I was talking to a friend of mine who is a pharmacist about what is in our liquids. She pointed out that propylene glycol is used by respiratory therapists as a simple breathing treatment to help patients 'hack something up' so they can send it to the lab. Meaning it's used in nebulizers that work the same basic way that the atomizers in our PVs, and patients breathe in the PG vapor and the patient eventually is able to 'hack something up'. There could be a connection with your use of propylene glycol and having clearer lungs. If the connection is this one, I would bet that you hacked up quite a bit of gunk when you suddenly switched to propylene glycol based e-juice from cigarette smoke.