My take on the "PG and VG can be harmful when inhaled" point is quite simple - If we, as vapers, maintain that it is harmless, we are doing ourselves and injustice. Many things are harmful when inhaled in sufficient quantities - the same applies to PG and VG. We need to, in my opinion, acknowledge (as KMS has done, above) that PG can cause irritation of the upper airway, and a slight increase in airway resistance, predominantly in no smokers, BUT when inhaling vapour is compared directly with inhaling smoke (of any origin, not just tobacco) it is less harmful physiologically by a very long margin.
I think
McAldo made a very good point in a similar thread a few days back that a lot of things are used in medicine out of necessity rather than choice - for example, some drugs cannot be dissolved in water for injection, so they are dissolved in PG and other solvents - at the other end of the scale there are drugs used in psychiatry which are suspended in sesame oil! A lot of drugs are not approved for the purpose of certain treatments - but they are still used - not because they are safe, but because they achieve the desired result for a particular illness or condition. A huge number of chemotherapy drugs are, in fact, carcinogenic.
The upshot is - we need to acknowledge that there are mild side effects, that animal research has shown that it is very unlikely for there to be any significant long-term adverse effects, and to reinforce our point with a like-for-like comparison. In my opinion, the vaping advocate that proclaims vaping as harmless is as uniformed as the non-vaper who proclaims it as bad as cigarette smoking. Common sense says that vaping is less harmful, and that is the point - it is LESS harmful. We need to supply information to support our perspective whilst at the same time acknowledging that nothing we breathe in is completely harm free, but there are somethings which are a lot better to breathe in, out of personal choice.