I'm not sure how many vapers - experienced or otherwise - are aware of the differences in quality and safety of flavoring oils that are used in mixing their e-juices. I've found a few sources that question the presence of diacetyl and eugenol in some liquids. Rocket Fuel Vapes mentioned this in one of their blog posts.
"Since e-liquids primarily get their taste from food flavorings, many e-liquid flavors are at risk of containing diacetyl. Although the risks are much lower for vapers inhaling diacetyl-containing e-liquids than for workers in manufacturing settings handling diacetyl, it is important to understand the potential consequences of extended diacetyl inhalation."
"… The latest study of e-liquids found that those containing diacetyl may cause harm to the respiratory system over time. It is important to note that of the e-liquids tested, the average carries around 100 times less diacetyl than the traditional tobacco cigarette. The scientific community considers vaping an avoidable risk and seeks to arm users with the knowledge to determine the best course of action for one's self."
Diacetyl is a volatile organic compound with a yellow or green color. It's generally used as food flavoring to bring out an intensely buttery taste in food items and beverages. It's also naturally found in apples, artichokes, beans, butter, coffee, dairy, fruits, honey, tobacco and vinegar in very small amounts.
In comparison, it seems that eugenol is as hazardous to respiratory health as diacetyl. A page at Medline Plus mentioned that this compound is often found in clove cigarettes:
Clove cigarettes, also called kreteks, generally contain 60% to 80% tobacco and 20% to 40% ground clove. Eugenol, one of the chemicals in clove, acts like menthol to reduce the harshness of tobacco smoke.
We all know that most e-juices available in the market today have not been thoroughly tested by the FDA. If you're unsure of the quality of your chosen flavorings and additives, then check out this Guide to Flavouring provide by Planet of the Vapes.
Experienced vapers have been using these commercial flavoring oils as ingredients in their custom e-juices, and so far, there haven't been any reports of someone falling grievously ill after inhaling those vaping liquids. Their flavoring strengths vary, but they usually fall into one of three ranges: between 5% and 10%, between 10% and 20%, and between 20% and 50%.
The decision to inhale these e-liquids is your personal responsibility, and you should take more than a few minutes pondering over the consequences of using some of these flavoring oils in creating your custom e-juices. In the end, however, it all comes down to your ability to make discerning judgments on what's good and bad for your health.