  Electronic cigarettes (ECIGS) are a class of tobacco products that emit a
nicotine-containing aerosol by heating and vaporizing a liquid. Apart from
initiating nicotine addiction in nonsmokers, a persistent concern about these
products is that their emissions often include high levels of carbonyl species,
toxicants thought to cause most non-cancer pulmonary disease in smokers.
Carbonyls are known thermal degradation products of the primary ECIG liquid
constituents: propylene glycol (PG) and vegetable glycerin (VG). To date, there
is no method available to predict the operating conditions for which an ECIG of
a given design will emit large quantities of carbonyls. This study examined
whether the phenomenon of film boiling can account for observations of high
carbonyl emissions under certain operating conditions, and if so, whether film
boiling theory can be invoked to predict conditions where such emissions are
likely. We measured the critical heat flux for several common heating materials
and liquids, and carbonyl emissions for several ECIG types while varying power.
We found that emissions rise drastically whenever power exceeds the value
corresponding to the critical heat flux. While limiting the heat flux to below
this threshold can greatly reduce carbonyl exposure, ECIG manufacturer
operating instructions often exceed it. Product regulations that limit heat
flux may reduce the public health burden of electronic cigarette use.
