets of aerosols based on glycols (see Wright et al). However, PG being more volatile evaporates more rapidly and efficiently than VG, with nicotine evaporation depending on its PH (or its acidity).

The rate of evaporation of PG and VG was quantified in the experiments of Parmentier et al with Raman scattering, whose main results are displayed in Figure 4 below

Figure 4 Evaporation of PG, VG and water. The graphs plot % of the total droplet mass. see Parmentier et a

The droplet looses 50% of its mass in 20 seconds (figure 4 of Parmentier et al, not displayed), with rapid decay % of PG in the droplet mass, while VG increases forming over 65% of the mass (nicotine evolution is displayed in Figure 5). However, the authors considered a single droplet of 4 micrograms diameter in laboratory conditions, which are not realistic even for an inhaled aerosol. Droplets in exhaled vape aerosol have smaller initial diameters and evolve in a disruptive turbulent and dynamic air medium, hence they evaporate faster and lead to smaller final diameters.

The time evolution of nicotine was also analyzed by Parmentier et a, as shown by Figure 5, the % respect to the droplet mass increases in protonated (acidic) nicotine and decreases in nicotine base

Figure 5 Evolution of the % of nicotine in the droplet mass. Parmentier et a

Both panels used e-liquid with 5% nicotine, the left one with 9.9 PH (nicotine base), the right one with 6.5 PH (nicotine salt).

Temporal and spatial variation of droplet distributions

Chamber studies. The evaporation of droplets in exhaled vape aerosols is clearly evident in several well conducted chamber studies analyzing the aerosol exhaled by one or several voluntary vapers. Chamber studies take place in a (typically) closed small room (< 200 cubic meters), with different ventilation regimes, authors often request the vaper to puff an e-cigarette with a predermined puffing plan. Instruments (typically impact cascade particle counters) are placed at various distances from the vaper, who most times is instructed to puff towards their direction. Often the experiment involves a smoker puffing a tobacco cigarette for comparison purpose.

Figure 6. Example of a chamber experiment. The blue arrow shows the direction of vaping jet. Source

Although these are idealized and non-realistic experiments, they can provide useful information if the vaping volunteer is not vaping naive and if the puffing plan is not unrealistic (it bears some minimal connection with real life vaping frequency).

Exhaled vape aerosols cannot be reproduced by vaping machines, since this makes it impossible to account for the change in its physical properties from the evolution of inhaled vape aerosols in the respiratory tracts (specially the high retention rates in both phases). Studies having used vaping machines to mimic exhaled vape aerosols are either useless or of extremely limited utility.

Vaping is intermittent. The temporal evolution and spatial (distance) dependence of exhaled vape aerosols is very important to fully understand its properties and the potential risks of bystander exposure. Unfortunately, the temporal and spatial variation of exhaled aerosols has been examined in vary few studies.

The fact that vaping (as opposed to smoking) is intermittent is based on the rapid evaporation of the droplets in exhaled vape aerosol. This is captured by the sawthooth profile of particle number density: rising abruptly in the inhale/exhale cycle and rapidly dropping to near background levels until the next puff. See figure 7 below taken from Martuzevicius et al

Figure 7 average time profile of particle number densities at 0.5, 1.0 and 2.0 m from the vaper in the direction of the jet (Figure 2 of Martuzevicius et al)..

Notice how the peaks decrease with the distance to the emission of the aerosol (the vaper’s mouth), at 2 meters the aerosol is unperceptible. The same sawthooth profile was obtained by Zhao et al for the mass density (micrograms per cubic meter)

Figure 8, average mass density time profile taken from Zhao et al

Notice the fading gray curve at the bottom (pointes by the red arrow) showing that at 1.5 meters away from the source the mass density of the aerosol is practically indistinguishable from the background. Zhao et al also show the decrease of particle density with distance reaching almost background levels at 2 meters

Figure 9. Decay of particle mass density with distance.

Comparison with environmental tobacco smoke. Oldham et al measured the timeline of particle number density in of 4 hours of ad libitum smoking and vaping

Figure 10. Time evolution of particle number density in 4 hours of smoking and vaping in a home scenario. Oldham et al

Notice that smoking also shows a sawtooth profile at much higher particle numbers, but it oscilates around a large value of 10^8 particles/cm3 during the 4 hours, while in vaping the sawtooth profile reaches almost background levels after each puff. This graph shows that ETS stays in the environment, while the exhaled vape aerosol is almost gone after puffing.

Vape aerosols do not “age”

An aerosol “ages” when it interacts chemically with external pollutants after standing sufficiently long time periods in the environment. As a consequence of this interaction a “secondary” aerosol is formed from the original (the “primary”). The secondary aerosol might exhibit signifficant differences in particle sizes, chemical composition and even optical properties. A typical case of chemical interaction are Secondary Organic Aerosols (SOA) that form from the oxidation chemical reactions with atmospheric volatile organic compounds (VOCs) (This has important implications in climate change modeling, see Srivatsava et al). A SOA can also be formed from smokes generated by burning biomass, including tobacco (see Ahern et al). Cooking aerosols also form SOAs (see Abdullahi et al).

While aging does occur in ETS, there is no connection between what is understood of “aging” and exhaled vape aerosols. Of course, an experiment can be made by organizing a vaping session in a small closed cabin and letting it stay for a long time after no one stays, but from the account of its physical properties and the evidence from laboratory and chamber studies, it is evident that this “abandoned” vape aerosol cannot generate the necessary chemical activity for the oxidation (or photo chemical) reactions to form a secondary aerosol.

In a hypothetical experiment to examine the long time standing state of exhaled vape aerosol, say in a “normal” indoor space, 10 vapers vape ad libitum for (say) 4 hours and then leave. What could be the signature of the vape aerosol after a long time with doors/windows closed and no ventilation? WE can guess: practically all PG and a significant part of the VG and nicotine residuals should have evaporated from the droplets into a gas phase, which itself must have been strongly diluted in air, while the residual droplets should be floating with a tiny fraction of otheir orginal mass and diameter and will certainly be dispersed everywhere, becoming completely undetectable and indistinguishable from the background PM that existed before vaping. In other words an “aged” vape aerosol is really an indetectable ghost.

Conclusions & further posts.

This is the first of a series of 3 posts looking at environmental vape aerosol. In the upcoming post I will examine the gas phase and the chemical properties of the exhaled aerosol, no longer taking evidence from chamber studies, but from studies that consider “vaping atmospheres” in indoor spaces. In the next (third) post I will critically review the literature and demolish some insiduous disinformation on vape “particulates”.