1,2-propylene glycol (propylene glycol, PG) is a colourless, tasteless viscous liquid. Because of its good atomisation effect, it is commonly used as a primary ingredient in electronic cigarette e-liquids and thus can present scenarios of potential long-term inhalation in humans. Its safety for chronic and high-dose inhalation exposures has therefore been of wide interest.

A 13-week nose-only inhalation experiment of aerosolised PG followed by 4-week recovery period in Wistar rats was conducted. A self-made capillary aerosol generator (CAG) was used to produce PG aerosol based on the electronic cigarette principle. Male and female Wistar rats were assigned into four groups (Sham, low-, middle- and high-dose). All exposure groups were exposed to PG aerosol at a concentration of 28 mg/L air and targeted doses of 100 mg/kg, 500 mg/kg, and 1500 m/kg (for low-, middle- and high-dose groups, respectively) were achieved by adjusting the exposure duration based on body weight. Biological endpoints after 13-week exposure and 4-week recovery period included hematology, coagulation, serum biochemistry, urinalysis, bronchoalveolar fluid (BALF) analysis, necropsy and histopathology.

The non-observed effect level (NOEL) values were determined to be 100 mg/kg based on body weight and food consumption data. All other statistically significant differences in hematology, coagulation, serum biochemistry, urinalysis, BALF analysis and organ weight data were considered to be attributed to biological variation and not regarded as adverse effects because all parameters with statistically significant changes were within normal ranges and there were no microscopic correlates. Histopathological examination of both sexes in all groups revealed the Sham group and high-dose group exhibited similar incidences and degrees of pulmonary pathological changes. No abnormal morphologic variations were observed in the nasal passage, trachea and larynx across groups.

In summary, exposure to PG aerosol for 13 weeks in rats results in relatively low toxicity.