Electronic cigarette refill liquids (e-liquids) are commonly composed of propylene glycol and glycerol as solvents, along with nicotine, an organic acid for pH adjustment, and a host of flavorants for aroma. One popular group of flavorants are flavor aldehydes such as vanillin, benzaldehyde, or cinnamaldehyde. As expected, these flavor aldehydes have been shown to react with the solvents propylene glycol and glycerol to form a variety of acetals in a simple mixture of the three components, without an active water removal step. Furthermore, acetals, such as vanillin propylene glycol acetal, have been detected in a wide variety of commercial e-liquids and a recent study has shown that flavor aldehyde propylene glycol acetals reach the aerosol and therefore the user’s airways during “vaping” of the e-cigarette, and that they are relatively stable in the aqueous environment of the airways. Toxicological studies have shown that vanillin propylene glycol is in fact more irritating to human lung cells than vanillin itself and that benzaldehyde propylene glycol acetal is more toxic to BEAS-2B lung cells than pure benzaldehyde. Taken together, this raises several questions on e-cigarette safety as well as labeling requirements.<br/><br/>In an effort to reduce the buildup of flavor aldehyde propylene glycol acetals, this study explored the extent to which the addition of various amounts of water (0 wt-%, 5 wt-%, 10 wt-%, 20 wt-%)  to a lab-made e-liquid suppressed the acetal formation therein. Initial results suggest that the equilibrium concentration between initially added flavor aldehyde and the corresponding acetal depends on the aldehyde. While for vanillin, the buildup reaches an equilibrium at an acetal mole fraction of ~0.5 in 3 weeks, benzaldehyde reacts much quicker and to an acetal mole fraction of up to 0.95. Since water is a by-product of the acetalization reaction, its addition is expected to reduce the total amount of acetal formed, and initial results confirm this: the addition of 20 wt-% of water can reduce the extent of flavor aldehyde acetal formation by a factor of up to 10.<br/><br/>Regulators as well as e-liquid manufacturers could utilize this strategy to lower flavor aldehyde acetal contents in their products by using one of the most benign materials, water. In addition, water-containing e-liquids would also likely retaining the aroma profile since the free flavor aldehydes tend to have a stronger aroma than their corresponding acetals.

Altering e-liquid composition and toxicology through the addition of water

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