Eucalyptus has been inhaled in vapor for hundreds of years (think aromatherapy and steam inhalation). The biggest issue with Eucalyptus is topical (use on skin) toxicity when used in high doses. The following information is from Wikipedia:
If consumed internally at low dosage as a flavoring component or in pharmaceutical products at the recommended rate, cineole based 'oil of eucalyptus' is safe for adults. However, systemic toxicity can result from ingestion or topical application at higher than recommended doses.[17]
The probable lethal dose of pure eucalyptus oil for an adult is in the range of 0.05 mL to 0.5 mL/per kg of body weight.[18] Because of their high body surface area to mass ratio, children are more vulnerable to poisons absorbed transdermally. Severe poisoning has occurred in children after ingestion of 4 mL to 5 mL of eucalyptus oil.[
Chinese
e-liquid (Dekang) has had oil based ingredients in it from the beginning. Commonly found are Rose oil (typically 4%) and tobacco essential oils (at 5%). This website lists testing and ingredients
https://www.........../images/main/msds/SH081111-190.pdf (you'll have to insert www (dot) e-cig(dot) com into the link as for some reason, the forum pulls it out of the links so they must have been spamming, LOL)
Scubabatdan has been using oils in his DIY for quite a while and hasn't reported any problems. Rolygate uses peppermint oil for well over a year and has had no reported issues.
Some oils (lipid oils) can cause Lipid Pneumonia (this is probably what your well intentioned warning was about), but this isn't a problem with aroma therapy and food grade flavoring oils. As long as you are using
food grade oils, and keep to a low amount added you shouldn't have any problems other than maybe your atomizers. However, do be aware that any ingredient used heavily can suddenly cause allergies, especially if you are using it 24/7. If you notice any lung issues or unexplained illness, stop using the liquid to see if the problem subsides. DIY does have risks.
When using oils, just because it says insoluble in water means just that WATER. However PG is not water. If you are using 1% oils (it is not necessary usually to use beyond that, as they are strong), oil base will mix just fine in PG with or without alcohol. At 1% that is about 4 drops of oil to 10ml of juice when using a pipette that has a ratio of 40 drops = 1ml. (this info. was from the forum, but when I copied it into my computer file, I forgot to note the original author).
Do a dilution of your 1% oil in a very small amount of alcohol just to thin it and allow an easier infusion process so you don't have floating oil in your mix.
Happy DIY Vaping!
