Toxicant content, physical properties and biological activity of waterpipe tobacco smoke and its tobacco-free alternatives

The narghile waterpipe

This review focuses on the narghile waterpipe which has been at the centre of the global waterpipe use epidemic witnessed in the past decade.³³ This type of waterpipe (figure 1), also commonly referred to as ‘shisha’ and ‘hookah’, is most commonly smoked using maassel, a preparation of shredded tobacco, glycerol and other additives, and sold in a broad range of flavours that mimic various fruits, candy and beverages. Because maassel is incapable of burning on its own, it is normally smoked using charcoal as a heat source. The charcoal is placed on top of the maassel, with a thin, perforated sheet of aluminium foil separating them. It is noteworthy that while maassel is the principle intended material of consumption, considerably more charcoal is consumed than maassel during a typical use session.¹⁰

During a puff, air drawn into the waterpipe passes over the charcoal and picks up coal combustion products and thermal energy. The heated air and combustion products then pass through the waterpipe head and entrain vapours emanating from the maassel. These vapours cool and recondense some distance downstream of the head to form an optically dense, white aerosol. Waterpipe smoke, therefore, consists of gases and particles emanating from the charcoal and the heated tobacco preparation. As with cigarette smoke, this aerosol includes constituents which are simply transferred from the raw material (eg, glycerol, nicotine, tobacco-specific nitrosamines (TSNAs) evaporating from the tobacco leaf), constituents which are chemically synthesised during smoking (eg, carbon monoxide, CO) and constituents which are both transferred and synthesised in situ (eg, polyaromatic hydrocarbons ³⁴).

A narghile waterpipe is assembled by pressing the head onto the metal body (figure 1), using tissue paper or a rubber gasket at the joint to make a seal. The interface between the body and the glass water bowl is similarly sealed, as is the interface between the body and hose. The flexible hose is most commonly made of leather or other fibrous material, and is porous, and therefore allows air to infiltrate and dilute the smoke during every puff. The inherent variability in hose porosity, and waterpipe sealing are challenges for making reproducible measurements of toxicants in waterpipe smoke, and have led to the manufacture of at least one analytical grade, air-tight waterpipe for laboratory testing purposes (Borgwaldt Shisha Smoker).

Smoke generation methods in the analytical laboratory

Studies of WTS smoke constituents and its in vitro effects normally involve generating smoke from a waterpipe using a smoking machine that is programmed to produce a given puffing regimen, and then sampling the particle and gas phases of the smoke for subsequent assays. Numerous factors can influence WTS properties and chemistry, including the composition of the product smoked, the quality of the charcoal used, the design and construction of the waterpipe (eg, volume of the water bubbler head space, the fresh air infiltration rate of the hose during a puff), and puff topography (ie, the number of puffs drawn, the puff volume and duration, and the interval between successive puffs).

To meet scientific conventions of reproducibility, analytical laboratory studies of WTS composition and biological activity necessarily involve specifying these and other variables, ideally in some combination that represents a common waterpipe use scenario. Most studies to date have adopted some variants of the Beirut method,³⁵ which specifies the amount of maassel tobacco preparation to be loaded in the waterpipe, a charcoal lighting and replacement regimen, an aluminium foil perforation pattern, and a steady periodic 1 h puffing regimen consisting of 171 puffs of 530 mL volume and 2.6 s duration, with a 17 s interpuff interval. This method was derived from aggregate measurements of puff topography from waterpipe users made during two field campaigns in Beirut area cafés where waterpipes were served.^{35 36} Importantly, the method was found to produce realistic yields of ‘tar’, nicotine and CO when compared with measurements made using an experimental device that allowed real-time sampling of smoke generated by consumers in the same Beirut area cafés.³⁵

Puff topography has been measured in other settings and populations, ^{37 38} and using different products, ³⁹ and found to differ considerably from the topography parameters used to derive the Beirut method. This factor alone highlights a key limitation inherent in any standard machine smoking protocol; rather than providing a method for estimating a given individual's toxicant intake, it provides a highly idealised but reproducible methodology for generating smoke for subsequent testing, and is particularly useful for exploratory work on identifying and quantifying new toxicants and biological effects in waterpipe smoke.

To examine toxicants and biological effects over a wider range of use behaviours and waterpipe products, some studies have used a ‘playback’ approach in which records of puff topography from sampled waterpipe-consuming individuals are reproduced in fine detail using a digital smoking machine.^{20 22} By comparing toxicant levels in playback machine-produced smoke to blood concentrations of these toxicants in the individuals whose topography was recorded, it has been shown that playback provides quantitative predictions of toxicant exposure for a given sample population of waterpipe users.⁴⁰ A key advantage of this approach is that it automatically accounts for interactions between product features (eg, nicotine content, flavour) and puff topography. For example, it has been reported that when provided a low-nicotine waterpipe tobacco, experienced waterpipe users puff at a higher frequency,⁴¹ likely resulting in different toxicant yields than would be measured with a puffing regimen that does not account for this behavioural difference. This ‘compensatory’ behaviour has been long recognised in cigarette smoking.⁴²

In summary, waterpipe smoke toxicant content and effects have been studied using different smoke generation protocols which fall into two broad categories: steady periodic and playback. The former has the advantage of holding constant all variables, allowing for optimisation of chemical assays and investigation of dose–response characteristics in biological assays, while the latter has the advantage of producing smoke that closely resembles that which is produced by a given population using a given product. Results reported below draw from both approaches.

Toxicant content

Approximately, 300 chemicals have been identified in waterpipe tobacco and smoke to date. This number amounts to a small fraction of the 9600 compounds listed in Rodgman and Perfetti's⁴³ most recent edition of “The Chemical Components of Tobacco and Tobacco Smoke”. This embryonic state of knowledge likely reflects the comparatively small number of investigators studying waterpipe tobacco and WTS, its recent adoption as a subject for sustained study and the fact that existing analytical procedures developed to study cigarettes often are insufficient for analysing WTS.

Unsurprisingly, studies of WTS have so far focused on identifying and quantifying chemicals that appear in the ‘Hoffmann list’,⁴⁴ a list of tobacco or tobacco smoke constituents thought to be major or contributing causative agents in tobacco-related diseases (table 1). Eighty-two toxicants have been quantified in WTS to date, including polyaromatic hydrocarbons (PAHs), heterocyclic compounds, PAAs, N-heterocyclic amines, TSNAs, carbonylic compounds, VOCs, and miscellaneous organic and inorganic compounds. In the following subsections, the information currently available on the yields of these WTS constituents is summarised.

Biological activity

Three studies have examined biological activity of WTS using in vitro culture systems.^23–25 It was found that Beirut method-generated WTS elicited deleterious effects on cell function in human lung epithelial cells²⁴ and vascular endothelial cells.²³ These studies implicated waterpipe smoke as a contributing factor in the pathogenesis of chronic obstructive pulmonary disease and vascular disease by impairing cellular growth (cell cycle arrest) and cell repair, and inducing inflammation, oxidative stress, impaired vasodilation, cellular senescence, increased levels of matrix metalloproteinases and attenuation of angiogenesis. Using playback WTS and smoke generated with tobacco-free products, it was also reported²⁵ that tobacco-free waterpipe smoke induced cellular dysfunction on human lung cells on par with tobacco-containing products.

Particle size distribution

Key carcinogens in tobacco smoke such as benzo(a)pyrene and heavy metals are predominantly delivered in submicron sized particles. The location and extent to which inhaled particles deposit in the respiratory tract is central to the question of toxicant delivery and subsequent effects; for example, the effective dose of a given toxicant to the lung is the product of the toxicant mass inhaled and the fraction that deposits in the lung. Experimental studies of cigarette smokers indicate that 22–89% of the mainstream smoke particles inhaled with each puff remain in the smoker.^{51 52} It is generally accepted that the detailed physical properties of tobacco smoke particulate matter determine the deposition fraction and location, which in turn determine the occurrence, type and location of tumours found in the respiratory system. Key among these properties is particle size distribution.

Two studies^{53 54} of mainstream WTS particle size distributions have been reported in the literature. Using a differential mobility analyser and laser spectrometer, Monn et al⁵³ scanned particle diameters ranging from 0.01 to 10 μm in machine-generated WTS (two 5 s puffs per minute, 1 L/puff) and cigarette smoke. The method involved injecting individual puffs of WTS or cigarette smoke into a 113 L chamber filled with fresh air, resulting in a diluted smoke sample for subsequent particle size analysis. They reported a number mean diameter of 0.04 μm for WTS, compared with about 0.3 μm for cigarette smoke. Becquemin et al⁵⁴ utilised an electrical low-pressure impactor to determine WTS particle mass distributions, in a diameter range of 0.028–10 μm. They reported a mass median diameter for 0.24 μm for WTS particles. Results from these studies suggest that WTS is a submicron aerosol with capability of depositing in the human respiratory tract in a manner similar to cigarette smoke. It should be noted however that both studies likely suffered from bias due to particle evaporation before or during sizing. Such evaporation would result in underestimation of the true particle size. In the case of Monn et al,⁵³ evaporation would be expected in the setup employed because the setup involved trapping and diluting WTS with a large quantity of fresh air prior to sizing. In the case of Becquemin et al,⁵⁴ particles likely partially evaporated during transit through the low-pressure stages of the impactor. Further studies are needed which account for the high volatility composition of WTS particles.

Is smoke generated by tobacco-free waterpipe products less toxic?

This question has been addressed in two studies^{7 22} that directly compared toxicant content of smoke generated using tobacco-containing and tobacco-free mixtures, and one study that directly compared in vitro biological effects of the two types of products.²⁵ Using the Beirut method, Hammal et al⁷ compared volatile aldehydes, PAHs, NO, CO, ‘tar’ and nicotine yields for smoke generated using a tobacco-based waterpipe product and three tobacco-free products. Shihadeh et al²² measured the same toxicants using a playback approach with puff topography recordings of 31 participants who smoked a waterpipe in a double-blind controlled laboratory study, once using a tobacco-based product of the participants choice, and once using flavour-matched tobacco-free product (table 2). While both studies found that nicotine was present only in the smoke of the tobacco-based products, every other toxicant measured using the tobacco-free products equalled or exceeded those measured using the tobacco-based products. Consistent with these findings, Shihadeh et al²⁵ found that waterpipe smoke produced using both types of waterpipe products markedly reduced cell proliferation, caused cell cycle arrest and increased cell doubling time in human alveolar cells. There were no significant differences across product in any measure. Taken together, these studies indicate that while using non-tobacco products presents no risk of nicotine exposure, smoking a tobacco-free waterpipe product very likely presents no less disease risk than smoking a conventional tobacco-based product.