This is an open and interesting question. I would like to assess if environmental “second hand” vaping aerosols can evolve and produce an analogue of “third hand smoke” (THS).
I will address this issue in two posts. In this one, part I, I will provide a precise explanation of what THS is. In the following post, part II, I will provide arguments to examine if vaping aerosols are compatible with the aging phenomena that defines THS in smoking. I will also criticize studies that have proposed it.
What is “third hand smoke”?
It is a combination of phenomena in indoor spaces produced by residues of aged environmental tobacco smoke ETS (the “second hand”). There are several reviews on THS, I recommend these two: Matt et al (2012), Jacob III et al (2017) and a full summary of the literature by Diez-Izquierdo et al (2018).
Aging of aerosols
The notion of “aging” can be (in principle) applied to any aerosol (not only ETS) conceived as a physical system composed of two interacting phases: the gaseous and the particulate. As the aerosol evolves in an indoor space the distribution between gas and particles (the “phase partition”) might change through reactions of the gas or particulates with other indoor and outdoor pollutants. The evolution of an aerosol can be quite complex and might involve sufficient phase changes to define the aged aerosol as a new “secondary aerosol” distinct from the “primary” one.
Understanding indoor aerosols is important because people spend most of the time indoors. While indoor conditions are affected by atmospheric aerosols, there is an important difference: atmospheric aerosols spread in large open volumes, while aerosols in indoor spaces occupy relatively small volumes containing many surfaces (walls, doors, etc). Therefore, interactions with surfaces are crucial for understanding indoor aerosols, since surfaces on which aerosols deposit may become “reservoirs” of compounds (specially gas phase compounds).
Equilibrium phase partitioning.
An importnt interaction of aerosols and surfaces is known as the “equilibrium phase partitioning” that occurs in many aerosols whose gas phase contains volatile and semi-volatile organic compounds (VOCs and SVOCs). As the aerosol deposits on surfaces kinetic processes in thermal equilibrium favors these gases to become adsorbed (ie attached) into the surfaces, forming thin liquid films on them. We identify the gases as “adsorbates” and the surfaces as “adsorbents”
The adsorbed molecules interact with surface molecules and might be re-emitted (desorption) to become airborne and reactive, possibly (pending on many factors) forming new gas compounds. Aerosol particles also interact (impact and adhesion) with surfaces, while re-emitted molecules might nucleate to form also new particles. These processes are mediated by oxidation and photochemical reactions with atmospheric pollutants (ozone, nitrogen oxides and acids).
All these processes happen with ETS (as adsorbate) and equilibrium phase partitioning is the main process that triggers the formation of THS. However, the passage of ETS to THS is far from unique, equilibrium phase partitioning also occurs with other indoor aerosols as adsorbates whose gas phase contains VOCs and SVOCs. This is a recent review on various organic primary and secondary aerosols, but none was ETS. Other examples are aerosols produced by cooking (see also here) cleaning liquids (see also here) and sprays. The seminal work on equilibrium phase partitioning of VOCs and SVOCs was published by Weschler and Nazaroff in 2008.
Fresh, just emitted, ETS is a mixture of the mainstream emission (MS exhaled by smokers) and the side-stream emission (SS from the burning tip of the cigarette). Most of the ETS initial mass comes from the SS that is continuously emitted, which implies that exposure is not intermittent. As freshly emitted ETS spreads in indoor spaces, reacts with pollutants that are present and it undergoes an aging process that involves equilibrium phase partitioning, and likely also re-emission of molecules and particles and further reactions with oxidant pollutants, all of which modifies its initial physical and chemical properties.
Aging of ETS
As ETS ages, equilibrium phase partitioning sets in: VOCs and SVOCs gas compounds (specially but not only nicotine) deposit on dust particles and on indoor surfaces as adsorbents (including furniture, clothing and even in the human body). Since these compounds remain adsorbed (attached to surfaces and dust particles as thin liquid films) for long periods (even months and years without anyone smoking), dermal and ingestion exposures become possible.
As mentioned before, some of the adsorbed material is re-emitted (desorbed) from both, dust particles and surfaces, reacting with oxidant indoor pollutants (nitrogen acids and oxides, ozone, free radicals). Depending on concentrations and intensity of the reactions desorbed material my produce new gas phase compounds and (possibly) new particulates that define a secondary aerosol. However, It is still not fully understood (beyond laboratory experiments) if THS is really a secondary aerosol derived from ETS or just residues of aged ETS adsorbed and desorbed from surfaces.
Adsorption and desorption of nicotine and detection of its byproducts are a clear and verifiable signal of THS that brings compounds that were absent in ETS and this is straightforward to verify (I will deal with this issue further ahead). The main problem with finding airborne THS compounds and particulates is the fact that these prospective THS compounds also appear in ETS. As far as I know, evidence of adsorption/desorption of other compounds is restricted to laboratory experiments and simulations.
The chemistry of THS
I summarize several experiments. Sleiman et al examined ETS generated by smoking 6 cigarettes aging at 2, 8 and 18 hours. They detected 58 VOC and SVOC compounds: (a) nitrogenated(amines and nitriles), (b) aromatic hydrocarbons, (c) carbonyls and chlorinated, and (d) alkanes and alkenes. Since all of them are present in ETS, they looked at compounds whose concentration increased from 2 to 18 hours and estimated their long term dilution, identifying Acetonitrile, 2,5-dimethyl furan, and 2-methyl furan as possible markers of the transition from ETS to THS. However, these findings have not been verified in field studies outside their experiment.
Singer et al 2002 examined injected ETS emissions (5, 10, 20 cigarettes/day) of 26 gas-phase VOCs and SVOCs identified as ETS markers in three ventilation regimes, into a 50 m3 model room furnished with typical household material. Compounds whose emission rates were stable in all ventilation and furnishing conditions (acrolein) were identified as non-sorptive, while compounds whose emission rates decreased under low ventilation and full furnishing (nicotine, 3-ethenylpyridine) were identified as highly adsorptive. Almost all nicotine and most cresol remained adsorbed 3 days after the smoke injection. Figure 2 of Singer et al illustrates these adsorption patterns
A second experiment by Singer et al 2004 specifically examined, under the same experimental conditions as the 2002 paper, adsorption/desorption rates for 20 VOCs and SVOCs common to indoor spaces, including nicotine and some ETS markers. The results can be summarized in their figure 7 that displays the fraction that remains in the gas phase (not adsorbed)
Desaillats et al 2006 examined in the laboratory the effects of ozone (O3) on nicotine desorption from cotton and teflon surfaces for a week. In comparison with baseline ozone free dry conditions, gas-phase nicotine concentrations decreased by 2 orders of magnitude for Teflon after 50 h at 20-45 ppb O3, and by a factor of 10 for cotton after 100 h with 13-15 ppb O3. Byproducts of nicotine and formaldehyde were identified. Reaction with O2 was inhibited in acidified solutions, in consistency with the fact that only free base nicotine is susceptible to oxidation. Surface ozone chemistry can dramatically reduce nicotine desorption rates in teflon in all conditions, but only in dry conditions in cotton.
These experiments prove that other ETS compounds besides nicotine can be precursors of THS. However, it remains extremely difficult to obtain a generic chemical signature of THS, or universal markers that signal the transition from ETS to THS. These problems arise from the complexity of tracing ETS residues, whose presence and concentrations depend on multiple factors: initial mass and injection rate of ETS, properties of the building materials of indoor spaces (buildings, homes), the presence of other anthropogenic aerosols, specific indoor and outdoor pollutants and degree of isolation from outdoor pollution.
Nicotine
Basically, the most solid evidence so far of THS is adsorption of nicotine in household surfaces and the presence of its byproducts produced by well known oxidation reactions. The special role of nicotine emerges because it is the most abundant SVOC in the gas phase of ETS and also because it is a very efficient adsorbate (adsorbs efficiently in many adsorbent surfaces).
Although nicotine adsorption in indoor surfaces happens also with fresh ETS, it is a clear signal of (at least the first stage) of THS formation. It is known that nicotine, either in gaseous form or adsorbed in dust and surfaces, sustains 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 NNNAs, specifically
N’-nitrosonornicotine (NNN),
3-ethenylpyridine (3-EP)
4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)
Its role as a tracer of THS is further illustrated by the fact that one of its byproducts, NNA (4-(methylnitrosamino)-4-(3-pyridyl) butanal), is absent in fresh ETS, but NNK is detected when analyzing oxidizing reactions on nicotine in ETS, all of which shows that it is a secondary product of aging, not a compound emitted in ETS.
However, as shown by the experiments summarized before, the formation of THS in adsorbed surfaces, airborne re-emission and reactions with oxidants involve other VOCs and SVOCs besides nicotine, for example polycyclic aromatic hydrocarbons (PAHs), which are also carcinogens.
Evidence from field studies.
Beyond the laboratory, practically all field work on THS consists in evaluating the presence of nicotine and byproducts (including TSNAs) on dust and on surfaces in a wide variety of indoor spaces: homes, cars and hotels. The method to evaluate nicotine presence is through surface wipe sampling, with wipes being standardized surfaces that act as calibrated environmental markers, in an analogous role as CO and cotinine in evaluating active and passive smoking (see details in Quintana et al (2013)).
Area densities of nicotine (in µg/m2) from surface wiping seems distinguishes between smoking and non smoking spaces. The following graph taken from Quintana et al (2013) displays nicotine area densities in a wide variety of indoor environments:
The graph suggest the value 100 µg/cm2 to separate smoking and non-smoking environments, while a low cutoff should be 1-10 µg/cm2. In one of the studies whose results appear in this figure, Matt et al (2011) et all showed that non-smokers buying houses whose previous owners smoked had 6 times more nicotine residues in their living room than houses bought from non-smokers: 10.04 µg/m2 vs 1.52 vs. µg/m2.
In another study on cabins of used cars Mat et al (2011) measured a nicotine area density in cars owned b smokers that ban smoking aboard was 5.09 µg/m2, without ban 8.61 µg/m2, while in nonsmokers’ cars with smoking ban was 0.06 µg/m2. A proposed cut-off of 0.14 µg/m2 separated all non-smoking cars and 82% of smokers’ cars that banned smoking.
Realistic exposures in humans.
Nicotine surface concentrations (even if high) can be misleading, specially in the small enclosed volume inside cars, where objects and surfaces have smaller sizes and areas than in homes or other indoor spaces. Consider a car’s front board in the passenger seat with dimensions 40 X 20 cm, or 800 cm2 = 0.08 m2. If the owner/driver of the car is a smoker allowing smoking in the cabin, then the front board should have roughly the average nicotine surface density of 8.61 µg/m2 (from measurements of Matt et al). The passenger would be exposed to 8.61 µg/m2 X 0.08 m2 = 0.68 µg = 680 ng of nicotine, this is less than one part in one million of 1 gram. Since typically TSNSa surface concentrations are three orders of magnitude less than nicotine concentrations, the amount of TSNAs in the car front board would be about 1 ng.
Evidently, areas (and thus densities and masses) would be much larger in surfaces in homes. For example, in a smoker’s home surface densities would average 200 µg/m2, a towel of 2 m2 would host 400 µg of nicotine and maybe 0.5 µg of TSNAs. Looks terrifying. However, measuring accumulated adsorbed nicotine and TSNAs in a surface is not an assessment of the actual human dermal exposure.
To estimate the actual human dermal exposure to adsorbed nicotine or TNSA in any indoor surface (large or small), it is necessary to estimate the time and fabric surface area to which human subjects are actually exposed in dermal contact. However, we must proceed under realistic assumptions, since under normal conditions exposure times to pollutants by dermal manipulation is not continuous nor prolonged: people may touch or dermally manipulate an item (a fabric, a door knob, a towel, clothing) for brief intermittent time periods and along reduced skin contact surfaces.
Suppose that a child is close to a cotton fabric of 1 m2 in a smoker’s house in which average nicotine density is 200 µg/m2. To claim that the fabric exposes the child to 200 µg of nicotine and 0.2 µg = 200 ng of TSNA, can only be translated into an actual human exposure under an extreme maximalist assumption: the child exerts continued dermal interaction with the fabric, touching or licking every inch for a sufficiently long period. This is utterly unrealistic, a child (or any person) will not be hours touching or licking 1 m2 of fabric. Total human exposure in this case (and in general) will be much less because the actual ingestion or dermal interaction is short timed and intermittent and contact or mouthed surfaces contain few grams of fabric.
Surfaces of objects hung on walls, regardless of the nicotine area density, are not likely subject to intense or prolonged dermal manipulation. The more risky situation is a carpet or rug on the floor, since these fabrics are made of very intricate networks of porous fibers, magnifying the sorption surface and making them extremely efficient sorbents.
A toddler crawling on very efficient sorptive carpet or rug is more likely to present a prolonged dermal exposure, not only to nicotine but to other adsorbed VOCs and SVOCs, but also to infalling particulate matter or dust. I believe that this is the only realistic worrying scenario in the context of dermal exposure from THS. Another risky scenario could probably happen in inner city non-ventilated indoor enclosures, full of ETS that has aged for years, generating sufficient adsorbed nicotine and other VOCs and SVOCs that desorb and react to form a secondary more toxic aerosol.
Epilogue
While THS has provided a motivation for very interesting research on indoor pollution, physical chemistry and aerosol science, its announcement as a new public health hazard has had limited success, as it is evident that it has caused much less harm than primary and passive smoking. THS has also lead in terms of media diffusion and public policies to heap more stigma on smokers, which are perceived not only as walking poisoners from their emission, but also as walking sources of pollution from their belongings, dwellings and even their bodies.
THS is clearly a US-centric issue that has little relevance outside the US. The overwhelming majority of studies on THS have been conducted in the US between 2008 and 2018. However, currently smoking prevalence has sharply decreased in the US and in many rich countries, where also most public and private indoor spaces have been practically smoke-free in the last 10-15 years. Under these developments, adsorbed surface nicotine and its byproducts and airborne indoor concentrations that could be assigned to THS are bound to become negligible and elusive. There might come a point when it might not be worthwhile funding an expensive study to detect in fabrics or surfaces few nanograms of nicotine or toxins or carcinogens per square meter (a nanogram is one billionth of a gram, 1 part in 100,000,000,000).
What is next?
In the next post I will show that the physical and chemical properties of environmental vaping aerosols do not allow under normal conditions to produce a THS analogue. I will also critically address the effort to assign to vaping a “third hand” exposure analogous to THS. Spoiler: studies that claim a that a “third hand vaping” exists have examined the aerosol under artificial laboratory experiments or under extremely unrealistic conditions.
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