Few animal studies have been performed to evaluate the potential harm of humectants in EC liquids (i.e. propylene glycol and glycerol) when given by inhalation. Robertson and colleagues tested the effects on primates of inhaling propylene glycol vapor for several months and found no evidence of toxicity on any organ (including the lungs) after post-mortem examination of the animals [
Robertson et al. 1947]. Similar observations were made in a recent study in rats and dogs [
Werley et al. 2011].
Concerns have been raised in human use, based on studies of people exposed to theatrical fog [Varughese et al. 2005; American Chemistry Council, 2003] or propylene glycol used in the aviation industry [Wieslander et al. 2001]. Irritation of the respiratory tract was found, but no permanent lung injury or other long-term health implications were detected. It should be reminded that, in these circumstances, nonpharmaceutical purity propylene glycol is used and in some cases oils are added, making it difficult to interpret the results in the context of EC use. Evidence for the potential harm of inhaled glycerol is sparse.
A study using Sprague–Dawley rats found minimal to mild squamous metaplasia of the epiglottis epithelium in the high-dose group only, without any changes observed in lungs or other organs [
Renne et al. 1992]. No comparative set of experiments with cigarette smoke was included, but it is well known that exposure to tobacco smoke in similar animal models leads to dramatic changes in the lungs, liver and kidneys [
Czekaj et al. 2002].