Introduction:
Glycerin, flavorings and sweeteners collectively constitute up to 70% of water-pipe tobacco smoking (WTS) mixtures. The combustion of such ingredients produces smoke particulates that are known toxins and carcinogens including nicotine, nitrosamines and carbonyl compounds. The type and quantity of toxic emissions generated in the vapor exhaled are highly dependable on the formulation of the smoked mixture. Glycerin combustion produces known toxic and carcinogenic emissions including acetaldehyde, benzaldehyde, acrolein, and acetone. Recent evidence concerning e-cigarette indicate that increasing glycerol concentrations liquid lead to significant increase in toxicant emissions. However, the literature is lacking evidence regarding glycerin addition effects in WTS mixtures. According to WHO, there are no currently approved international upper limits regulations on WTS mixture ingredients. Therefore, the objective of this study aims to assess toxicant emission levels in response to increasing glycerin concentration in WTS mixtures.

Material and Methods:
Laboratory experimental study. The experiment measures levels of toxic compounds in emission (smoke) resulting from WTS using tobacco mixture samples with varying glycerin concentrations and fixed other main ingredients. Tobacco samples were prepared by two leading brands in the market and an equivalent generic lab-made (prepared with same tobacco mixture main ingredients).

Results:
Of all toxicant emissions observed, acrolein emission was associated with glycerin addition in WTS mixture indicated by lab-made samples throughout all glycerin concentrations (10%, 20%, 40% and 60%), and brand samples with glycerin concentrations 10% to 20%. In contrast to lab-made samples, brand samples showed no increase in acrolein emissions corresponding to the increase in glycerin concentrations from 20% to 60%.

Conclusions:
According to two water-pipe tobacco brand products, increased glycerin concentration from 20% to 60% does not yield increasing levels of acrolein emission, while lab-made tobacco sample shows significant correlation between glycerin addition and increased acrolein emission. Regulatory recommendations for water-pipe smoking products require further investigations regarding potential confounders in acrolein emissions and health effects of glycerin concentrations with corresponding toxicant emissions.

MAIN MESSAGES - Combustion of ingredients in tobacco mixtures used in water-pipe smoking (WTS) and e-cigarettes produces toxins in exhaled smoke (e.g. carbonyl compounds), of which the type and amount are highly dependable on tobacco mixture formula. Glycerin in tobacco mixture contribute to enhanced taste, vapor and smoking experience quality, however, its’ combustion produces toxicants such as acrolein. - According to WHO, there are no approved international upper limits regulations on WTS mixture ingredients. - This study assessed toxicant emission levels corresponding to increasing glycerin concentration in WTS mixtures. - Acrolein emission was significantly associated with glycerin addition. Two popular market products revealed that increased glycerin concentration from 20% to 60% did not yield increasing levels of acrolein emission. - In contrast, equivalent generic lab-made product (prepared with same basic compounds as market products) showed significant directly proportionate correlation between acrolein emission and glycerin concentration in tobacco mixtures.The contrast in findings between brand products and generic lab-made equivalent may point to unexplored differences in manufacturing and/or add-on ingredients potentially affecting exhaled/inhaled toxicants. - Regulatory recommendations for WTS products require further investigations regarding potential confounders such as brand tobacco supplementary additives and its effects on emitted/trapped toxins. STUDY SIGNIFICANCE AND IMPLICATIONS This study explores the effect of glycerin addition to tobacco mixtures on toxic emissions in smoke. Potential increase in toxic emissions indicates higher toxicity of glycerin, in which case may direct further research to health effects of high glycerin levels in tobacco and subsequent regulatory recommendations for tobacco policy makers. Our findings suggest that regulatory recommendations for WTS products require further investigations regarding potential confounders such as brand tobacco supplementary additives and its effects on emitted toxins. In summary, this study’s insights may aid the research field concerning the development of tobacco manufacturing regulations and policies towards potential harm reduction.