Fog machines also use a glycol solution, and have been shown to be harmful. Thus vaping glycol may not be safe. https://www.cdc.gov/niosh/hhe/reports/pdfs/1990-0355-2449.pd...
"Based on the results of this study, there is no evidence that theatrical "smoke," at the levels found in the theaters studied, is a cause of occupational asthma among performers. Some of the constituents of theatrical "smoke," such as the aerolized glycols and mineral oil, could have irritative or mucous membrane drying properties in some individuals. Therefore, it is reasonable to minimize exposures by such means as relocating "smoke" machines to avoid exposing actors to the direct, concentrated release of the aerosols, minimizing the amount of "smoke" necessary for the production, and using only fog fluids approved by the manufacturers of the machines. The glycols used should be at the level of "food grade" or "high grade." Glycol-based systems should also be designed to heat the fog fluids only to the lowest temperature needed that achieve proper aerolsolization. This would help to avoid overheating the fluid and minimize the generation of decomposition products."
> Based on the results of this study, there is no evidence that theatrical "smoke," at the levels found in the theaters studied, is a cause of occupational asthma among performers.
That being said, chronic exposure is probably not good for you. From a health perspective, don't inhale anything other than air!
When compared to actors from the non-"smoke" productions, actors from two or more of the four productions utilizing theatrical "smoke" reported experiencing a significantly greater prevalence of nasal symptoms (sneezing, runny or stuffy nose), respiratory symptoms (cough, wheeze, breathlessness, chest tightness), and mucous membrane symptoms (sore throat, hoarseness, dry throat, itchy/burning eyes, dry eyes) during their performances for the week prior to the survey.
The final line is: The quantity and frequency of use of the various fogs during a performance should be minimized.
PS: A method of action was suggested to be the breakdown of glycol due to high temperature.
Fog machines cause noticeable effects on breathing at significantly lower exposure levels and concentrations than vaping. IOW, it provides a lower bound for the purported safety of vaping.