Investigating the transfer rate of waterpipe additives to smoke as an integral part of toxicological risk assessments

Waterpipe, also known as hookah, narghile or narghila, shisha or hubbly bubbly, is a tobacco-smoking device. Waterpipe tobacco is heated and consumed by a process of inhaling tobacco smoke, that bubbles through water before being inhaled. To date, limited studies have examined the transfer of waterpipe additives from tobacco to smoke. This study was designed to investigate the filtration ability of water in the waterpipe’s bowl to define exposure to additives in waterpipe smoke, which is an essential requirement to perform toxicological risk assessments of waterpipe additives. Within this study, a standard smoking protocol (ISO 22486) was used to evaluate the transfer of > 40 additives from experimental and commercially available samples. These results are the first to provide such an extensive dataset of information showing transfer rates varying between 6% and 61% depending on the additive. Various physicochemical parameters of the additives including water solubility, partition coefficient, molecular weight, boiling point, and vapor pressure were also evaluated to seek to identify any correlation to transfer rate that may be later used to predict transfer. The amount of additive transfer from waterpipe tobacco to the smoke was found to be moderately correlated to vapor pressure (Pearson correlation coefficient = 0.33) with subsequent multivariate analysis using step-wise selection indicating 39% of the transfer rate variance can be explained collectively by the additive boiling point, molecular weight, vapor pressure and water solubility. These findings underscore the complexity of additive transfer and highlight the necessity of exposure assessment for meaningful waterpipe additive risk assessments.