That article is riddled with errors. There are too many to list, but here are a couple:
".....it alters the structure of the skin by allowing chemicals to penetrate deep beneath it while increasing their ability to reach the blood stream."
This is the direct opposite of the operation of PG with regard to compounds in dilution: in fact there is an effect we call 'PG lock-in' whereby it slows down bioavailability. In cigarette manufacture it is known that the higher the PG content, the less nicotine is available; in ecig refills, the more PG is added to a refill, the longer you have to wipe it off the skin after a spill (pure nic spilled on the skin is highly toxic or fatal, but nic with PG spilled onto the skin is blocked from absorption for a time increasing with the PG percentage).
".....propylene glycol ..... has been shown to be linked to cancer.....".
No it hasn't. There are 70 years of safe usage and clinical research data that show it is far safer than the usual 'acceptably safe' level needed for pharmaceuticals.
It is universally licensed for pharmaceutical use, for topical / ingestion / inhalation / intravenous duty. You can breathe it in or inject it without issue. It has been used as the excipient in asthma inhalers for decades; it is the excipient used in the nebulizer treatments given to lung transplant patients. It is the diluent used for immiscible drugs such as diazepam - when a drug does not mix with water, PG is used instead for the injectable medium.
There are few materials with such a long history of research without issue, or safe use, or such a wide usage profile in modern medicine and industrial use. For example it is used to treat the water in large building air-conditioning plant, as it kills waterborne and airborne pathogens, which is why you don't get Legionnaire's Disease. Since the disease pathogens circulate in the building air after dispersal as an aerosol from an untreated aircon plant's water system, you can reliably assume that so do the PG molecules, and therefore that people in large buildings breathe PG. This is why hospitals don't use PG for aerosol air treatment to reduce infection any more, the building air plant already does this.
There are minimal personal tolerance issues for PG: more than than 1 in 100 but less than 1 in 10 have a throat / upper respiratory tract drying-out issue, with ecig or inhaler use, that may recede with use. Any other effect is so unusual that it has not been possible to reliably identify / isolate it as being due to PG (skin dryness issues etc). Glycerine is gradually replacing PG as the most-common inhaler diluent (search: Dow Optim) but as far as we know this is only because of the humectant effect of PG on the throat of some users.
There are hundreds of millions of user-years that say PG is safe. There is no body of clinical research that appears to show otherwise. Individual clinical trials or studies are meaningless until the results are repeated.
The MSDS can be completely ignored as it has no relevance at all except for transport and storage contractors. The LD50 of PG is about the same as water, 50 gallons (you have to drown in it...). PG can be regarded as either inert in the human organism, or as a complex carbohydrate part-excreted and part-metabolysed to lactic acid and excreted - the effect is about the same.
