From a quick Google search, research into the inhalation of VG & PG is limited.
Limited initial research though apparently does suggest that the inhalation of PG can increase inflammatory cells in the lungs over time. Genome-wide expression, microRNA, and mRNA test results showed that Inflammatory cell counts and cytokines were determined in bronchoalveolar lavage (BAL) fluids as being higher in those who inhaled more PG. Apparently this was a short duration study and more research is needed.
EDIT: Just started reading this, but here's a study
Toxicology of E-Cigarette Constituents - Public Health Consequences of E-Cigarettes - NCBI Bookshelf (nih.gov)
Also:
Ethylene Glycol and Propylene Glycol Toxicity: What is Propylene Glycol? | Environmental Medicine | ATSDR (cdc.gov)
One research paper says: "
Through the randomized clinical trial of healthy never-smokers over a month, we found that an increase in urinary propylene glycol, a marker of inhalation-e-cig intake, was significantly correlated with increased inflammatory response in the lung.”.
I don't think it has been proven to be safe. Not from what I've read in the last 5mins anyway.