Use of the e-liquid flavourings
diacetyl and
acetyl propionyl has raised concerns that they might cause
respiratory diseases amongst vapers. Product surveys show that these compounds, plus a less toxic alternative,
acetoin, are widely used in e-liquids. We have investigated the chemistry of
acetoin,
acetyl propionyl and
diacetyl in e-liquids. They are reactive, with concentrations falling substantially over time.
Acetyl propionyl is the most reactive,
diacetyl less so, and
acetoin significantly more stable. Their reactivity is pH-enhanced when
nicotine is present in the e-liquid. Of major concern, we found that
acetoin generates
diacetyl in e-liquids. We found
diacetyl formation in all
acetoin-containing e-liquids, but it is not an
acetoin-contaminant.
Diacetyl concentrations were proportional to
acetoin content, grew over time, and formation was accelerated by
nicotine. E-liquids stored for up to 18 months contained significant
diacetyl, and reduced
acetoin levels, showing that
acetoin is a long-term
diacetyl source. Other reaction pathways operate, and we advance mechanisms to explain this area of e-liquid chemistry.
Acetoin use in e-liquids is an inevitable source of
diacetyl exposure for e-cigarette users.
Acetoin,
acetyl propionyl and
diacetyl are avoidable hazards for vapers, and we recommend e-liquid manufacturers move away from their use in e-liquid formulations.