Is there "third hand vaping"?

Part II. The case for/against Third Hand Vaping

Introduction.

This is the second part of the discussion on the existence of “third hand vaping”. It is the continuation of the previous post (Part I) where I addressed “Third Hand Smoke” (THS). That first part was necessary, as it is important to understand what is THS in order to see if this phenomenon is applicable to vaping.

I start this post by I providing a quick summary of the previous post that was dedicated to THS, which is often described as a “third hand” extending “second hand” smoking (environmental tobacco smoke, ETS). Then, I will discuss if this phenomenon is compatible with the physical and chemical properties of what would be the “second hand” in vaping (environmental vaping aerosol, EVA). I also review the litrature on this issue.

Background on EVA

• Post 8: Physical and chemical properties of EVA and ETS

• Post 9: Chamber studies and vaping atmospheres.

• Post 10: The physical and chemical nature of the ultra-fine “particles” of EVA.

• Post 11. On the global identification of “vapor free” policies. Part 1. Criticism of the attempting to extend to vaping the same smoke-free policies implemented on active and passive smoking.

• Post 12. On the global identification of “vapor free” policies. Part 2. Dissection of a study in which the authors ignore their own experimental results, issuing alarmist conclusions on risks for children and vulnerable individuals from inhaling ultra-fine particles (UFPs) from indoor vaping. .

Spoiler: From a practical point of view THERE IS NO “THIRD HAND VAPING”.

The physical and chemical properties of EVA (previously discussed) are incompatible with a significant manifestation of the phenomena that characterizes THS under normal consitions in which vaping takes place. In order to obtain with EVA some of the features of THS it is necessary to (1) produce and constrain artificial aerosols in the lab or (2) conduct measurements in conditions of abnormally intense vaping activity in vape shops.

Summary of THS

This is a brief summary, see previous post for detail.

• THS is a series of interrelated phenomena that includes nicotine adsorption and other processes involving the residues of aging ETS (environmental tobacco smoke).

• The first and most noticeable phenomena that triggers THS is adsorption of nicotine: gaseous free base nicotine in ETS attaches to indoor surfaces forming thin liquid films.

• Free base nicotine is a semi-volatile organic compound (SVOC) that efficiently adsorbs in many surfaces.

• ETS contains many reactive compounds (ready to chemically react) that react in these films with other surface material and with environmental gases and particles present in the environment (with ETS if smoking continues).

• Some molecules in the adsorbed material might detach (desorb) and become re-emitted in the environment.

• Once airborne they interact mostly through oxidation reactions (involving oxygen) with gases and particles present in the environment (with fresh supplied ETS if smoking continues, but this is not necessary)

• Nicotine adsorption is stable, it continues even if smoking has ended, either from deposition of gases and particles present in the environment or with extra ETS.

• Other SVOCs in ETS besides are involved. This might produce more desorption and more interactions.

• The accumulation of these processes in ETS (or other aerosols) is a long term process known as “aging”.

• It might produce in the long term (days to years) sufficient changes in the initial aerosol that the aged one can be identified as a new “secondary” aerosol.

Notice:

• The aging of an aerosol is a long term complex phenomena. It requires the aerosol to remain airborne in the environment for sufficiently long periods that surpass the time frames of adsorption and of oxidation and other reactions.

• Aging that probably leads to a secondary aerosol is not exclusive to ET. It can occur without anyone smoking in other aerosols (for example cooking or atmospheric aerosols) whose gaseous part (phase) havs volatile or semi-volatile compounds (VOCs and SVOCs).

• There is experimental evidence in laboratories that ETS may generate THS as a secondary aerosol (in the processes described). However, there is no solid evidence that this happens in field studies on indoor spaces.

• The formation of THS involves a very complicated chain of processes that vary in different environments, depending on many local factors that are difficult to sample and control outside experiments.

• There is no consensus on standardized chemical signature to determine when aged ETS has become a secondary aerosol identified with THS.

• Therefore, it is not clear if evidence shows that THS is a secondary aerosol formed from ETS or just a series of phenomena of aged ETS.

Nicotine

• Nicotine in its free base form (non-protonated) is a SVOC. It is a very efficient adsorbate (can be easily adsorbed). It plays a dominant role in triggering the formation of THS.

• Protonated (salt based) nicotine is not volatile, it is not an efficient adsorbate (difficult to adsorb). It is inhibits oxidation reactions. Its role in aging ETS is not well known and should be completely different.

The most solid evidence from laboratory and field studies on THS is nicotine adsorbed in dust and surfaces sustaining oxidation reactions with nitrogen acids and oxides and ozone (see Schick et al and Sleiman et al) to form many byproducts, including carcinogenic tobacco specific nitrosamines TSNAs, specifically

• N’-nitrosonornicotine (NNN),

• 3-ethenylpyridine (3-EP)

• 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

Nicotine can be regarded as the main (but not unique) tracer of THS, as it is the most abundant SVOC in the gas phase of ETS and a very efficient adsorbate (adsorbs efficiently in many adsorbent surfaces).

Nicotine adsorption occurs also in freshly emitted ETS and in vaping aersols generated from e-liquids with free base nicotine.

THS and vaping

Nicotine is the obvious common abundant SVOC in environmental vaping aerosol (EVA) and ETS. The fact that nicotine adsorption and its byproducts play such an important role in defining THS would suggest that something analogous to THS should take place in vaping.

However, the reasoning by analogy expressed above is misleading, since environmental vaping aerosols (EVAs) are chemically and physically very different from ETS. In fact, nicotine being an important constituent in both aerosols could be the only significant similarity between them. The following is a summary of these differences (can be further consulted in previous posts).

EVA releases much less nicotine than ETS

The first striking difference is the fact that EVA has no side-stream (SS) emission (no burning tip continuously releasing emissions). EVA (and nicotine in EVA) is exclusively released to the environment by the user exhalation. Also, users retain about 95% of the inhaled nicotine (see St Helen et al) with devices available before 2016, but similar retention has been observed in currently used pod devices and Juul (see Hua et al) that generate aerosol mainly with protonated nicotine.

As a contrast, a much larger nicotine mass to be adsorbed should be released to the environment with ETS: from the smoker exhalation (mainstream emission) and the continuous SS emission that makes 60-80% of ETS.

The type of nicotine matters.

Experiments on THS were conducted with ETS, in which nicotine is predominantly non-protonated (free base), which is very volatile and is almost entirely contained in the gas phase.

All these features are favorable to equilibrium phase partitioning and thus for adsorption into surfaces, specially household furniture, carpets, wood and upholstery.

There is abundant field evidence found by THS researchers of this adsorption in homes and many indoor venues, even inside cars (see the previous post and also Quintana et al, Northrup et al).

Practically all vaping before 2018 relied on e-liquids with free base nicotine, which is the same type of nicotine as in the THS studies. While free base nicotine can be also in the droplets (particulate phase), it evaporates into the gas phase almost immediately after exhalation (see experimental evidence).

Before 2018 many vapers puffed high powered devices that released larger aerosol masses, including larger mass of nicotine (despite the fact that these devices are used with e-liquids with low nicotine concentrations).

However, since 2018 the vaping market changed, e-liquids with protonated (salt-based) nicotine and higher nicotine concentrations becoming popular after the success of Juul.

The market shifted from a majority of tank models and free base nicotine to salt based nicotine in low powered pods and disposables. Protonated nicotine is not volatile, it is mostly contained in the droplets and barely evaporates.

This is the worse case scenario for nicotine adsorption following the equilibrium phase partitioning: an EVA with a low mass content of non-volatile nicotine.

EVA has a much simpler chemistry than ETS

The complexity of the chemistry of ETS is very well known. More than 4000 compounds have been detected in ETS, most toxic and carcinogen compounds in the MS and SS emissions are present in diluted concentrations in ETS, plus extra compounds from reactions with environmental pollutants.

As a contrast, around 100 compounds are detected in EVA, with 4 (PG, VG, nicotine and water) making almost the total aerosol mass, 3-4 aldehydes (formaldehde, acetaldehyde, acrolein) appear in minute concentrations and the rest of the compounds appear barely above detection limit.

A detailed discussion of EVA is found in Post 8, Post 9, Post 10). In particular, van Drooge et al (see Post 9) showed that the particulate and gas phases of EVA are practically indistinguishable from background environmenlal levels without vaping.

Of the 26 tracers of ETS displayed in the figures of Singer et al (see previous post), only nicotine is significantly present in EVA, the rest are either not detected or (acrolein) detected in negligible quantities that that are almost indistinguishable from non-vaping background level.

Nicotine adsorption from EVA and from ETS are different processes

Although free base nicotine can be adsorbed like nicotine from EVA and from ETS, the adsorption process is different. As shown in experiments of Singer et al (see previous post), nicotine from ETS is adsorbed together with numerous other VOCs and SVOCs (polycyclic aromatic hydrocarbons, isoprene, etc) that are absent in EVA.

As contrast, when free base nicotine is adsorbed from EVA it is accompanied only by propylene glycol, glycerol and by trace level quantities of formaldehyde, acetaldehyde and other aldehydes.

EVA remains much less time in the environment

EVA evolves in respiratory tracts at 37°C and 95% RH, since users retain > 80% of glycerol, > 90% of propylene glycol and nicotine, it is exhaled in diluted form into a (in general) colder and drier environmental air.

These conditions strongly favor a rapid evaporation of very volatile PG and free base nicotine in the liquid droplets, with diminished droplets made of a SVOCs glycerol (and nicotine if protonated).

The rapid evaporation of droplets (half life of < 20 s per puff) and low molecular mass of gas compounds rapidly dilutes and disperses EVA to achieve thermal equilibrium with environmental air (a thermal reservoir).

Only under abnormal conditions of intense vaping activity (vape fests or vape shops), evaporation, dispersion and dilution of EVA become slower, leaving residual levels of aerosol for hours.

However, a slower evaporation rate because of continuous aerosol injection at ambient temperature and pressure does change the chemistry of the aerosol, since reactions require to become efficient a sufficient mass of reactives and permanence time. Therefore, even in excess EVA does not favor sufficiently energetic oxidation reactions with sufficient intensity for a significantly measurable chemical activity.

As a contrast, ETS is a complex mixture of two combustion originated aerosols (the MS and SS emissions) and environmental pollutants. ETS contains in its gas a particulate phases many reactive compounds with a high share of non-volatile content, all of which favors remaining long time in the environment. Its half life is 30-60 minutes per puff.

Evidently, the physicochemical properties of ETS that favor its long term permanence in the environment also strongly favor oxidation reactions.

Evidence of a “third hand” vaping aerosol

The implications of the previous arguments point to a clear incompatibility between EVA and any sort of “third hand” phenomenon analogous to THS. The only common phenomenon is nicotine adsorption and it occurs under different chemical conditions.

However, there are several published studies claiming that these phenomena taking place in vaping are not only plausible, but fully demonstrable. These studies can be classified as follows

• Laboratory studies. Primary vaping aerosol is artificially generated with syringes and injected into a chamber with household fabrics and materials inside, leaving these samples exposed to the aerosol for prolonged time.

• Studies on adsorption from EVA in vape shops. Researchers measure area density of nicotine and its byproducts on fixed surfaces and also on fabrics they bring to place them on the surfaces. Area density on the fixed surfaces and the fabrics are measured for extended periods (up to 3 months).

In all studies adsorption and desorption of nicotine and its byproducts are reported. The authors describe this adsorption as evidence that “third hand vaping” has been discovered.

Laboratory studies

• Goniewicz et al 2015. 100 puffs delivered in 1.5 hours from 3 brands of e-cigarettes were injected directly into an exposure chamber through the e-cigarettes attached to a 100 ml syringe via rubber connector. Surface wipe samples were taken from 5 indoor 100 cm2 surfaces (window, walls, floor, wood, and metal). Significant increases in the amount of nicotine was measured on surfaces, largest in the floor and glass windows. Average amount of nicotine on the was 205 µg/m2 (range 0 to to 550 µg/m2).

• Marcham et al 2019. The authors placed glass and cotton samples on a Petri dish in a laboratory chamber of volume 1 m3. Aerosol was injected by 49 puffs in 15 minutes through a 500 ml syringe connected to a sub-ohm vaping device. The exposed samples were left for 45 minutes and then were extracted for analysis. Mean amount of adsorbed nicotine was 0.75 µg/cm2 (cotton) and 0.125 µg/cm2 (glass). Statistical modeling predicted surface concentrations to reach background levels after 4 and 16 days for glass and terrycloth, respectively.

The merit of these experiments is finding special laboratory conditions to be able to measure significant levels of nicotine adsorption from vaping. However, the experiments are unrealistic: no vaper vapes compulsively into samples, less so inside a chamber.

The aerosol injection in these experiments is also excessive (100 puffs in 1.5 hours and 49 in 15 minutes). This excess puffing together with the need to lock the aerosol in a chamber are necessary to slow the evaporation and force as much nicotine as possible to deposit on the samples. More normal conditions would probably yield negligible adsorption.

There is another experiment claiming to study the aging of vaping aerosols. It is even more unrealistic. I placed a Pub Peer comment criticizing it (scroll down to find it).

Studies on vape shops

There are three studies published in 2019 and 2020 quantifying nicotine adsorption in vape shops:

• Son, Y., Giovenco, D. P., Delnevo, C., Khlystov, A., Samburova, V., & Meng, Q. (2020). Indoor air quality and passive e-cigarette aerosol exposures in vape-shops. Nicotine and Tobacco Research, 22(10), 1772-1779.
  https://doi.org/10.1093/ntr/ntaa094

• Khachatoorian, C., Jacob III, P., Sen, A., Zhu, Y., Benowitz, N. L., & Talbot, P. (2019). Identification and quantification of electronic cigarette exhaled aerosol residue chemicals in field sites. Environmental research, 170, 351-358.
  https://doi.org/10.1016/j.envres.2018.12.02

• Khachatoorian, C., Jacob Iii, P., Benowitz, N. L., & Talbot, P. (2019). Electronic cigarette chemicals transfer from a vape shop to a nearby business in a multiple-tenant retail building. Tobacco control, 28(5), 519-525.
  https://tobaccocontrol.bmj.com/content/28/5/519

I will discuss only the first two, since all useful information in the third one is contained in the second one.

Some comments are necessary to provide context. These vape shops at the time allowed vaping in the premises. They were indoor spaces with relatively small volumes (typically < 200 m3) in which an abnormally intense vaping activity took place, as many vapers (dozens to hundreds per day) congregated in short time periods and vaped while buying liquids or supplies. Under this combination of intense vaping in modest indoor volumes, it is not surprising that these studies measured large yields of adsorbed nicotine and byproducts.

Busy vape shop, many vapers vaping, adsorbed nicotine (N) accumulates in surfaces, TSNAs (T) are measurable

However, it is evident that the vape shops these studies examined were (and still are) a very special scenario distinct from a normal routine vaping experience. Vapers only sporadically visit vape shops and do not stay hours in them. The authors do not recognize this, or are unaware of the fact that they made measurements under abnormal and infrequent conditions unrepresentative of mainstream vaping.

The study by Son et al (2010)

These comments are necessary: The claims of this study are unsustainable.

• This study deserves very strong criticism. The authors deliberately introduced external fabrics in their protocol to produce an artificial enhancement of nicotine adsorption.
  
  The reason: likely to produce the most extreme scenario that would justify their claim of having found “third hand vaping” that is completely analogous to THS. Authors justify this claim by references on THS research that do not apply to vaping.

• Also, as happens in a lot of research following an anti-vaping agenda, the authors use their results to spread fear for the exposure of children, when vape shops were (and still are) adult venues.

To see how inappropriate it is to alude to children when sampling nicotine and TSNAs in a vape shop, we should ponder what are the odds to find this scenario?

What are odds of chldren being in a busy vape shop? What are the odds for them to be exposed to surface nicotine and TSNA?

The authors examined environmental variables (CO2, NO2, PM2.5 aldehydes and air nicotine) in 5 vape shops. However, their main interest was to measure surface density of adsorbed nicotine and detection of NNA and NNK byproducts. This was their protocol:

Samples of surface nicotine and TSNAs were collected by wipes in for places in each vape shop: a show case, a TV set, a picture frame and at the floor.

• Notice an important fact: these are natural inherent surfaces of the vape shops. Area density of adsorbed nicotine on them is indicative of adsorption on surfaces that belong and are normally in the vape shops.

However, the authors brought their own surfaces (the “materials”) to measure adsorbed nicotine and TSNAs on them. These external materials were bought in local retailers and brought externally

- a piece of glass (5 in 8 in, purchased from a local retailor)

- a piece of paper (5 in × 8 in)

- Whatman rectangular cellulose filter paper

- a piece of baby clothing material (5 in × 8 in, 100% cotton cloth

- a rubber ball (5 cm in diameter) and a fur ball (polyester) purchased from a baby toys retailer.

Notice two important facts.

• These items are not inherent objects of the vape shops, they were introduced by the researchers. These items would not be there had the researchers not brought them. Therefore, nicotine adsorption on them is not indicative of nicotine adsorption in the vape shops

• Some items are material only used by children: baby clothing and the rubber and fur balls. Why should children’s items be brought to sample nicotine and TSNAs in an adult venue?

• Some of these materials are highly adsorptive: baby clothing made by cotton and the fur ball are much more efficient adsorbents (easy to adsorb) than the inherent objects that were sampled (a show case, a TV set, a picture frame and at the floor). It is evident that higher surface nicotine values will be measured on them

But if placing these external objects was not enough manipulation, look where the authors placed them (in their own words):

“The five materials were left in each vape-shop for 14 days on a shelf near a main countertop where e-cigarette users were frequently seen vaping”

This is the crunch of the manipulation: the authors placed highly adsorptive extraneous materials just in the right spots were vapers vape frequently.

It is evident that placing these external materials inside the vape shops is an artifact used by the authors to artificially enhance the readings of nicotine and TSNAs.

This protocol artifact fulfilled its purpose. These were the results on the fixed surfaces

• surface nicotine 223.6 ± 313.2 µg/m2,

• NNA 4.78 ± 11.8 ng/m2,

• NNK 44.8 ± 102.3 ng/m2.

and, as expected from their artifact, much larger yields (10 x) of nicotine and TSNAs were found deposited on the materials externally brought to the vape shops:

• nicotine up to 2073 µg/m2,

• NNA up to 474.4 ng/m2

• NNK up to 184.0 ng/m2.

It is evident that the only real legitimate values on nicotine adsorption and TSNAs presence are those collected from the fixed surface, which are much lower than those from the authors manipulation. As an important note: I have revised now a good number of field studies on THS. None of these studies introduced an external fabric to enhance their results.

The study examined correctly the environmental variables, but they are completely wrong on the issue of “third hand vaping”, to which they dedicate a full subsection. I comment on several key paragraphs:

• ”Exhaled e-cigarette aerosol could also deposit to indoor surfaces, leading to “thirdhand” e-cigarette aerosol (THA) exposure. Similar to thirdhand smoke exposure, THA exposure includes not only contacting residual e-cigarette aerosols on indoor surfaces, but also pathways such as aerosolization and/or evaporation and conversion to secondary toxic chemicals”. [11].

This is just the authors’ opinion. Their results (even with the manipulation) do not provide any evidence of “aerosolization and/or evaporation and conversion to secondary toxic chemicals”. The reference [11] they cited concerns only THS in laboratory conditions. As I mentioned in my previous post, this conversion to a secondary aerosol has only been observed in laboratory experiments, not in THS field studies.

Despite their hype and overconfidence, the authors have not found a “third hand vaping”. All what they found (despite their manipulation) is that surface density of adsorbed nicotine and TSNAa (clear signals of THS) are substantial and measurable only in an indoor environment with abnormally intense vaping activity. But (as mentioned before) these are special environments unrepresentative of normal routine vaping.

This study is not much useful for the millions of vapers who have very occasionally (or never) visited a vape shop. Although the authors deserve credit of recommending vape shop owners to provide appropriate ventilation, they presented a very biased account of these businesses.

This is the final punchline on this study: even if (as the authors’ study shows) excessively intense vaping activity can produce measurable levels of adsorbed nicotine and TSNAs, this does not prove having found “third hand vaping”, since measurable levels of adsorbed nicotine and TSNAs can also be found with fresh (not aged) ETS after 1-2 days of being exhaled and this is not identified as THS.

• “Our study also demonstrates that nicotine can deposit or be adsorbed on baby’s clothes and toys, and that tobacco-specific nitrosamines can form and retain on baby’s clothes, highlighting children’s exposure to environmental e-cigarette aerosol and THA at home is of a particular concern.”

Again “the children”. Concern for children’s safety is legitimate and commendable, but for assessing children’s safety the authors should have conducted their study in the dwellings where children live (homes, schools, nurseries), not in adult venues like vape sops. The safety of bars serving alcoholic beverages is not evaluated with standards of children safety. The authors did not examine nicotine and TSNAs in homes, schools or nurseries.

• “However, thirdhand exposure to nicotine and carcinogenic TSNAs could be much higher in vape-shops than that caused by cigarette smoking (Table 3). Surface nicotine levels in vape-shops could even exceed nicotine levels observed in cigarette smokers’ homes and cars.”

Besides the fallacy of the “thirdhand exposure”, this is a very misleading statement. The authors sampled 5 businesses where dozens or hundreds of vapers vaped every day while shopping. They compare the amount of surface nicotine in that environment with surface nicotine produced by family members smoking in private homes and cars. Just the number of nicotine emitters make it obvious which set up will produce more nicotine, but the comparison is desceptful. There is no smoking analogue of a vape shop. The right comparison with smokers in THS studies would have to be with vapers vaping at home or in their car.

A final critique: the actual exposure. The authors mention risks and potential harm from dermal exposure to surface densities of accumulated nicotine and TSNAs, but this is far from evaluating actual human exposure from a dermal route. As mentioned in my previous post, dermal exposure normally involves short timed dermal manipulation of the surfaces exerted on small proportions of its total area.

Thus, being in contact with a 1 m2 fabric with (say) 2000 µg/m2 surface density of nicotine does not mean dermal exposure to 2000 µg of nicotine. The exposure dose depends on the time and area involved in the dermal maniplation and under realistic assumptions the exposure should be well below the total content of nicotine in the fabric. The exposure dose should be even smaller by orders of magnitude for a nicotine density obtained in a home of a vaper or any environment without intense vaping activity.

The study by Khachatoorian et al (2019)

Khachatoorian et al (2019a) and Khachatoorian et al (2019b) were published by the same group of researchers. I will only review the second study that contains most of the useful data. The authors define adsorbed nicotine, its byproducts including TSNAs as “E-cigarette aerosol residues” (ECLEAR).

The same criticism to the paper by Son et al (2010) that I reviewd above applies to these two studies: they introduced external fabrics that artificially enhance nicotine adsorption and TSNAs. To these authors’ credit, they also sampled “ECLEAR” in the living room at the home of a vaper, but on an external cotton fabric, not on the instrinsic surfaces. Also, their conclusions are more cautious than Son et al, they do not declare with excess overconfidence having found “third hand vaping”.

The authors express the same concern for chidren that “could be” exposed to nicotine loaded fabrics, when vape shops are adult venues and are also visited for short time lapses. Also, the authors failed to understand that accumulated adsorbed nicotine does not provide an exposure dose from the dermal route, which involves very short times and minuscule area of the fabric. Their concern for the posibility of a hypothetical toddler licking 0.3 m2 (54 cm side for a a square shape) of a fabric in their study is unwarranted: a hypothetical toddler will not lick such an area of a towel.

The study placed in the vape shops commonly used materials (cotton, polyester, or terrycloth towel) placed inside (1) the living room of a vaper for 6 months and (2) in a vape shop collected in 6, 7, 18, 24, 48 hours, 1 week and 1 month. I only summarize their results for the the terrycloth towel that produced the largest yields. I convert their units ng/g, to ng/cm2 multiplying times the towel area density g/26.125 cm2 in the vape shop and g/44 cm2 at the vaper’s living room. These were their results:

• In the vaper’s living room nicotine oscilated in 6 monts between 2000-5000 ng/g (0.4 to 1.1 mg/m2) , with cotinine and other byproducts barely detectable. No TSNAs were detected.

• On the back of the shop nicotine passed between 6 h to 1 month from 24 ng/cm2 to 679 ng/cm2. Lounge area 19-786 ng/cm2. The largest density was in the display case, from 32 ng/cm2 to 10,914 ng/cm2. Cotinine varied from 7-8 ng/cm2, while NNA reached in 1 month 3 ng/cm2 in the display case.

Area density of nicotine on the vaper’s living room was relatively small and negligible for all byproducts, but (despite the artifact of an external fabric) these are much more representative of normal routine vaping than a vape shop. Also densities were also small in surfaces inherent to the vape shops (lounge area, display case) even after one month observation.

However, the nicotine surface concentration detected in the terrycloth towel on the display case is truly enormous: 108 mg/m2, larger than any adsorbed nicotine surface density registered in THS research.

Although this result looks worrying, as commented before in the critique of Son et al, it is a completely artificial and unrealistic result that does not reflect nicotine adsorption on intrinsic surfaces. As a comparison, surface density on the terrycloth towel in the vape shop was 60 times the density of the fabric placed in the vaper’s living room.

Sampling nicotine on a fixed surface is the only realistic approach because these are structural natural parts of the vape shop, but placing fabrics or towels on these surfaces would not be done by owners as a normal voluntary initiative. It is only an artifact used by the researchers to enhance surface density and byproducts.

It is specially artificial and unrealistic to place or hang a terrycloth towel in a vape shop, as people (including vape shop owners) would place this towel in a bathroom or a gym, not in the display case of their business. Since this type of towel is very hygroscopic (efficient to absorb water), placing it for one month in a vape shop where many customers are vaping will lead to enormous but artificial yields of adsorbed nicotine, it is like study trying to count how many flies stand on a wall and placing on it sugar oil. This will produce huge swarms of flies, but will not provide information on normal tendencies.

There is another study by Melstrom et al (2017) that sampled nicotine adsorption from emissions of aerosol emitted by 3 vapers vaping ad libitum for 2 hours in a small room of 52 m3, resulting in a surface area density of 6.9 µg/m2. A much lower value than the values obtained by the studies reviewed.

Epilogue and conclusion

On the grounds of available theoretical, experimental and observational evidence, we can safely state that there is no “third hand vaping” under the environmental conditions in which vaping takes place among millions of users. These conditions are mostly private homes, outdoor spaces and in indoor public spaces where vaping is allowed.

The study by Khachatoorian et al (2019a) showed that after 6 months accumulation the adsorbed nicotine in a cotton fabric placed in the living room in the home of a user in was a fraction of 1/60 with respect to a similar fabric in the vape shop. This comparison between two artificially enhanced surface densities highlights the fact that nicotine adsorption in normal conditions should be small and even barely detectable.

The studies I reviewed in this Substack entry were published between 2019 and 2020. At the time high powered tank devices were becoming less popular but were still used. Practically all vaping involved free-base nicotine and there were plenty of vape shops congregating vapers.

Things have changed: the market trend is low powered pods and disposables with protonated nicotine (which adsorbs less efficiently if at all). Many vape shops have closed and they are much less congregation hubs. It is very likely that nicotine adsorption measuements today will show negligible levels.

Nicotine adsorption and formation of byproducts (including TSNAs) is the only observed and measured phenomenon in vaping that is common to THS. However, it is only possible to obtain non-negligible measurements under abnormal conditions, such as laboratory experiments with artificially generated aerosols, or in vape shops that are places with localized excessively intense vaping activity.

Moreover, even under these abnormal conditions, there is no evidence of desorption and re-emission of adsorbed nicotine and byproducs, or evidence of other adsorbed compounds (besides nicotine) and their byproducts. In other words, even in laboratory experiments there is no signal of a chemical aging process as is observed in THS.

Final conclusion, we can state confidently that in practical terms and normal usage conditions, there is no “third hand vaping”.

Finally. In 2018 I wrote a critique of one of the studies of Khachatoorian et al, it appeared as a guest post in Brad Rodu’s blog Tobacco Truth. I recommend reading it to understand the papers I reviewed here. It is also a valid critique of some aspects of research on vaping in general and on THS.